Handle streaming PGNs, like from Lichess (although this might break non-streaming...

[remoteglot] / www / js / remoteglot.js

(function() {
'use strict';

/**
 * Version of this script. If the server returns a version larger than
 * this, it is a sign we should reload to upgrade ourselves.
 *
 * @type {number}
 * @const
 * @private */
let SCRIPT_VERSION = 2023122700;

/**
 * The current backend URL.
 *
 * @type {!string}
 * @private
 */
let backend_url = "/analysis.pl";
let backend_hash_url = "/hash";

/** @type {window.ChessBoard} @private */
let board = null;

/** @type {boolean} @private */
let board_is_animating = false;

/**
 * The most recent analysis data we have from the server
 * (about the most recent position).
 *
 * @type {?Object}
 * @private */
let current_analysis_data = null;

/**
 * If we are displaying previous analysis or from hash, this is non-null,
 * and will override most of current_analysis_data.
 *
 * @type {?Object}
 * @private
 */
let displayed_analysis_data = null;

/**
 * Games currently in progress, if any.
 *
 * @type {?Array.<{
 *      name: string,
 *      url: string,
 *      hashurl: string,
 *      id: string,
 *      score: Object=,
 *      result: string=,
 * }>}
 * @private
 */
let current_games = null;

/** @type {Array.<{
 *      from_col: number,
 *      from_row: number,
 *      to_col: number,
 *      to_row: number,
 *      line_width: number,
 *      arrow_size: number,
 *      fg_color: string
 * }>}
 * @private
 */
let arrows = [];

/** @type {Array.<Array.<boolean>>} */
let occupied_by_arrows = [];

/** Currently displayed refutation lines (on-screen).
 * Can either come from the current_analysis_data, displayed_analysis_data,
 * or hash_refutation_lines (choose_displayed_refutation_lines() chooses which one).
 *
 * @typedef {{
 *     score: Array,
 *     depth: string,
 *     pv: Array.<string>,
 *     move: string
 * }}
 * @private
 */
var RefutationLine;

/** Refutation lines from current hash probe.
 * If non-null, will override refutation lines from the base position.
 *
 * @type {Array.<RefutationLine>}
 */
let hash_refutation_lines = null;

/**
 * What FEN hash_refutation_lines is relative to.
 */
let hash_refutation_lines_base_fen = null;

/** @type {number} @private */
let ims = 0;

/** @type {boolean} @private */
let truncate_display_history = true;

/** @type {!string|undefined} @private */
let highlight_from = undefined;

/** @type {!string|undefined} @private */
let highlight_to = undefined;

/** The HTML object of the move currently being highlighted (in red).
 * @type {?Element}
 * @private */
let highlighted_move = null;

/** Currently suggested/recommended move when dragging.
 * @type {?{from: !string, to: !string}}
 * @private
 */
let recommended_move = null;

/** If reverse-dragging (dragging from the destination square to the
 * source square), the destination square.
 * @type {?string}
 * @private
 */
let reverse_dragging_from = null;

/** @type {?number} @private */
let unique = null;

/** @type {?string} @private */
let admin_password = null;

/** @type {boolean} @private */
let enable_sound = false;

/** @type {!number} @private */
let delay_ms = 0;

/**
 * Our best estimate of how many milliseconds we need to add to 
 * new Date() to get the true UTC time. Calibrated against the
 * server clock.
 *
 * @type {?number}
 * @private
 */
let client_clock_offset_ms = null;

let clock_timer = null;

/** The current position being analyzed, represented as a FEN string.
 * Note that this is not necessarily the same as display_fen.
 * @type {?string}
 * @private
 */
let base_fen = null;

/** The current position on the board, represented as a FEN string.
 * Note that board.fen() does not contain e.g. who is to play.
 * @type {?string}
 * @private
 */
let display_fen = null;

/** @typedef {{
 *    start_fen: string,
 *    pv: Array.<string>,
 *    scores: Array<{first_move: number, score: Object}>,
 *    start_display_move_num: number
 * }}
 *
 * "start_display_move_num" is the (half-)move number to start displaying the PV at,
 * i.e., the index into pv.
 *
 * "score" is also evaluated at this point. scores can be empty and is frequently
 * sparse; it's generally only really useful for history (we obviously don't know
 * much about how the score will * move during e.g. a PV, except that a mate/TB
 * counter might go down).
 */
var DisplayLine;

/** All PVs that we currently know of.
 *
 * Element 0 is history (or null if no history).
 * Element 1 is current main PV, or explored line if nowhere else on the screen.
 * All remaining elements are refutation lines (multi-PV).
 *
 * @type {Array.<DisplayLine>}
 * @private
 */
let display_lines = [];

/** @type {?DisplayLine} @private */
let current_display_line = null;

/** @type {boolean} @private */
let current_display_line_is_history = false;

/** @type {?number} @private
 *
 * The highlighted/used/shown move in current_display_line.pv, in terms of absolute index
 * (not relative to e.g. the start FEN).
 */
let current_display_move = null;

/**
 * The current backend request to get main analysis (not history), if any,
 * so that we can abort it.
 *
 * @type {?AbortController}
 * @private
 */
let current_analysis_xhr = null;

/**
 * The current timer to fire off a request to get main analysis (not history),
 * if any, so that we can abort it.
 *
 * @type {?number}
 * @private
 */
let current_analysis_request_timer = null;

/**
 * The current backend request to get historic data, if any.
 *
 * @type {?AbortController}
 * @private
 */
let current_historic_xhr = null;

/**
 * The current backend request to get hash probes, if any, so that we can abort it.
 *
 * @type {?AbortController}
 * @private
 */
let current_hash_xhr = null;

/**
 * The current timer to display hash probe information (it could be waiting on the
 * board to stop animating), if any, so that we can abort it.
 *
 * @type {?number}
 * @private
 */
let current_hash_display_timer = null;

function supports_html5_storage() {
        try {
                return 'localStorage' in window && window['localStorage'] !== null;
        } catch (e) {
                return false;
        }
}

// Make the unique token persistent so people refreshing the page won't count twice.
// Of course, you can never fully protect against people deliberately wanting to spam.
function get_unique() {
        let use_local_storage = supports_html5_storage();
        if (use_local_storage && window['localStorage']['unique']) {
                return window['localStorage']['unique'];
        }
        let unique = Math.random();
        if (use_local_storage) {
                window['localStorage']['unique'] = unique;
        }
        return unique;
}

function request_update() {
        current_analysis_request_timer = null;

        let handle_err = () => {
                // Backend error or similar. Wait ten seconds, then try again.
                current_analysis_request_timer = setTimeout(function() { request_update(); }, 10000);
        };

        current_analysis_xhr = new AbortController();
        const signal = current_analysis_xhr.signal;
        fetch(backend_url + "?ims=" + ims + "&unique=" + unique, { signal })
                .then((response) => response.json().then(data => ({ok: response.ok, headers: response.headers, json: data})))  // ick
                .then((obj) => {
                        if (!obj.ok) {
                                handle_err();
                                return;
                        }

                        if (delay_ms === 0) {
                                process_update_response(obj.json, obj.headers);
                        } else {
                                setTimeout(function() { process_update_response(obj.json, obj.headers); }, delay_ms);
                        }

                        // Next update.
                        if (!backend_url.match(/history/)) {
                                let timeout = 100;
                                current_analysis_request_timer = setTimeout(function() { request_update(); }, timeout);
                        }
                })
                .catch((err) => {
                        if (err.name === 'AbortError') {
                                // Aborted because we are switching backends. Abandon and don't retry,
                                // because another one is already started for us.
                        } else {
                                console.log(err);
                                handle_err(err);
                        }
                })
                .finally(() => {
                        // Display.
                        document.body.style.opacity = null;
                })

}

function process_update_response(data, headers) {
        sync_server_clock(headers.get('Date'));
        ims = headers.get('X-RGLM');
        let num_viewers = headers.get('X-RGNV');
        let new_data;
        if (Array.isArray(data)) {
                new_data = JSON.parse(JSON.stringify(current_analysis_data));
                JSON_delta.patch(new_data, data);
        } else {
                new_data = data;
        }

        let minimum_version = headers.get('X-RGMV');
        if (minimum_version && minimum_version > SCRIPT_VERSION) {
                // Upgrade to latest version with a force-reload.
                location.reload(true);
        }

        // Verify that the PV makes sense.
        let valid = true;
        if (new_data['pv']) {
                let hiddenboard = new Chess(new_data['position']['fen']);
                for (let i = 0; i < new_data['pv'].length; ++i) {
                        if (hiddenboard.move(new_data['pv'][i]) === null) {
                                valid = false;
                                break;
                        }
                }
        }

        if (valid) {
                possibly_play_sound(current_analysis_data, new_data);
                current_analysis_data = new_data;
                update_board();
                update_num_viewers(num_viewers);
        } else {
                console.log("Received invalid update, waiting five seconds and trying again.");
                setTimeout(function() { location.reload(true); }, 5000);
        }
}

function possibly_play_sound(old_data, new_data) {
        if (!enable_sound) {
                return;
        }
        if (old_data === null) {
                return;
        }
        let ding = document.getElementById('ding');
        if (ding && ding.play) {
                if (old_data['position'] && old_data['position']['fen'] &&
                    new_data['position'] && new_data['position']['fen'] &&
                    old_data['position']['fen'] !== new_data['position']['fen']) {
                        ding.play();
                }
        }
}

/**
 * @type {!string} server_date_string
 */
function sync_server_clock(server_date_string) {
        let server_time_ms = new Date(server_date_string).getTime();
        let client_time_ms = new Date().getTime();
        let estimated_offset_ms = server_time_ms - client_time_ms;

        // In order not to let the noise move us too much back and forth
        // (the server only has one-second resolution anyway), we only
        // change an existing skew if we are at least five seconds off.
        if (client_clock_offset_ms === null ||
            Math.abs(estimated_offset_ms - client_clock_offset_ms) > 5000) {
                client_clock_offset_ms = estimated_offset_ms;
        }
}

function clear_arrows() {
        for (let i = 0; i < arrows.length; ++i) {
                if (arrows[i].svg) {
                        arrows[i].svg.remove();
                        delete arrows[i].svg;
                }
        }
        arrows = [];

        occupied_by_arrows = [];
        for (let y = 0; y < 8; ++y) {
                occupied_by_arrows.push([false, false, false, false, false, false, false, false]);
        }
}

function redraw_arrows() {
        for (let i = 0; i < arrows.length; ++i) {
                position_arrow(arrows[i]);
        }
}

/** @param {!string} fen
 * @return {!string}
 *
 * Return whose side it is to play (w or b), given a FEN.
 */
function find_toplay(fen) {
        return fen.split(' ')[1];
}

/** @param {!string} fen
 * @return {!number}
 *
 * Return the move clock, starting from 1. See also find_toplay().
 */
function find_move_num(fen) {
        return parseInt(fen.split(' ')[5]);
}

/** @param {!string} fen
 * @return {!number}
 *
 * Return the half-move clock, starting from 0 (and never resetting).
 */
function find_halfmove_num(fen) {
        let move_num = find_move_num(fen);
        let toplay = find_toplay(fen);
        return (move_num - 1) * 2 + (toplay === 'w' ? 0 : 1);
}

/** @param {!number} x
 * @return {!number}
 */
function sign(x) {
        if (x > 0) {
                return 1;
        } else if (x < 0) {
                return -1;
        } else {
                return 0;
        }
}

/** See if drawing this arrow on the board would cause unduly amount of confusion.
 * @param {!string} from The square the arrow is from (e.g. e4).
 * @param {!string} to The square the arrow is to (e.g. e4).
 * @return {boolean}
 */
function interfering_arrow(from, to) {
        let from_col = from.charCodeAt(0) - "a1".charCodeAt(0);
        let from_row = from.charCodeAt(1) - "a1".charCodeAt(1);
        let to_col   = to.charCodeAt(0) - "a1".charCodeAt(0);
        let to_row   = to.charCodeAt(1) - "a1".charCodeAt(1);

        occupied_by_arrows[from_row][from_col] = true;

        // Knight move: Just check that we haven't been at the destination before.
        if ((Math.abs(to_col - from_col) == 2 && Math.abs(to_row - from_row) == 1) ||
            (Math.abs(to_col - from_col) == 1 && Math.abs(to_row - from_row) == 2)) {
                return occupied_by_arrows[to_row][to_col];
        }

        // Sliding piece: Check if anything except the from-square is seen before.
        let dx = sign(to_col - from_col);
        let dy = sign(to_row - from_row);
        let x = from_col;
        let y = from_row;
        do {
                x += dx;
                y += dy;
                if (occupied_by_arrows[y][x]) {
                        return true;
                }
                occupied_by_arrows[y][x] = true;
        } while (x != to_col || y != to_row);

        return false;
}

/** Find a point along the coordinate system given by the given line,
 * <t> units forward from the start of the line, <u> units to the right of it.
 * @param {!number} x1
 * @param {!number} x2
 * @param {!number} y1
 * @param {!number} y2
 * @param {!number} t
 * @param {!number} u
 * @return {!string} The point in "x y" form, suitable for SVG paths.
 */
function point_from_start(x1, y1, x2, y2, t, u) {
        let dx = x2 - x1;
        let dy = y2 - y1;

        let norm = 1.0 / Math.sqrt(dx * dx + dy * dy);
        dx *= norm;
        dy *= norm;

        let x = x1 + dx * t + dy * u;
        let y = y1 + dy * t - dx * u;
        return x + " " + y;
}

/** Find a point along the coordinate system given by the given line,
 * <t> units forward from the end of the line, <u> units to the right of it.
 * @param {!number} x1
 * @param {!number} x2
 * @param {!number} y1
 * @param {!number} y2
 * @param {!number} t
 * @param {!number} u
 * @return {!string} The point in "x y" form, suitable for SVG paths.
 */
function point_from_end(x1, y1, x2, y2, t, u) {
        let dx = x2 - x1;
        let dy = y2 - y1;

        let norm = 1.0 / Math.sqrt(dx * dx + dy * dy);
        dx *= norm;
        dy *= norm;

        let x = x2 + dx * t + dy * u;
        let y = y2 + dy * t - dx * u;
        return x + " " + y;
}

function position_arrow(arrow) {
        if (arrow.svg) {
                if (arrow.svg.parentElement) {
                        arrow.svg.parentElement.removeChild(arrow.svg);
                }
                delete arrow.svg;
        }
        if (current_display_line !== null && !current_display_line_is_history) {
                return;
        }

        // We always draw as if the board is 400x400, the viewBox will adjust that for us
        let line_width = arrow.line_width;
        let arrow_size = arrow.arrow_size;

        let square_width = 400 / 8;
        let from_y, to_y, from_x, to_x;
        if (board.orientation() === 'black') {
                from_y = (arrow.from_row + 0.5)*square_width;
                to_y = (arrow.to_row + 0.5)*square_width;
                from_x = (7 - arrow.from_col + 0.5)*square_width;
                to_x = (7 - arrow.to_col + 0.5)*square_width;
        } else {
                from_y = (7 - arrow.from_row + 0.5)*square_width;
                to_y = (7 - arrow.to_row + 0.5)*square_width;
                from_x = (arrow.from_col + 0.5)*square_width;
                to_x = (arrow.to_col + 0.5)*square_width;
        }

        let SVG_NS = "http://www.w3.org/2000/svg";
        let XHTML_NS = "http://www.w3.org/1999/xhtml";
        let svg = document.createElementNS(SVG_NS, "svg");
        svg.setAttribute("width", "100%");
        svg.setAttribute("height", "100%");
        svg.setAttribute("viewBox", "0 0 400 400");
        svg.setAttribute("version", "1.1");
        svg.setAttribute("class", "c1");
        svg.setAttribute("xmlns", XHTML_NS);

        let x1 = from_x;
        let y1 = from_y;
        let x2 = to_x;
        let y2 = to_y;

        // Draw the line.
        let outline = document.createElementNS(SVG_NS, "path");
        outline.setAttribute("d", "M " + point_from_start(x1, y1, x2, y2, arrow_size / 2, 0) + " L " + point_from_end(x1, y1, x2, y2, -arrow_size / 2, 0));
        outline.setAttribute("xmlns", XHTML_NS);
        outline.setAttribute("stroke", "#666");
        outline.setAttribute("stroke-width", line_width + 2);
        outline.setAttribute("fill", "none");
        svg.appendChild(outline);

        let path = document.createElementNS(SVG_NS, "path");
        path.setAttribute("d", "M " + point_from_start(x1, y1, x2, y2, arrow_size / 2, 0) + " L " + point_from_end(x1, y1, x2, y2, -arrow_size / 2, 0));
        path.setAttribute("xmlns", XHTML_NS);
        path.setAttribute("stroke", arrow.fg_color);
        path.setAttribute("stroke-width", line_width);
        path.setAttribute("fill", "none");
        svg.appendChild(path);

        // Then the arrow head.
        let head = document.createElementNS(SVG_NS, "path");
        head.setAttribute("d",
                "M " +  point_from_end(x1, y1, x2, y2, 0, 0) +
                " L " + point_from_end(x1, y1, x2, y2, -arrow_size, -arrow_size / 2) +
                " L " + point_from_end(x1, y1, x2, y2, -arrow_size * .623, 0.0) +
                " L " + point_from_end(x1, y1, x2, y2, -arrow_size, arrow_size / 2) +
                " L " + point_from_end(x1, y1, x2, y2, 0, 0));
        head.setAttribute("xmlns", XHTML_NS);
        head.setAttribute("stroke", "#000");
        head.setAttribute("stroke-width", "1");
        head.setAttribute("fill", arrow.fg_color);
        svg.appendChild(head);

        svg.style.position = 'absolute';
        svg.style.top = '0px';  /* Border for .board-b72b1. */
        svg.style.left = '0px';
        svg.style.pointerEvents = 'none';
        document.getElementById('board').appendChild(svg);
        arrow.svg = svg;
}

/**
 * @param {!string} from_square
 * @param {!string} to_square
 * @param {!string} fg_color
 * @param {number} line_width
 * @param {number} arrow_size
 */
function create_arrow(from_square, to_square, fg_color, line_width, arrow_size) {
        let from_col = from_square.charCodeAt(0) - "a1".charCodeAt(0);
        let from_row = from_square.charCodeAt(1) - "a1".charCodeAt(1);
        let to_col   = to_square.charCodeAt(0) - "a1".charCodeAt(0);
        let to_row   = to_square.charCodeAt(1) - "a1".charCodeAt(1);

        // Create arrow.
        let arrow = {
                from_col: from_col,
                from_row: from_row,
                to_col: to_col,
                to_row: to_row,
                line_width: line_width,
                arrow_size: arrow_size,
                fg_color: fg_color
        };

        position_arrow(arrow);
        arrows.push(arrow);
}

function compare_by_score(refutation_lines, invert, a, b) {
        let sa = compute_score_sort_key(refutation_lines[b]['score'], refutation_lines[b]['depth'], invert);
        let sb = compute_score_sort_key(refutation_lines[a]['score'], refutation_lines[a]['depth'], invert);
        return sa - sb;
}

/**
 * Fake multi-PV using the refutation lines. Find all “relevant” moves,
 * sorted by quality, descending.
 *
 * @param {!Object} data
 * @param {number} margin The maximum number of centipawns worse than the
 *     best move can be and still be included.
 * @param {boolean} invert Whether black is to play.
 * @return {Array.<string>} The FEN representation (e.g. Ne4) of all
 *     moves, in score order.
 */
function find_nonstupid_moves(data, margin, invert) {
        // First of all, if there are any moves that are more than 0.5 ahead of
        // the primary move, the refutation lines are probably bunk, so just
        // kill them all. 
        let best_score = undefined;
        let pv_score = undefined;
        for (let move in data['refutation_lines']) {
                let line = data['refutation_lines'][move];
                let score = compute_score_sort_key(line['score'], line['depth'], invert);
                if (move == data['pv'][0]) {
                        pv_score = score;
                }
                if (best_score === undefined || score > best_score) {
                        best_score = score;
                }
                if (line['depth'] < 8) {
                        return [];
                }
        }

        if (best_score - pv_score > 50) {
                return [];
        }

        // Now find all moves that are within “margin” of the best score.
        // The PV move will always be first.
        let moves = [];
        for (let move in data['refutation_lines']) {
                let line = data['refutation_lines'][move];
                let score = compute_score_sort_key(line['score'], line['depth'], invert);
                if (move != data['pv'][0] && best_score - score <= margin) {
                        moves.push(move);
                }
        }
        let toplay = find_toplay(data['position']['fen']);
        moves = moves.sort(function(a, b) { return compare_by_score(data['refutation_lines'], toplay, a, b) });
        moves.unshift(data['pv'][0]);

        return moves;
}

/**
 * @param {number} x
 * @return {!string}
 */
function thousands(x) {
        return String(x).split('').reverse().join('').replace(/(\d{3}\B)/g, '$1,').split('').reverse().join('');
}

/**
 * @param {!string} start_fen
 * @param {Array.<string>} pv
 * @param {Array<{ first_move: number, score: Object }>} scores
 * @param {number} start_display_move_num
 * @param {number=} opt_limit
 * @param {boolean=} opt_showlast
 */
function add_pv(start_fen, pv, scores, start_display_move_num, opt_limit, opt_showlast) {
        display_lines.push({
                start_fen: start_fen,
                pv: pv,
                scores: scores,
                start_display_move_num: start_display_move_num
        });
        let splicepos = null;
        if (scores !== null && scores.length >= 1 &&
            scores[scores.length - 1].score !== undefined &&
            scores[scores.length - 1].score !== null &&
            (scores[scores.length - 1].score[0] === 'T' ||
             scores[scores.length - 1].score[0] === 't')) {
                splicepos = scores[scores.length - 1].score[1];
        }
        return print_pv(display_lines.length - 1, splicepos, opt_limit, opt_showlast);
}

/**
 * @param {number} line_num
 * @param {?number} splicepos If non-null, where the tablebase-spliced portion of the TB starts.
 * @param {number=} opt_limit If set, show at most this number of moves.
 * @param {boolean=} opt_showlast If limit is set, show the last moves instead of the first ones.
 */
function print_pv(line_num, splicepos, opt_limit, opt_showlast) {
        let display_line = display_lines[line_num];
        let pv = display_line.pv;
        let halfmove_num = find_halfmove_num(display_line.start_fen) + 2;  // From two, for simplicity.
        let start_halfmove_num = halfmove_num;

        let ret = document.createDocumentFragment();

        // Truncate PV at the start if needed.
        let to_skip = display_line.start_display_move_num;
        if (opt_limit && opt_showlast && pv.length - to_skip > opt_limit) {
                // Explicit (UI-visible) truncation from the start, for the history.
                ret.appendChild(document.createTextNode('('));
                let link = document.createElement('a');
                link.className = 'move';
                link.href = 'javascript:collapse_history(false)';
                link.textContent = '…';
                ret.appendChild(link);
                ret.appendChild(document.createTextNode(') '));
                to_skip = pv.length - opt_limit;
                to_skip += to_skip % 2;  // Make sure it starts on a white move.
        }
        if (to_skip > 0) {
                pv = pv.slice(to_skip);
                halfmove_num += to_skip;
                if (splicepos !== null) {
                        splicepos -= to_skip;
                        if (splicepos < 0) {
                                splicepos = 0;
                        }
                }
        }

        // The initial move number needs to go before any (TB: …) marker.
        if (halfmove_num % 2 == 1) {
                // Black move.
                ret.appendChild(document.createTextNode((halfmove_num - 1) / 2 + '. … '));
        } else {
                // White move.
                ret.appendChild(document.createTextNode(halfmove_num / 2 + '. '));
        }
        let in_tb = false;
        for (let i = 0; i < pv.length; ++i, ++halfmove_num) {
                let prefix = '';
                if (splicepos === i) {
                        prefix = '(TB:';
                        in_tb = true;
                }

                if (halfmove_num % 2 == 0 && i != 0) {
                        if (i > opt_limit && !opt_showlast) {
                                if (in_tb) {
                                        prefix += ')';
                                }
                                ret.appendChild(document.createTextNode(prefix + ' (…)'));
                                return ret;
                        }
                        prefix += ' ' + (halfmove_num / 2) + '. ';
                } else {
                        prefix += ' ';
                }
                ret.appendChild(document.createTextNode(prefix));

                let link = document.createElement('a');
                link.className = 'move';
                link.setAttribute('id', 'automove' + line_num + '-' + (halfmove_num - start_halfmove_num));
                link.textContent = pv[i];
                link.href = 'javascript:show_line(' + line_num + ', ' + (halfmove_num - start_halfmove_num) + ');';
                ret.appendChild(link);
        }
        if (in_tb) {
                ret.appendChild(document.createTextNode(')'));
        }
        return ret;
}

/** Update the highlighted to/from squares on the board.
 * Based on the global "highlight_from" and "highlight_to" letiables.
 */
function update_board_highlight() {
        document.getElementById("board").querySelectorAll('.square-55d63').forEach((square) => square.classList.remove('nonuglyhighlight'));
        if ((current_display_line === null || current_display_line_is_history) &&
            highlight_from !== undefined && highlight_to !== undefined) {
                document.getElementById("board").querySelector('.square-' + highlight_from).classList.add('nonuglyhighlight');
                document.getElementById("board").querySelector('.square-' + highlight_to).classList.add('nonuglyhighlight');
        }
}

function update_history() {
        let history = document.getElementById('history');
        if (display_lines[0] === null || display_lines[0].pv.length == 0) {
                history.textContent = 'No history';
        } else if (truncate_display_history) {
                history.replaceChildren(print_pv(0, null, 8, true));
        } else {
                history.textContent = '(';
                let link = document.createElement('a');
                link.className = 'move';
                link.href = 'javascript:collapse_history(true)';
                link.textContent = 'collapse';
                history.appendChild(link);
                history.appendChild(document.createTextNode(') '));
                history.append(print_pv(0, null));
        }
}

/**
 * @param {!boolean} truncate_history
 */
function collapse_history(truncate_history) {
        truncate_display_history = truncate_history;
        update_history();
}
window['collapse_history'] = collapse_history;

function choose_displayed_refutation_lines() {
        if (hash_refutation_lines) {
                // If we're in hash exploration, that takes precedence.
                return [hash_refutation_lines, hash_refutation_lines_base_fen];
        } else {
                let data = displayed_analysis_data || current_analysis_data;
                return [data['refutation_lines'], data['position']['fen']];
        }
}

/** Update the HTML display of multi-PV.
 *
 * Also recreates the global "display_lines".
 */
function update_refutation_lines() {
        const [refutation_lines, refutation_lines_base_fen] = choose_displayed_refutation_lines();
        if (!refutation_lines) {
                return;
        }
        if (display_lines.length > 2) {
                // Truncate so that only the history and PV is left.
                display_lines = [ display_lines[0], display_lines[1] ];
        }
        let tbl = document.getElementById("refutationlines");
        tbl.replaceChildren();

        if (display_lines.length < 2) {
                // Update the move highlight, as we've rewritten all the HTML.
                update_move_highlight();
                return;
        }

        // Find out where the lines start from.
        let base_line = [];
        let base_scores = display_lines[1].scores;
        let start_display_move_num = 0;
        if (hash_refutation_lines) {
                base_line = current_display_line.pv.slice(0, current_display_move + 1);
                base_scores = current_display_line.scores;
                start_display_move_num = base_line.length;
        }

        let moves = [];
        for (let move in refutation_lines) {
                moves.push(move);
        }

        let invert = (find_toplay(refutation_lines_base_fen) === 'b');
        if (current_display_line && current_display_move % 2 == 0 && !current_display_line_is_history) {
                invert = !invert;
        }
        moves = moves.sort(function(a, b) { return compare_by_score(refutation_lines, invert, a, b) });
        for (let i = 0; i < moves.length; ++i) {
                let line = refutation_lines[moves[i]];

                let tr = document.createElement("tr");

                let move_td = document.createElement("td");
                tr.appendChild(move_td);
                move_td.classList.add("move");

                let scores = base_scores.concat([{ first_move: start_display_move_num, score: line['score'] }]);

                if (line['pv'].length == 0) {
                        // Not found, so just make a one-move PV.
                        let link = document.createElement('a');
                        link.className = 'move';
                        link.href = 'javascript:show_line(' + display_lines.length + ', ' + 0 + ')';
                        link.textContent = line['move'];
                        move_td.replaceChildren(link);

                        let score_td = document.createElement("td");
                        score_td.classList.add("score");
                        score_td.textContent = "—";
                        tr.appendChild(score_td);

                        let depth_td = document.createElement("td");
                        tr.appendChild(depth_td);
                        depth_td.classList.add("depth");
                        depth_td.textContent = "—";

                        let pv_td = document.createElement("td");
                        tr.appendChild(pv_td);
                        pv_td.classList.add("pv");
                        pv_td.append(add_pv(refutation_lines_base_fen, base_line.concat([ line['move'] ]), scores, start_display_move_num));

                        tbl.append(tr);
                        continue;
                }

                let move_link = document.createElement("a");
                move_link.classList.add("move");
                move_link.setAttribute("href", "javascript:show_line(" + display_lines.length + ", 0)");
                move_link.textContent = line['move'];
                move_td.appendChild(move_link);

                let score_td = document.createElement("td");
                tr.appendChild(score_td);
                score_td.classList.add("score");
                score_td.textContent = format_short_score(line['score']);

                let depth_td = document.createElement("td");
                tr.appendChild(depth_td);
                depth_td.classList.add("depth");
                if (line['depth'] && line['depth'] >= 0) {
                        depth_td.textContent = "d" + line['depth'];
                } else {
                        depth_td.textContent = "—";
                }

                let pv_td = document.createElement("td");
                tr.appendChild(pv_td);
                pv_td.classList.add("pv");
                pv_td.append(add_pv(refutation_lines_base_fen, base_line.concat(line['pv']), scores, start_display_move_num, 10));

                tbl.append(tr);
        }

        // Update the move highlight, as we've rewritten all the HTML.
        update_move_highlight();
}

/**
 * Create a Chess.js board object, containing the given position plus the given moves,
 * up to the given limit.
 *
 * @param {?string} fen
 * @param {Array.<string>} moves
 * @param {number} last_move
 */
function chess_from(fen, moves, last_move) {
        let hiddenboard = new Chess();
        if (fen !== null && fen !== undefined) {
                hiddenboard.load(fen);
        }
        for (let i = 0; i <= last_move; ++i) {
                if (moves[i] === '0-0') {
                        hiddenboard.move('O-O');
                } else if (moves[i] === '0-0-0') {
                        hiddenboard.move('O-O-O');
                } else {
                        hiddenboard.move(moves[i]);
                }
        }
        return hiddenboard;
}

function update_game_list(games) {
        document.getElementById("games").textContent = "";
        if (games === null) {
                return;
        }

        let games_div = document.getElementById('games');
        for (let game_num = 0; game_num < games.length; ++game_num) {
                let game = games[game_num];
                let game_span = document.createElement("span");
                game_span.setAttribute("class", "game");

                let game_name = document.createTextNode(game['name']);
                if (game['url'] === backend_url) {
                        // This game.
                        game_span.appendChild(game_name);

                        if (current_analysis_data && current_analysis_data['position']) {
                                let score;
                                if (current_analysis_data['position']['result']) {
                                        score = " (" + current_analysis_data['position']['result'] + ")";
                                } else {
                                        score = " (" + format_short_score(current_analysis_data['score']) + ")";
                                }
                                game_span.appendChild(document.createTextNode(score));
                        }
                } else {
                        // Some other game.
                        let game_a = document.createElement("a");
                        game_a.setAttribute("href", "#" + game['id']);
                        game_a.appendChild(game_name);
                        game_span.appendChild(game_a);

                        let score;
                        if (game['result']) {
                                score = " (" + game['result'] + ")";
                        } else {
                                score = " (" + format_short_score(game['score']) + ")";
                        }
                        game_span.appendChild(document.createTextNode(score));
                }

                games_div.appendChild(game_span);
        }
}

/**
 * Try to find a running game that matches with the current hash,
 * and switch to it if we're not already displaying it.
 */
function possibly_switch_game_from_hash() {
        let history_match = window.location.hash.match(/^#history=([a-zA-Z0-9_-]+)/);
        if (history_match !== null) {
                let game_id = history_match[1];
                let fake_game = {
                        url: '/history/' + game_id + '.json',
                        hashurl: '',
                        id: 'history=' + game_id
                };
                switch_backend(fake_game);
                return;
        }

        if (current_games === null) {
                return;
        }

        let hash = window.location.hash.replace(/^#/,'');
        for (let i = 0; i < current_games.length; ++i) {
                if (current_games[i]['id'] === hash) {
                        if (backend_url !== current_games[i]['url']) {
                                switch_backend(current_games[i]);
                        }
                        return;
                }
        }
}

/**
 * If this is a Chess960 castling which doesn't move the king,
 * move the rook instead.
*/
function patch_move(move) {
        if (move === null) return null;
        if (move.from !== move.to) return move;

        let f = move.rook_sq & 15;
        let r = move.rook_sq >> 4;
        let from = ('abcdefgh'.substring(f,f+1) + '87654321'.substring(r,r+1));
        let to = move.to;

        if (move.to === 'g1') {
                to = 'f1';
        } else if (move.to === 'g8') {
                to = 'f8';
        } else if (move.to === 'b1') {
                to = 'c1';
        } else if (move.to === 'b8') {
                to = 'c8';
        }

        return { from: from, to: to };
}

/** Update all the HTML on the page, based on current global state.
 */
function update_board() {
        document.body.style.opacity = null;

        let data = displayed_analysis_data || current_analysis_data;
        let current_data = current_analysis_data;  // Convenience alias.

        display_lines = [];

        // Print the history. This is pretty much the only thing that's
        // unconditionally taken from current_data (we're not interested in
        // historic history).
        if (current_data['position']['history']) {
                let start = (current_data['position'] && current_data['position']['start_fen']) ? current_data['position']['start_fen'] : 'rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1';
                add_pv(start, current_data['position']['history'], null, 0, 8, true);
        } else {
                display_lines.push(null);
        }
        update_history();

        // Games currently in progress, if any.
        if (current_data['games']) {
                current_games = current_data['games'];
                possibly_switch_game_from_hash();
        } else {
                current_games = null;
        }
        update_game_list(current_games);

        // The headline. Names are always fetched from current_data;
        // the rest can depend a bit.
        let headline;
        if (current_data &&
            current_data['position']['player_w'] && current_data['position']['player_b']) {
                headline = current_data['position']['player_w'] + '–' +
                        current_data['position']['player_b'] + ', analysis';
        } else {
                headline = 'Analysis';
        }

        // Credits: The engine name/version.
        if (current_data['engine'] && current_data['engine']['name'] !== null) {
                document.getElementById("engineid").textContent = current_data['engine']['name'];
        }

        // Credits: The engine URL.
        if (current_data['engine'] && current_data['engine']['url']) {
                document.getElementById("engineid").setAttribute("href", current_data['engine']['url']);
        } else {
                document.getElementById("engineid").removeAttribute("href");
        }

        // Credits: Engine details.
        if (current_data['engine'] && current_data['engine']['details']) {
                document.getElementById("enginedetails").textContent = " (" + current_data['engine']['details'] + ")";
        } else {
                document.getElementById("enginedetails").textContent = "";
        }

        // Credits: Move source, possibly with URL.
        if (current_data['move_source'] && current_data['move_source_url']) {
                document.getElementById("movesource").textContent = "Moves provided by ";
                let movesource_a = document.createElement("a");
                movesource_a.setAttribute("href", current_data['move_source_url']);
                let movesource_text = document.createTextNode(current_data['move_source']);
                movesource_a.appendChild(movesource_text);
                let movesource_period = document.createTextNode(".");
                document.getElementById("movesource").appendChild(movesource_a);
                document.getElementById("movesource").appendChild(movesource_period);
        } else if (current_data['move_source']) {
                document.getElementById("movesource").textContent = "Moves provided by " + current_data['move_source'] + ".";
        } else {
                document.getElementById("movesource").textContent = "";
        }

        let last_move;
        if (displayed_analysis_data) {
                // Displaying some non-current position, pick out the last move
                // from the history. This will work even if the fetch failed.
                if (current_display_move !== -1) {
                        last_move = format_halfmove_with_number(
                                current_display_line.pv[current_display_move],
                                current_display_move);
                        headline += ' after ' + last_move;
                }
        } else if (data['position']['last_move'] !== 'none') {
                // Find the previous move.
                let previous_move_num, previous_toplay;
                let fen = data['position']['fen'];
                if (find_toplay(fen) === 'b') {
                        previous_move_num = find_move_num(fen);
                        previous_toplay = 'w';
                } else {
                        previous_move_num = find_move_num(fen) - 1;
                        previous_toplay = 'b';
                }

                last_move = format_move_with_number(
                        data['position']['last_move'],
                        previous_move_num,
                        previous_toplay == 'w');
                headline += ' after ' + last_move;
        } else {
                last_move = null;
        }
        document.getElementById("headline").textContent = headline;

        // The <title> contains a very brief headline.
        let title_elems = [];
        if (data['position'] && data['position']['result']) {
                title_elems.push(data['position']['result']);
        } else if (data['score']) {
                title_elems.push(format_short_score(data['score']));
        }
        if (last_move !== null) {
                title_elems.push(last_move);
        }

        if (title_elems.length != 0) {
                document.title = '(' + title_elems.join(', ') + ') analysis.sesse.net';
        } else {
                document.title = 'analysis.sesse.net';
        }

        // The last move (shown by highlighting the from and to squares).
        if (data['position'] && data['position']['last_move_uci']) {
                highlight_from = data['position']['last_move_uci'].substr(0, 2);
                highlight_to = data['position']['last_move_uci'].substr(2, 2);
        } else if (current_display_line_is_history && current_display_line && current_display_move >= 0) {
                // We don't have historic analysis for this position, but we
                // can reconstruct what the last move was by just replaying
                // from the start.
                let position = (data['position'] && data['position']['start_fen']) ? data['position']['start_fen'] : null;
                let hiddenboard = chess_from(position, current_display_line.pv, current_display_move);
                let moves = hiddenboard.history({ verbose: true });
                last_move = moves.pop();
                highlight_from = last_move.from;
                highlight_to = last_move.to;
        } else {
                highlight_from = highlight_to = undefined;
        }
        update_board_highlight();

        if (data['failed']) {
                document.getElementById("score").textContent = "No analysis for this move";
                document.getElementById("pvtitle").textContent = "PV:";
                document.getElementById("pv").replaceChildren();
                document.getElementById("searchstats").innerHTML = "&nbsp;";
                document.getElementById("refutationlines").replaceChildren();
                document.getElementById("whiteclock").replaceChildren();
                document.getElementById("blackclock").replaceChildren();
                update_refutation_lines();
                clear_arrows();
                update_displayed_line();
                update_move_highlight();
                return;
        }

        if (clock_timer !== null) {
                clearTimeout(clock_timer);
        }
        update_clock();

        // The score.
        if (current_display_line && !current_display_line_is_history) {
                let score;
                if (current_display_line.scores && current_display_line.scores.length > 0) {
                        for (let i = 0; i < current_display_line.scores.length; ++i) {
                                if (current_display_move < current_display_line.scores[i].first_move) {
                                        break;
                                }
                                score = current_display_line.scores[i].score;
                        }
                }
                if (score) {
                        document.getElementById("score").textContent = format_long_score(score);
                } else {
                        document.getElementById("score").textContent = "No score for this line";
                }
        } else if (data['score']) {
                document.getElementById("score").textContent = format_long_score(data['score']);
        }

        // The search stats.
        if (data['searchstats']) {
                document.getElementById("searchstats").textContent = data['searchstats'];
        } else if (data['nodes'] && data['nps'] && data['depth']) {
                let stats = thousands(data['nodes']) + ' nodes, ' + thousands(data['nps']) + ' nodes/sec, depth ' + data['depth'] + ' ply';
                if (data['seldepth']) {
                        stats += ' (' + data['seldepth'] + ' selective)';
                }
                if (data['tbhits'] && data['tbhits'] > 0) {
                        if (data['tbhits'] == 1) {
                                stats += ', one Syzygy hit';
                        } else {
                                stats += ', ' + thousands(data['tbhits']) + ' Syzygy hits';
                        }
                }

                document.getElementById("searchstats").textContent = stats;
        } else {
                document.getElementById("searchstats").textContent = "";
        }
        if (admin_password !== null) {
                document.getElementById("searchstats").innerHTML += " | <span style=\"color: red;\">ADMIN MODE (if password is right) | <a href=\"javascript:undo_move()\">Undo move</a></span>";
        }

        // Update the board itself.
        base_fen = data['position']['fen'];
        update_displayed_line();

        // Print the PV.
        document.getElementById("pvtitle").textContent = "PV:";

        let scores = [{ first_move: -1, score: data['score'] }];
        document.getElementById("pv").replaceChildren(add_pv(data['position']['fen'], data['pv'], scores, 0));

        // Update the PV arrow.
        clear_arrows();
        let toplay = find_toplay(data['position']['fen']);
        if (data['pv'].length >= 1) {
                let hiddenboard = new Chess(base_fen);

                // draw a continuation arrow as long as it's the same piece
                let last_to;
                for (let i = 0; i < data['pv'].length; i += 2) {
                        let move = patch_move(hiddenboard.move(data['pv'][i]));

                        if ((i >= 2 && move.from != last_to) ||
                             interfering_arrow(move.from, move.to)) {
                                break;
                        }
                        create_arrow(move.from, move.to, '#f66', 6, 20);
                        last_to = move.to;
                        hiddenboard.move(data['pv'][i + 1]);  // To keep continuity.
                }

                let alt_moves = find_nonstupid_moves(data, 30, toplay === 'b');
                for (let i = 1; i < alt_moves.length && i < 3; ++i) {
                        hiddenboard = new Chess(base_fen);
                        let move = patch_move(hiddenboard.move(alt_moves[i]));
                        if (move !== null) {
                                create_arrow(move.from, move.to, '#f66', 1, 10);
                        }
                }
        }

        // See if all semi-reasonable moves have only one possible response.
        if (data['pv'].length >= 2) {
                let nonstupid_moves = find_nonstupid_moves(data, 300, toplay === 'b');
                let hiddenboard = new Chess(base_fen);
                hiddenboard.move(data['pv'][0]);
                let response = hiddenboard.move(data['pv'][1]);
                for (let i = 0; i < nonstupid_moves.length; ++i) {
                        if (nonstupid_moves[i] == data['pv'][0]) {
                                // ignore the PV move for refutation lines.
                                continue;
                        }
                        if (!data['refutation_lines'] ||
                            !data['refutation_lines'][nonstupid_moves[i]] ||
                            !data['refutation_lines'][nonstupid_moves[i]]['pv'] ||
                            data['refutation_lines'][nonstupid_moves[i]]['pv'].length < 2) {
                                // Incomplete PV, abort.
                                response = undefined;
                                break;
                        }
                        let line = data['refutation_lines'][nonstupid_moves[i]];
                        hiddenboard = new Chess(base_fen);
                        hiddenboard.move(line['pv'][0]);
                        let this_response = hiddenboard.move(line['pv'][1]);
                        if (this_response === null) {
                                console.log("BUG: ", i);
                                console.log(data);
                                console.log(line['pv']);
                        }
                        if (response.from !== this_response.from || response.to !== this_response.to) {
                                // Different response depending on lines, abort.
                                response = undefined;
                                break;
                        }
                }

                if (nonstupid_moves.length > 0 && response !== undefined) {
                        create_arrow(response.from, response.to, '#66f', 6, 20);
                }
        }

        base_fen = data['position']['fen'];
        update_refutation_lines();

        // Update the sparkline last, since its size depends on how everything else reflowed.
        update_sparkline(data);
}

function update_sparkline(data) {
        let scorespark = document.getElementById('scoresparkcontainer');
        scorespark.textContent = '';
        if (data && data['score_history']) {
                let first_move_num = undefined;
                for (let halfmove_num in data['score_history']) {
                        halfmove_num = parseInt(halfmove_num);
                        if (first_move_num === undefined || halfmove_num < first_move_num) {
                                first_move_num = halfmove_num;
                        }
                }
                if (first_move_num !== undefined) {
                        let fen = data['position']['fen'];
                        let last_move_num = find_move_num(fen) * 2 - 3;
                        if (find_toplay(fen) === 'b') {
                                ++last_move_num;
                        }

                        // Possibly truncate some moves if we don't have enough width.
                        let max_moves = Math.floor(scorespark.getBoundingClientRect().width / 5) - 3;
                        if (last_move_num - first_move_num > max_moves) {
                                first_move_num = last_move_num - max_moves;
                        }

                        let min_score = -1;
                        let max_score = 1;
                        let last_score = null;
                        let scores = [];
                        for (let halfmove_num = first_move_num; halfmove_num <= last_move_num; ++halfmove_num) {
                                if (data['score_history'][halfmove_num]) {
                                        let score = compute_plot_score(data['score_history'][halfmove_num]);
                                        last_score = score;
                                        if (score < min_score) min_score = score;
                                        if (score > max_score) max_score = score;
                                }
                                scores.push(last_score);
                        }
                        if (data['score']) {
                                let score = compute_plot_score(data['score']);
                                scores.push(score);
                                if (score < min_score) min_score = score;
                                if (score > max_score) max_score = score;
                        }
                        if (max_score - min_score < 100) {
                                if (Math.abs(max_score) >= Math.abs(min_score)) {
                                        max_score = min_score + 100;
                                } else {
                                        min_score = max_score - 100;
                                }
                        }

                        const h = scorespark.getBoundingClientRect().height;

                        let base_y = h - h * min_score / (min_score - max_score);
                        for (let i = 0; i < scores.length; ++i) {
                                let rect = document.createElementNS('http://www.w3.org/2000/svg', 'rect');
                                //rect.setAttributeNS(null, 'stroke', "#000000");
                                rect.setAttributeNS(null, 'x', i * 5)
                                rect.setAttributeNS(null, 'width', 4);
                                let extent = scores[i] * h / (max_score - min_score);
                                if (extent > 0 && extent < 1) {
                                        extent = 1;
                                } else if (extent < 0 && extent > -1) {
                                        extent = -1;
                                }

                                let color;
                                if (scores[i] === 0) {
                                        color = [0.5, 0.5, 0.5];
                                        rect.setAttributeNS(null, 'y', base_y);
                                        rect.setAttributeNS(null, 'height', 1);
                                } else if (scores[i] > 0) {
                                        color = [0.2, 0.4, 0.8];
                                        rect.setAttributeNS(null, 'y', base_y - extent);
                                        rect.setAttributeNS(null, 'height', extent + 1);
                                } else {
                                        color = [1.0, 0.267, 0.267];
                                        rect.setAttributeNS(null, 'y', base_y);
                                        rect.setAttributeNS(null, 'height', -extent + 1);
                                }
                                let hlcolor = [color[0], color[1], color[2]];
                                if (scores[i] !== 0) { 
                                        hlcolor[0] = Math.min(hlcolor[0] * 1.4, 1.0);
                                        hlcolor[1] = Math.min(hlcolor[1] * 1.4, 1.0);
                                        hlcolor[2] = Math.min(hlcolor[2] * 1.4, 1.0);
                                }
                                rect.style.fill = 'rgb(' + color[0]*100.0 + '%, ' + color[1]*100.0 + '%, ' + color[2]*100.0 + '%)';

                                // score_history contains the Nth _position_, but format_tooltip
                                // wants to format the Nth _move_; thus the -1.
                                const tooltip = format_tooltip(data, i + first_move_num - 1);
                                rect.addEventListener('mouseenter', (e) => draw_hover(e, hlcolor, tooltip));
                                rect.addEventListener('mousemove', (e) => draw_hover(e, hlcolor, tooltip));
                                rect.addEventListener('mouseleave', (e) => hide_hover(e, color));
                                rect.addEventListener('click', (e) => { hide_hover(e, color); show_line(0, i + first_move_num - 1) });
                                scorespark.appendChild(rect);
                        }
                }
        }
}

function draw_hover(e, color, tooltip) {
        e.target.style.fill = 'rgb(' + color[0]*100.0 + '%, ' + color[1]*100.0 + '%, ' + color[2]*100.0 + '%)';

        let hover = document.getElementById('sparklinehover');
        hover.textContent = tooltip;
        hover.style.display = 'initial';

        let left = Math.max(e.pageX + 10, window.pageXOffset);
        let top = Math.max(e.pageY - hover.getBoundingClientRect().height, window.pageYOffset);
        left = Math.min(left, window.pageXOffset + document.documentElement.clientWidth - hover.getBoundingClientRect().width);

        hover.style.left = left + 'px';
        hover.style.top = top + 'px';

}
function hide_hover(e, color) {
        e.target.style.fill = 'rgb(' + color[0]*100.0 + '%, ' + color[1]*100.0 + '%, ' + color[2]*100.0 + '%)';
        document.getElementById('sparklinehover').style.display = 'none';
}

/**
 * @param {number} num_viewers
 */
function update_num_viewers(num_viewers) {
        let text = "";
        if (num_viewers === null) {
                text = "";
        } else if (num_viewers == 1) {
                text = "You are the only current viewer";
        } else {
                text = num_viewers + " current viewers";
        }
        if (display_fen !== null) {
                let counter = Math.floor(display_fen.split(" ")[4] / 2);
                if (counter >= 20) {
                        text = text.replace("current ", "");
                        text += " | 50-move rule: " + counter;
                }
        }
        document.getElementById("numviewers").textContent = text;
}

function update_clock() {
        clock_timer = null;

        let data = displayed_analysis_data || current_analysis_data;
        if (!data) return;

        if (data['position']) {
                let result = data['position']['result'];
                if (result === '1-0') {
                        document.getElementById("whiteclock").textContent = "1";
                        document.getElementById("blackclock").textContent = "0";
                        document.getElementById("whiteclock").classList.remove("running-clock");
                        document.getElementById("blackclock").classList.remove("running-clock");
                        return;
                }
                if (result === '1/2-1/2') {
                        document.getElementById("whiteclock").textContent = "1/2";
                        document.getElementById("blackclock").textContent = "1/2";
                        document.getElementById("whiteclock").classList.remove("running-clock");
                        document.getElementById("blackclock").classList.remove("running-clock");
                        return;
                }       
                if (result === '0-1') {
                        document.getElementById("whiteclock").textContent = "0";
                        document.getElementById("blackclock").textContent = "1";
                        document.getElementById("whiteclock").classList.remove("running-clock");
                        document.getElementById("blackclock").classList.remove("running-clock");
                        return;
                }
        }

        let white_clock_ms = null;
        let black_clock_ms = null;

        // Static clocks.
        if (data['position'] &&
            data['position']['white_clock'] &&
            data['position']['black_clock']) {
                white_clock_ms = data['position']['white_clock'] * 1000;
                black_clock_ms = data['position']['black_clock'] * 1000;
        }

        // Dynamic clock (only one, obviously).
        let color;
        if (data['position']['white_clock_target']) {
                color = "white";
                document.getElementById("whiteclock").classList.add("running-clock");
                document.getElementById("blackclock").classList.remove("running-clock");
        } else if (data['position']['black_clock_target']) {
                color = "black";
                document.getElementById("whiteclock").classList.remove("running-clock");
                document.getElementById("blackclock").classList.add("running-clock");
        } else {
                document.getElementById("whiteclock").classList.remove("running-clock");
                document.getElementById("blackclock").classList.remove("running-clock");
        }
        let remaining_ms;
        if (color) {
                let now = new Date().getTime() + client_clock_offset_ms;
                remaining_ms = data['position'][color + '_clock_target'] * 1000 - now;
                if (color === "white") {
                        white_clock_ms = remaining_ms;
                } else {
                        black_clock_ms = remaining_ms;
                }
        }

        if (white_clock_ms === null || black_clock_ms === null) {
                document.getElementById("whiteclock").replaceChildren();
                document.getElementById("blackclock").replaceChildren();
                return;
        }

        // If either player has twenty minutes or less left, add the second counters.
        // This matches what DGT clocks do.
        let show_seconds = (white_clock_ms < 60 * 20 * 1000 || black_clock_ms < 60 * 20 * 1000);

        if (color && remaining_ms > 0) {
                // See when the clock will change next, and update right after that.
                let next_update_ms;
                if (show_seconds) {
                        next_update_ms = remaining_ms % 1000 + 100;
                } else {
                        next_update_ms = remaining_ms % 60000 + 100;
                }
                clock_timer = setTimeout(update_clock, next_update_ms);
        }

        document.getElementById("whiteclock").textContent = format_clock(white_clock_ms, show_seconds);
        document.getElementById("blackclock").textContent = format_clock(black_clock_ms, show_seconds);
}

/**
 * @param {number} remaining_ms
 * @param {boolean} show_seconds
 */
function format_clock(remaining_ms, show_seconds) {
        if (remaining_ms <= 0) {
                if (show_seconds) {
                        return "00:00:00";
                } else {
                        return "00:00";
                }
        }

        let remaining = Math.floor(remaining_ms / 1000);
        let seconds = remaining % 60;
        remaining = (remaining - seconds) / 60;
        let minutes = remaining % 60;
        remaining = (remaining - minutes) / 60;
        let hours = remaining;
        if (show_seconds) {
                return format_2d(hours) + ":" + format_2d(minutes) + ":" + format_2d(seconds);
        } else {
                return format_2d(hours) + ":" + format_2d(minutes);
        }
}

/**
 * @param {number} x
 */
function format_2d(x) {
        if (x >= 10) {
                return x;
        } else {
                return "0" + x;
        }
}

/**
 * @param {string} move
 * @param {number} move_num Move number of this move.
 * @param {boolean} white_to_play Whether white is to play this move.
 */
function format_move_with_number(move, move_num, white_to_play) {
        let ret;
        if (white_to_play) {
                ret = move_num + '. ';
        } else {
                ret = move_num + '… ';
        }
        ret += move;
        return ret;
}

/**
 * @param {string} move
 * @param {number} halfmove_num Half-move number that is to be played,
 *   starting from 0.
 */
function format_halfmove_with_number(move, halfmove_num) {
        return format_move_with_number(
                move,
                Math.floor(halfmove_num / 2) + 1,
                halfmove_num % 2 == 0);
}

/**
 * @param {Object} data
 * @param {number} halfmove_num
 */
function format_tooltip(data, halfmove_num) {
        if (data['score_history'][halfmove_num + 1] ||
            (halfmove_num + 1) === data['position']['history'].length) {
                // Position is in the history, or it is the current position
                // (which is implicitly tacked onto the history).
                let move;
                let short_score;
                if ((halfmove_num + 1) === data['position']['history'].length) {
                        move = data['position']['last_move'];
                        short_score = format_short_score(data['score']);
                } else {
                        move = data['position']['history'][halfmove_num];
                        short_score = format_short_score(data['score_history'][halfmove_num + 1]);
                }
                if (halfmove_num === -1) {
                        return "Start position: " + short_score;
                } else {
                        let move_with_number = format_halfmove_with_number(move, halfmove_num);
                        return "After " + move_with_number + ": " + short_score;
                }
        } else {
                for (let i = halfmove_num; i --> -1; ) {
                        if (data['score_history'][i]) {
                                let move = data['position']['history'][i];
                                if (i === -1) {
                                        return "[Analysis kept from start position]";
                                } else {
                                        return "[Analysis kept from " + format_halfmove_with_number(move, i) + "]";
                                }
                        }
                }
        }
}

/**
 * @param {number} line_num
 * @param {number} move_num
 */
function show_line(line_num, move_num) {
        if (line_num == -1) {
                current_display_line = null;
                current_display_move = null;
                hash_refutation_lines = null;
                if (displayed_analysis_data) {
                        // TODO: Support exiting to history position if we are in an
                        // analysis line of a history position.
                        displayed_analysis_data = null;
                }
                update_board();
                return;
        } else {
                current_display_line = {...display_lines[line_num]};  // Shallow clone.
                current_display_move = move_num;
        }
        current_display_line_is_history = (line_num == 0);

        update_historic_analysis();
        update_displayed_line();
        update_board_highlight();
        update_move_highlight();
        redraw_arrows();
}
window['show_line'] = show_line;

function prev_move() {
        if (current_display_line &&
            current_display_move >= current_display_line.start_display_move_num) {
                --current_display_move;
        }
        update_historic_analysis();
        update_displayed_line();
        update_move_highlight();
}
window['prev_move'] = prev_move;

function next_move() {
        if (current_display_line &&
            current_display_move < current_display_line.pv.length - 1) {
                ++current_display_move;
        }
        update_historic_analysis();
        update_displayed_line();
        update_move_highlight();
}
window['next_move'] = next_move;

function next_game() {
        if (current_games === null) {
                return;
        }

        // Try to find the game we are currently looking at.
        for (let game_num = 0; game_num < current_games.length; ++game_num) {
                let game = current_games[game_num];
                if (game['url'] === backend_url) {
                        let next_game_num = (game_num + 1) % current_games.length;
                        switch_backend(current_games[next_game_num]);
                        return;
                }
        }

        // Couldn't find it; give up.
}

function update_historic_analysis() {
        if (!current_display_line_is_history) {
                return;
        }
        if (current_display_move == current_display_line.pv.length - 1) {
                displayed_analysis_data = null;
                update_board();
        }

        // Fetch old analysis for this line if it exists.
        let hiddenboard = chess_from(current_display_line.start_fen, current_display_line.pv, current_display_move);
        let filename = "/history/move" + (current_display_move + 1) + "-" +
                hiddenboard.fen().replace(/ /g, '_').replace(/\//g, '-') + ".json";

        let handle_err = () => {
                displayed_analysis_data = {'failed': true};
                update_board();
        };

        current_historic_xhr = new AbortController();
        const signal = current_analysis_xhr.signal;
        fetch(filename, { signal })
                .then((response) => response.json().then(data => ({ok: response.ok, json: data})))  // ick
                .then((obj) => {
                        if (!obj.ok) {
                                handle_err();
                                return;
                        }
                        displayed_analysis_data = obj.json;
                        update_board();
                })
                .catch((err) => {
                        if (err.name === 'AbortError') {
                                // Aborted because we are switching backends. Abandon and don't retry,
                                // because another one is already started for us.
                        } else {
                                console.log(err);
                                handle_err();
                        }
                });
}

/**
 * @param {string} fen
 */
function update_imbalance(fen) {
        let imbalance = {'k': 0, 'q': 0, 'r': 0, 'b': 0, 'n': 0, 'p': 0};
        for (const c of fen) {
                if (c === ' ') {
                        // End of board
                        break;
                }
                if (c != c.toUpperCase()) {
                        --imbalance[c];
                } else if (c != c.toLowerCase()) {
                        ++imbalance[c.toLowerCase()];
                }
        }

        let white_imbalance = document.getElementById('whiteimbalance');
        let black_imbalance = document.getElementById('blackimbalance');
        white_imbalance.textContent = '';
        black_imbalance.textContent = '';
        for (let piece in imbalance) {
                for (let i = 0; i < imbalance[piece]; ++i) {
                        let i1 = document.createElement('img');
                        i1.src = svg_pieces['w' + piece.toUpperCase()];
                        i1.setAttribute('alt', piece.toUpperCase());
                        i1.classList.add('imbalance-piece');
                        white_imbalance.appendChild(i1);

                        let i2 = document.createElement('img');
                        i2.src = svg_pieces['b' + piece.toUpperCase()];
                        i2.setAttribute('alt', piece.toUpperCase());
                        i2.classList.add('imbalance-inverted-piece');
                        white_imbalance.appendChild(i2);
                }
                for (let i = 0; i < -imbalance[piece]; ++i) {
                        let i1 = document.createElement('img');
                        i1.src = svg_pieces['b' + piece.toUpperCase()];
                        i1.setAttribute('alt', piece.toUpperCase());
                        i1.classList.add('imbalance-piece');
                        black_imbalance.appendChild(i1);

                        let i2 = document.createElement('img');
                        i2.src = svg_pieces['w' + piece.toUpperCase()];
                        i2.setAttribute('alt', piece.toUpperCase());
                        i2.classList.add('imbalance-inverted-piece');
                        black_imbalance.appendChild(i2);
                }
        }
}

/** Mark the currently selected move in red.
 * Also replaces the PV with the current displayed line if it's not shown
 * anywhere else on the screen.
 */
function update_move_highlight() {
        if (highlighted_move !== null) {
                highlighted_move.classList.remove('highlight'); 
        }
        if (current_display_line) {
                let display_line_num = find_display_line_matching_num();
                if (display_line_num === null) {
                        // Replace the PV with the (complete) line.
                        document.getElementById("pvtitle").textContent = "Exploring:";
                        current_display_line.start_display_move_num = 0;
                        display_lines.push(current_display_line);
                        document.getElementById("pv").replaceChildren(print_pv(display_lines.length - 1, null));  // FIXME
                        display_line_num = display_lines.length - 1;

                        // Clear out the PV, so it's not selected by anything later.
                        display_lines[1].pv = [];
                }

                highlighted_move = document.getElementById("automove" + display_line_num + "-" + current_display_move);
                if (highlighted_move !== null) {
                        highlighted_move.classList.add('highlight');
                }
        }
}

/**
 * See if the current displayed line is identical to any of the ones
 * we have on screen. (It might not be if e.g. the analysis reloaded
 * since we started looking.)
 *
 * @return {?number}
 */
function find_display_line_matching_num() {
        for (let i = 0; i < display_lines.length; ++i) {
                let line = display_lines[i];
                if (line.start_display_move_num > 0) continue;
                if (current_display_line.start_fen !== line.start_fen) continue;
                if (current_display_line.pv.length !== line.pv.length) continue;
                let ok = true;
                for (let j = 0; j < line.pv.length; ++j) {
                        if (current_display_line.pv[j] !== line.pv[j]) {
                                ok = false;
                                break;
                        }
                }
                if (ok) {
                        return i;
                }
        }
        return null;
}

/** Update the board based on the currently displayed line.
 * 
 * TODO: This should really be called only whenever something changes,
 * instead of all the time.
 */
function update_displayed_line() {
        if (current_display_line === null) {
                document.getElementById("linenav").style.display = 'none';
                document.getElementById("linemsg").style.display = 'revert';
                display_fen = base_fen;
                set_board_position(base_fen);
                update_imbalance(base_fen);
                return;
        }

        document.getElementById("linenav").style.display = 'revert';
        document.getElementById("linemsg").style.display = 'none';

        if (current_display_move <= 0) {
                document.getElementById("prevmove").textContent = "Previous";
        } else {
                document.getElementById("prevmove").innerHTML = "<a href=\"javascript:prev_move();\">Previous</a></span>";
        }
        if (current_display_move == current_display_line.pv.length - 1) {
                document.getElementById("nextmove").textContent = "Next";
        } else {
                document.getElementById("nextmove").innerHTML = "<a href=\"javascript:next_move();\">Next</a></span>";
        }

        let hiddenboard = chess_from(current_display_line.start_fen, current_display_line.pv, current_display_move);
        set_board_position(hiddenboard.fen());
        if (display_fen !== hiddenboard.fen() && !current_display_line_is_history) {
                // Fire off a hash request, since we're now off the main position
                // and it just changed.
                explore_hash(hiddenboard.fen());
        }
        display_fen = hiddenboard.fen();
        update_imbalance(hiddenboard.fen());
}

function set_board_position(new_fen) {
        board_is_animating = true;
        let old_fen = board.fen();
        let animate = old_fen !== '8/8/8/8/8/8/8/';
        board.position(new_fen, animate);
        if (board.fen() === old_fen) {
                board_is_animating = false;
        }
}

/**
 * @param {boolean} param_enable_sound
 */
function set_sound(param_enable_sound) {
        enable_sound = param_enable_sound;
        if (enable_sound) {
                document.getElementById("soundon").innerHTML = "<strong>On</strong>";
                document.getElementById("soundoff").innerHTML = "<a href=\"javascript:set_sound(false)\">Off</a>";

                // Seemingly at least Firefox prefers MP3 over Opus; tell it otherwise,
                // and also preload the file since the user has selected audio.
                let ding = document.getElementById('ding');
                if (ding && ding.canPlayType && ding.canPlayType('audio/ogg; codecs="opus"') === 'probably') {
                        ding.src = 'ding.opus';
                        ding.load();
                }
        } else {
                document.getElementById("soundon").innerHTML = "<a href=\"javascript:set_sound(true)\">On</a>";
                document.getElementById("soundoff").innerHTML = "<strong>Off</strong>";
        }
        if (supports_html5_storage()) {
                window['localStorage']['enable_sound'] = enable_sound ? 1 : 0;
        }
}
window['set_sound'] = set_sound;

/** Send off a hash probe request to the backend.
 * @param {string} fen
 */
function explore_hash(fen) {
        // If we already have a backend response going, abort it.
        if (current_hash_xhr) {
                current_hash_xhr.abort();
        }
        if (current_hash_display_timer) {
                clearTimeout(current_hash_display_timer);
                current_hash_display_timer = null;
        }
        document.getElementById("refutationlines").replaceChildren();

        current_hash_xhr = new AbortController();
        const signal = current_analysis_xhr.signal;
        fetch(backend_hash_url + "?fen=" + fen, { signal })
                .then((response) => response.json())
                .then((data) => { show_explore_hash_results(data, fen); })
                .catch((err) => {
                        // Truncate the lines, since we already cleared the display.
                        display_lines = [ display_lines[0], display_lines[1] ];
                        update_move_highlight();
                });
}

/** Process the JSON response from a hash probe request.
 * @param {!Object} data
 * @param {string} fen
 */
function show_explore_hash_results(data, fen) {
        if (board_is_animating) {
                // Updating while the animation is still going causes
                // the animation to jerk. This is pretty crude, but it will do.
                current_hash_display_timer = setTimeout(function() { show_explore_hash_results(data, fen); }, 100);
                return;
        }
        current_hash_display_timer = null;
        hash_refutation_lines = data['lines'];
        hash_refutation_lines_base_fen = fen;
        update_board();
}

// almost all of this stuff comes from the chessboard.js example page
function onDragStart(source, piece, position, orientation) {
        let pseudogame = new Chess(display_fen);
        if (pseudogame.game_over() === true ||
            (pseudogame.turn() === 'w' && piece.search(/^b/) !== -1) ||
            (pseudogame.turn() === 'b' && piece.search(/^w/) !== -1)) {
                return false;
        }

        recommended_move = get_best_move(pseudogame, source, null, pseudogame.turn() === 'b');
        if (recommended_move) {
                let squareEl = document.querySelector('#board .square-' + recommended_move.to);
                squareEl.classList.add('highlight1-32417');
        }
        return true;
}

function mousedownSquare(e) {
        if (!e.target || !e.target.getAttribute('data-square')) {
                return;
        }

        reverse_dragging_from = null;
        let square = e.target.getAttribute('data-square');

        let pseudogame = new Chess(display_fen);
        if (pseudogame.game_over() === true) {
                return;
        }

        // If the square is empty, or has a piece of the side not to move,
        // we handle it. If not, normal piece dragging will take it.
        let position = board.position();
        if (!position.hasOwnProperty(square) ||
            (pseudogame.turn() === 'w' && position[square].search(/^b/) !== -1) ||
            (pseudogame.turn() === 'b' && position[square].search(/^w/) !== -1)) {
                reverse_dragging_from = square;
                recommended_move = get_best_move(pseudogame, null, square, pseudogame.turn() === 'b');
                if (recommended_move) {
                        let squareEl = document.querySelector('#board .square-' + recommended_move.from);
                        squareEl.classList.add('highlight1-32417');
                        squareEl = document.querySelector('#board .square-' + recommended_move.to);
                        squareEl.classList.add('highlight1-32417');
                }
        }
}

function mouseupSquare(e) {
        if (!e.target || !e.target.getAttribute('data-square')) {
                return;
        }
        if (reverse_dragging_from === null) {
                return;
        }
        let source = e.target.getAttribute('data-square');
        let target = reverse_dragging_from;
        reverse_dragging_from = null;
        if (onDrop(source, target) !== 'snapback') {
                onSnapEnd(source, target);
        }
        document.getElementById("board").querySelectorAll('.square-55d63.highlight1-32417').forEach((square) => {
                square.classList.remove('highlight1-32417');
        });
}

function get_best_move(game, source, target, invert) {
        let moves = game.moves({ verbose: true });
        if (source !== null) {
                moves = moves.filter(function(move) { return move.from == source; });
        }
        if (target !== null) {
                moves = moves.filter(function(move) { return move.to == target; });
        }
        if (moves.length == 0) {
                return null;
        }
        if (moves.length == 1) {
                return moves[0];
        }

        // More than one move. Use the display lines (if we have them)
        // to disambiguate; otherwise, we have no information.
        let move_hash = {};
        for (let i = 0; i < moves.length; ++i) {
                move_hash[moves[i].san] = moves[i];
        }

        // See if we're already exploring some line.
        if (current_display_line &&
            current_display_move < current_display_line.pv.length - 1) {
                let first_move = current_display_line.pv[current_display_move + 1];
                if (move_hash[first_move]) {
                        return move_hash[first_move];
                }
        }

        // History and PV take priority over the display lines.
        for (let i = 0; i < 2; ++i) {
                let line = display_lines[i];
                let first_move = line.pv[line.start_display_move_num];
                if (move_hash[first_move]) {
                        return move_hash[first_move];
                }
        }

        let best_move = null;
        let best_move_score = null;

        let refutation_lines = choose_displayed_refutation_lines()[0];
        for (let move in refutation_lines) {
                let line = refutation_lines[move];
                if (!line['score']) {
                        continue;
                }
                let first_move = line['pv'][0];
                if (move_hash[first_move]) {
                        let score = compute_score_sort_key(line['score'], line['depth'], invert);
                        if (best_move_score === null || score > best_move_score) {
                                best_move = move_hash[first_move];
                                best_move_score = score;
                        }
                }
        }
        return best_move;
}

function onDrop(source, target) {
        if (source === target) {
                if (recommended_move === null) {
                        return 'snapback';
                } else {
                        // Accept the move. It will be changed in onSnapEnd.
                        return;
                }
        } else {
                // Suggestion not asked for.
                recommended_move = null;
        }

        // see if the move is legal
        let pseudogame = new Chess(display_fen);
        let move = pseudogame.move({
                from: source,
                to: target,
                promotion: 'q' // NOTE: always promote to a queen for example simplicity
        });

        // illegal move
        if (move === null) return 'snapback';
}

/**
 * If we are in admin mode, send this move to the backend.
 *
 * @param {string} fen
 * @param {string} move
 */
function send_chosen_move(fen, move) {
        if (admin_password !== null) {
                let history = current_analysis_data['position']['history'];
                let url = '/manual-override.pl';
                url += '?fen=' + encodeURIComponent(fen);
                url += '&history=' + encodeURIComponent(JSON.stringify(history));
                url += '&move=' + encodeURIComponent(move);
                url += '&player_w=' + encodeURIComponent(current_analysis_data['position']['player_w']);
                url += '&player_b=' + encodeURIComponent(current_analysis_data['position']['player_b']);
                url += '&password=' + encodeURIComponent(admin_password);

                console.log(fen, history);
                fetch(url);  // Ignore the result.
        }
}

function undo_move() {
        if (admin_password !== null) {
                let history = current_analysis_data['position']['history'];
                history = history.slice(0, history.length - 1);

                let position = current_analysis_data['position']['start_fen'];
                let hiddenboard = chess_from(position, history, history.length);
                let fen = hiddenboard.fen();

                let url = '/manual-override.pl';
                url += '?fen=' + encodeURIComponent(fen);
                url += '&history=' + encodeURIComponent(JSON.stringify(history));
                url += '&move=null';
                url += '&player_w=' + encodeURIComponent(current_analysis_data['position']['player_w']);
                url += '&player_b=' + encodeURIComponent(current_analysis_data['position']['player_b']);
                url += '&password=' + encodeURIComponent(admin_password);

                console.log(fen, history);
                fetch(url);  // Ignore the result.
        }
}
window['undo_move'] = undo_move;

function onSnapEnd(source, target) {
        if (source === target && recommended_move !== null) {
                source = recommended_move.from;
                target = recommended_move.to;
        }
        recommended_move = null;
        let pseudogame = new Chess(display_fen);
        let move = pseudogame.move({
                from: source,
                to: target,
                promotion: 'q' // NOTE: always promote to a queen for example simplicity
        });

        if (admin_password !== null) {
                send_chosen_move(display_fen, move.san);
                return;
        }

        // Move ahead on the line we're on -- this includes history if we've
        // gone backwards.
        if (current_display_line &&
            current_display_move < current_display_line.pv.length - 1 &&
            current_display_line.pv[current_display_move + 1] === move.san) {
                next_move();
                return;
        }

        // Walk down the displayed lines until we find one that starts with
        // this move, then select that. Note that this gives us a good priority
        // order (PV, then multi-PV lines; history was already dealt with above,
        // as it's the only line that originates backwards).
        for (let i = 1; i < display_lines.length; ++i) {
                if (i == 1 && current_display_line) {
                        // Do not choose PV if not on it.
                        continue;
                }
                let line = display_lines[i];
                if (line.pv[line.start_display_move_num] === move.san) {
                        show_line(i, line.start_display_move_num);
                        return;
                }
        }

        // Shouldn't really be here if we have hash probes, but there's really
        // nothing we can do.
        // FIXME: Just make a new line, probably (even if we don't have hash moves).
        // As it is, we can actually drag (but not click) such a move in the UI,
        // but it has no effect on what we're probing.
}
// End of dragging-related code.

function fmt_cp(v) {
        if (v === 0) {
                return "0.00";
        } else if (v > 0) {
                return "+" + (v / 100).toFixed(2);
        } else {
                v = -v;
                return "-" + (v / 100).toFixed(2);
        }
}

function format_short_score(score) {
        if (!score) {
                return "???";
        }
        if (score[0] === 'T' || score[0] === 't') {
                let ret = "TB\u00a0";
                if (score[2]) {  // Is a bound.
                        ret = score[2] + "\u00a0TB\u00a0";
                }
                if (score[0] === 'T') {
                        return ret + Math.ceil(score[1] / 2);
                } else {
                        return ret + "-" + Math.ceil(score[1] / 2);
                }
        } else if (score[0] === 'M' || score[0] === 'm') {
                let sign = (score[0] === 'm') ? '-' : '';
                if (score[2]) {  // Is a bound.
                        return score[2] + "\u00a0M " + sign + score[1];
                } else {
                        return "M " + sign + score[1];
                }
        } else if (score[0] === 'd') {
                return "TB =0";
        } else if (score[0] === 'cp') {
                if (score[2]) {  // Is a bound.
                        return score[2] + "\u00a0" + fmt_cp(score[1]);
                } else {
                        return fmt_cp(score[1]);
                }
        }
        return null;
}

function format_long_score(score) {
        if (!score) {
                return "???";
        }
        if (score[0] === 'T') {
                if (score[1] == 0) {
                        return "Won for white (tablebase)";
                } else {
                        return "White wins in " + Math.ceil(score[1] / 2);
                }
        } else if (score[0] === 't') {
                if (score[1] == -1) {
                        return "Won for black (tablebase)";
                } else {
                        return "Black wins in " + Math.ceil(score[1] / 2);
                }
        } else if (score[0] === 'M') {
                if (score[1] == 0) {
                        return "White wins by checkmate";
                } else {
                        return "White mates in " + score[1];
                }
        } else if (score[0] === 'm') {
                if (score[1] == 0) {
                        return "Black wins by checkmate";
                } else {
                        return "Black mates in " + score[1];
                }
        } else if (score[0] === 'd') {
                return "Theoretical draw";
        } else if (score[0] === 'cp') {
                return "Score: " + format_short_score(score);
        }
        return null;
}

function compute_plot_score(score) {
        if (score[0] === 'M' || score[0] === 'T') {
                return 500;
        } else if (score[0] === 'm' || score[0] === 't') {
                return -500;
        } else if (score[0] === 'd') {
                return 0;
        } else if (score[0] === 'cp') {
                if (score[1] > 500) {
                        return 500;
                } else if (score[1] < -500) {
                        return -500;
                } else {
                        return score[1];
                }
        }
        return null;
}

/**
 * @param score The score digest tuple.
 * @param {?number} depth Depth the move has been computed to, or null.
 * @param {boolean} invert Whether black is to play.
 * @return {number}
 */
function compute_score_sort_key(score, depth, invert) {
        let s;
        if (!score) {
                return -10000000;
        }
        if (score[0] === 'T') {
                // White reaches TB win.
                s = 89999 - score[1];
        } else if (score[0] === 't') {
                // Black reaches TB win.
                s = -(89999 - score[1]);
        } else if (score[0] === 'M') {
                // White mates.
                s = 99999 - score[1];
        } else if (score[0] === 'm') {
                // Black mates.
                s = -(99999 - score[1]);
        } else if (score[0] === 'd') {
                s = 0;
        } else if (score[0] === 'cp') {
                s = score[1];
        }
        if (s) {
                if (invert) s = -s;
                return s;
        } else {
                return null;
        }
}

/**
 * @param {Object} game
 */
function switch_backend(game) {
        // Stop looking at historic data.
        current_display_line = null;
        current_display_move = null;
        displayed_analysis_data = null;
        if (current_historic_xhr) {
                current_historic_xhr.abort();
        }

        // If we already have a backend response going, abort it.
        if (current_analysis_xhr) {
                current_analysis_xhr.abort();
        }
        if (current_hash_xhr) {
                current_hash_xhr.abort();
        }

        // Otherwise, we should have a timer going to start a new one.
        // Kill that, too.
        if (current_analysis_request_timer) {
                clearTimeout(current_analysis_request_timer);
                current_analysis_request_timer = null;
        }
        if (current_hash_display_timer) {
                clearTimeout(current_hash_display_timer);
                current_hash_display_timer = null;
        }

        // Request an immediate fetch with the new backend.
        backend_url = game['url'];
        backend_hash_url = game['hashurl'];
        window.location.hash = '#' + game['id'];
        current_analysis_data = null;
        ims = 0;
        request_update();
}
window['switch_backend'] = switch_backend;

window['flip'] = function() { board.flip(); redraw_arrows(); };
window['set_delay_ms'] = function(ms) { delay_ms = ms; console.log('Delay is now ' + ms + ' ms.'); };

// Mostly from Wikipedia's chess set as of October 2022, but some pieces are from
// the 2013 version, as I like those better (and it matches the 2014 PNGs; nobody
// really likes change, do they?). That is wK, bK, bQ. wQ is also slightly different,
// but not enough to notice.
const svg_pieces = {
        'wK': 'data:image/svg+xml,<?xml version=%221.0%22 encoding=%22UTF-8%22 standalone=%22no%22?>%0A<!DOCTYPE svg PUBLIC %22-//W3C//DTD SVG 1.1//EN%22 %22http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd%22>%0A<svg xmlns=%22http://www.w3.org/2000/svg%22 version=%221.1%22 width=%2245%22 height=%2245%22><g style=%22fill:none;fill-opacity:1;fill-rule:evenodd;stroke:%23000000;stroke-width:1.5;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none;stroke-opacity:1%22><path d=%22M 22.5,11.63 L 22.5,6%22 style=%22fill:none;stroke:%23000000;stroke-linejoin:miter%22/><path d=%22M 20,8 L 25,8%22 style=%22fill:none;stroke:%23000000;stroke-linejoin:miter%22/><path d=%22M 22.5,25 C 22.5,25 27,17.5 25.5,14.5 C 25.5,14.5 24.5,12 22.5,12 C 20.5,12 19.5,14.5 19.5,14.5 C 18,17.5 22.5,25 22.5,25%22 style=%22fill:%23ffffff;stroke:%23000000;stroke-linecap:butt;stroke-linejoin:miter%22/><path d=%22M 11.5,37 C 17,40.5 27,40.5 32.5,37 L 32.5,30 C 32.5,30 41.5,25.5 38.5,19.5 C 34.5,13 25,16 22.5,23.5 L 22.5,27 L 22.5,23.5 C 19,16 9.5,13 6.5,19.5 C 3.5,25.5 11.5,29.5 11.5,29.5 L 11.5,37 z %22 style=%22fill:%23ffffff;stroke:%23000000%22/><path d=%22M 11.5,30 C 17,27 27,27 32.5,30%22 style=%22fill:none;stroke:%23000000%22/><path d=%22M 11.5,33.5 C 17,30.5 27,30.5 32.5,33.5%22 style=%22fill:none;stroke:%23000000%22/><path d=%22M 11.5,37 C 17,34 27,34 32.5,37%22 style=%22fill:none;stroke:%23000000%22/></g></svg>',
        'wQ': 'data:image/svg+xml,<?xml version=%221.0%22 encoding=%22UTF-8%22 standalone=%22no%22?>%0A<!DOCTYPE svg PUBLIC %22-//W3C//DTD SVG 1.1//EN%22 %22http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd%22>%0A<svg xmlns=%22http://www.w3.org/2000/svg%22 version=%221.1%22 width=%2245%22 height=%2245%22><g style=%22fill:%23ffffff;stroke:%23000000;stroke-width:1.5;stroke-linejoin:round%22><path d=%22M 9,26 C 17.5,24.5 30,24.5 36,26 L 38.5,13.5 L 31,25 L 30.7,10.9 L 25.5,24.5 L 22.5,10 L 19.5,24.5 L 14.3,10.9 L 14,25 L 6.5,13.5 L 9,26 z%22/><path d=%22M 9,26 C 9,28 10.5,28 11.5,30 C 12.5,31.5 12.5,31 12,33.5 C 10.5,34.5 11,36 11,36 C 9.5,37.5 11,38.5 11,38.5 C 17.5,39.5 27.5,39.5 34,38.5 C 34,38.5 35.5,37.5 34,36 C 34,36 34.5,34.5 33,33.5 C 32.5,31 32.5,31.5 33.5,30 C 34.5,28 36,28 36,26 C 27.5,24.5 17.5,24.5 9,26 z%22/><path d=%22M 11.5,30 C 15,29 30,29 33.5,30%22 style=%22fill:none%22/><path d=%22M 12,33.5 C 18,32.5 27,32.5 33,33.5%22 style=%22fill:none%22/><circle cx=%226%22 cy=%2212%22 r=%222%22/><circle cx=%2214%22 cy=%229%22 r=%222%22/><circle cx=%2222.5%22 cy=%228%22 r=%222%22/><circle cx=%2231%22 cy=%229%22 r=%222%22/><circle cx=%2239%22 cy=%2212%22 r=%222%22/></g></svg>',
        'wR': 'data:image/svg+xml,<?xml version=%221.0%22 encoding=%22UTF-8%22 standalone=%22no%22?>%0A<!DOCTYPE svg PUBLIC %22-//W3C//DTD SVG 1.1//EN%22 %22http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd%22>%0A<svg xmlns=%22http://www.w3.org/2000/svg%22 version=%221.1%22 width=%2245%22 height=%2245%22><g style=%22opacity:1;fill:%23ffffff;fill-opacity:1;fill-rule:evenodd;stroke:%23000000;stroke-width:1.5;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none;stroke-opacity:1%22 transform=%22translate(0,0.3)%22><path d=%22M 9,39 L 36,39 L 36,36 L 9,36 L 9,39 z %22 style=%22stroke-linecap:butt%22/><path d=%22M 12,36 L 12,32 L 33,32 L 33,36 L 12,36 z %22 style=%22stroke-linecap:butt%22/><path d=%22M 11,14 L 11,9 L 15,9 L 15,11 L 20,11 L 20,9 L 25,9 L 25,11 L 30,11 L 30,9 L 34,9 L 34,14%22 style=%22stroke-linecap:butt%22/><path d=%22M 34,14 L 31,17 L 14,17 L 11,14%22/><path d=%22M 31,17 L 31,29.5 L 14,29.5 L 14,17%22 style=%22stroke-linecap:butt;stroke-linejoin:miter%22/><path d=%22M 31,29.5 L 32.5,32 L 12.5,32 L 14,29.5%22/><path d=%22M 11,14 L 34,14%22 style=%22fill:none;stroke:%23000000;stroke-linejoin:miter%22/></g></svg>',
        'wB': 'data:image/svg+xml,<?xml version=%221.0%22 encoding=%22UTF-8%22 standalone=%22no%22?>%0A<!DOCTYPE svg PUBLIC %22-//W3C//DTD SVG 1.1//EN%22 %22http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd%22>%0A<svg xmlns=%22http://www.w3.org/2000/svg%22 version=%221.1%22 width=%2245%22 height=%2245%22><g style=%22opacity:1;fill:none;fill-rule:evenodd;fill-opacity:1;stroke:%23000000;stroke-width:1.5;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none;stroke-opacity:1%22 transform=%22translate(0,0.6)%22><g style=%22fill:%23ffffff;stroke:%23000000;stroke-linecap:butt%22><path d=%22M 9,36 C 12.39,35.03 19.11,36.43 22.5,34 C 25.89,36.43 32.61,35.03 36,36 C 36,36 37.65,36.54 39,38 C 38.32,38.97 37.35,38.99 36,38.5 C 32.61,37.53 25.89,38.96 22.5,37.5 C 19.11,38.96 12.39,37.53 9,38.5 C 7.65,38.99 6.68,38.97 6,38 C 7.35,36.54 9,36 9,36 z%22/><path d=%22M 15,32 C 17.5,34.5 27.5,34.5 30,32 C 30.5,30.5 30,30 30,30 C 30,27.5 27.5,26 27.5,26 C 33,24.5 33.5,14.5 22.5,10.5 C 11.5,14.5 12,24.5 17.5,26 C 17.5,26 15,27.5 15,30 C 15,30 14.5,30.5 15,32 z%22/><path d=%22M 25 8 A 2.5 2.5 0 1 1 20,8 A 2.5 2.5 0 1 1 25 8 z%22/></g><path d=%22M 17.5,26 L 27.5,26 M 15,30 L 30,30 M 22.5,15.5 L 22.5,20.5 M 20,18 L 25,18%22 style=%22fill:none;stroke:%23000000;stroke-linejoin:miter%22/></g></svg>',
        'wN': 'data:image/svg+xml,<?xml version=%221.0%22 encoding=%22UTF-8%22 standalone=%22no%22?>%0A<!DOCTYPE svg PUBLIC %22-//W3C//DTD SVG 1.1//EN%22 %22http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd%22>%0A<svg xmlns=%22http://www.w3.org/2000/svg%22 version=%221.1%22 width=%2245%22 height=%2245%22><g style=%22opacity:1;fill:none;fill-opacity:1;fill-rule:evenodd;stroke:%23000000;stroke-width:1.5;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none;stroke-opacity:1%22 transform=%22translate(0,0.3)%22><path d=%22M 22,10 C 32.5,11 38.5,18 38,39 L 15,39 C 15,30 25,32.5 23,18%22 style=%22fill:%23ffffff;stroke:%23000000%22/><path d=%22M 24,18 C 24.38,20.91 18.45,25.37 16,27 C 13,29 13.18,31.34 11,31 C 9.958,30.06 12.41,27.96 11,28 C 10,28 11.19,29.23 10,30 C 9,30 5.997,31 6,26 C 6,24 12,14 12,14 C 12,14 13.89,12.1 14,10.5 C 13.27,9.506 13.5,8.5 13.5,7.5 C 14.5,6.5 16.5,10 16.5,10 L 18.5,10 C 18.5,10 19.28,8.008 21,7 C 22,7 22,10 22,10%22 style=%22fill:%23ffffff;stroke:%23000000%22/><path d=%22M 9.5 25.5 A 0.5 0.5 0 1 1 8.5,25.5 A 0.5 0.5 0 1 1 9.5 25.5 z%22 style=%22fill:%23000000;stroke:%23000000%22/><path d=%22M 15 15.5 A 0.5 1.5 0 1 1 14,15.5 A 0.5 1.5 0 1 1 15 15.5 z%22 transform=%22matrix(0.866,0.5,-0.5,0.866,9.693,-5.173)%22 style=%22fill:%23000000;stroke:%23000000%22/></g></svg>',
        'wP': 'data:image/svg+xml,<?xml version=%221.0%22 encoding=%22UTF-8%22 standalone=%22no%22?>%0A<!DOCTYPE svg PUBLIC %22-//W3C//DTD SVG 1.1//EN%22 %22http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd%22>%0A<svg xmlns=%22http://www.w3.org/2000/svg%22 version=%221.1%22 width=%2245%22 height=%2245%22><path d=%22m 22.5,9 c -2.21,0 -4,1.79 -4,4 0,0.89 0.29,1.71 0.78,2.38 C 17.33,16.5 16,18.59 16,21 c 0,2.03 0.94,3.84 2.41,5.03 C 15.41,27.09 11,31.58 11,39.5 H 34 C 34,31.58 29.59,27.09 26.59,26.03 28.06,24.84 29,23.03 29,21 29,18.59 27.67,16.5 25.72,15.38 26.21,14.71 26.5,13.89 26.5,13 c 0,-2.21 -1.79,-4 -4,-4 z%22 style=%22opacity:1;fill:%23ffffff;fill-opacity:1;fill-rule:nonzero;stroke:%23000000;stroke-width:1.5;stroke-linecap:round;stroke-linejoin:miter;stroke-miterlimit:4;stroke-dasharray:none;stroke-opacity:1%22/></svg>',
       'bK': 'data:image/svg+xml,<?xml version=%221.0%22 encoding=%22UTF-8%22 standalone=%22no%22?>%0A<!DOCTYPE svg PUBLIC %22-//W3C//DTD SVG 1.1//EN%22 %22http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd%22>%0A<svg xmlns=%22http://www.w3.org/2000/svg%22 version=%221.1%22 width=%2245%22 height=%2245%22><g style=%22fill:none;fill-opacity:1;fill-rule:evenodd;stroke:%23000000;stroke-width:1.5;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none;stroke-opacity:1%22><path d=%22M 22.5,11.63 L 22.5,6%22 style=%22fill:none;stroke:%23000000;stroke-linejoin:miter%22 id=%22path6570%22/><path d=%22M 22.5,25 C 22.5,25 27,17.5 25.5,14.5 C 25.5,14.5 24.5,12 22.5,12 C 20.5,12 19.5,14.5 19.5,14.5 C 18,17.5 22.5,25 22.5,25%22 style=%22fill:%23000000;fill-opacity:1;stroke-linecap:butt;stroke-linejoin:miter%22/><path d=%22M 11.5,37 C 17,40.5 27,40.5 32.5,37 L 32.5,30 C 32.5,30 41.5,25.5 38.5,19.5 C 34.5,13 25,16 22.5,23.5 L 22.5,27 L 22.5,23.5 C 19,16 9.5,13 6.5,19.5 C 3.5,25.5 11.5,29.5 11.5,29.5 L 11.5,37 z %22 style=%22fill:%23000000;stroke:%23000000%22/><path d=%22M 20,8 L 25,8%22 style=%22fill:none;stroke:%23000000;stroke-linejoin:miter%22/><path d=%22M 32,29.5 C 32,29.5 40.5,25.5 38.03,19.85 C 34.15,14 25,18 22.5,24.5 L 22.51,26.6 L 22.5,24.5 C 20,18 9.906,14 6.997,19.85 C 4.5,25.5 11.85,28.85 11.85,28.85%22 style=%22fill:none;stroke:%23ffffff%22/><path d=%22M 11.5,30 C 17,27 27,27 32.5,30 M 11.5,33.5 C 17,30.5 27,30.5 32.5,33.5 M 11.5,37 C 17,34 27,34 32.5,37%22 style=%22fill:none;stroke:%23ffffff%22/></g></svg>',
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        'bB': 'data:image/svg+xml,<?xml version=%221.0%22 encoding=%22UTF-8%22 standalone=%22no%22?>%0A<!DOCTYPE svg PUBLIC %22-//W3C//DTD SVG 1.1//EN%22 %22http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd%22>%0A<svg xmlns=%22http://www.w3.org/2000/svg%22 version=%221.1%22 width=%2245%22 height=%2245%22><g style=%22opacity:1;fill:none;fill-rule:evenodd;fill-opacity:1;stroke:%23000000;stroke-width:1.5;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none;stroke-opacity:1%22 transform=%22translate(0,0.6)%22><g style=%22fill:%23000000;stroke:%23000000;stroke-linecap:butt%22><path d=%22M 9,36 C 12.39,35.03 19.11,36.43 22.5,34 C 25.89,36.43 32.61,35.03 36,36 C 36,36 37.65,36.54 39,38 C 38.32,38.97 37.35,38.99 36,38.5 C 32.61,37.53 25.89,38.96 22.5,37.5 C 19.11,38.96 12.39,37.53 9,38.5 C 7.65,38.99 6.68,38.97 6,38 C 7.35,36.54 9,36 9,36 z%22/><path d=%22M 15,32 C 17.5,34.5 27.5,34.5 30,32 C 30.5,30.5 30,30 30,30 C 30,27.5 27.5,26 27.5,26 C 33,24.5 33.5,14.5 22.5,10.5 C 11.5,14.5 12,24.5 17.5,26 C 17.5,26 15,27.5 15,30 C 15,30 14.5,30.5 15,32 z%22/><path d=%22M 25 8 A 2.5 2.5 0 1 1 20,8 A 2.5 2.5 0 1 1 25 8 z%22/></g><path d=%22M 17.5,26 L 27.5,26 M 15,30 L 30,30 M 22.5,15.5 L 22.5,20.5 M 20,18 L 25,18%22 style=%22fill:none;stroke:%23ffffff;stroke-linejoin:miter%22/></g></svg>',
        'bN': 'data:image/svg+xml,<?xml version=%221.0%22 encoding=%22UTF-8%22 standalone=%22no%22?>%0A<!DOCTYPE svg PUBLIC %22-//W3C//DTD SVG 1.1//EN%22 %22http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd%22>%0A<svg xmlns=%22http://www.w3.org/2000/svg%22 version=%221.1%22 width=%2245%22 height=%2245%22><g style=%22opacity:1;fill:none;fill-opacity:1;fill-rule:evenodd;stroke:%23000000;stroke-width:1.5;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none;stroke-opacity:1%22 transform=%22translate(0,0.3)%22><path d=%22M 22,10 C 32.5,11 38.5,18 38,39 L 15,39 C 15,30 25,32.5 23,18%22 style=%22fill:%23000000;stroke:%23000000%22/><path d=%22M 24,18 C 24.38,20.91 18.45,25.37 16,27 C 13,29 13.18,31.34 11,31 C 9.958,30.06 12.41,27.96 11,28 C 10,28 11.19,29.23 10,30 C 9,30 5.997,31 6,26 C 6,24 12,14 12,14 C 12,14 13.89,12.1 14,10.5 C 13.27,9.506 13.5,8.5 13.5,7.5 C 14.5,6.5 16.5,10 16.5,10 L 18.5,10 C 18.5,10 19.28,8.008 21,7 C 22,7 22,10 22,10%22 style=%22fill:%23000000;stroke:%23000000%22/><path d=%22M 9.5 25.5 A 0.5 0.5 0 1 1 8.5,25.5 A 0.5 0.5 0 1 1 9.5 25.5 z%22 style=%22fill:%23ffffff;stroke:%23ffffff%22/><path d=%22M 15 15.5 A 0.5 1.5 0 1 1 14,15.5 A 0.5 1.5 0 1 1 15 15.5 z%22 transform=%22matrix(0.866,0.5,-0.5,0.866,9.693,-5.173)%22 style=%22fill:%23ffffff;stroke:%23ffffff%22/><path d=%22M 24.55,10.4 L 24.1,11.85 L 24.6,12 C 27.75,13 30.25,14.49 32.5,18.75 C 34.75,23.01 35.75,29.06 35.25,39 L 35.2,39.5 L 37.45,39.5 L 37.5,39 C 38,28.94 36.62,22.15 34.25,17.66 C 31.88,13.17 28.46,11.02 25.06,10.5 L 24.55,10.4 z %22 style=%22fill:%23ffffff;stroke:none%22/></g></svg>',
        'bP': 'data:image/svg+xml,<?xml version=%221.0%22 encoding=%22UTF-8%22 standalone=%22no%22?>%0A<!DOCTYPE svg PUBLIC %22-//W3C//DTD SVG 1.1//EN%22 %22http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd%22>%0A<svg xmlns=%22http://www.w3.org/2000/svg%22 version=%221.1%22 width=%2245%22 height=%2245%22><path d=%22m 22.5,9 c -2.21,0 -4,1.79 -4,4 0,0.89 0.29,1.71 0.78,2.38 C 17.33,16.5 16,18.59 16,21 c 0,2.03 0.94,3.84 2.41,5.03 C 15.41,27.09 11,31.58 11,39.5 H 34 C 34,31.58 29.59,27.09 26.59,26.03 28.06,24.84 29,23.03 29,21 29,18.59 27.67,16.5 25.72,15.38 26.21,14.71 26.5,13.89 26.5,13 c 0,-2.21 -1.79,-4 -4,-4 z%22 style=%22opacity:1;fill:%23000000;fill-opacity:1;fill-rule:nonzero;stroke:%23000000;stroke-width:1.5;stroke-linecap:round;stroke-linejoin:miter;stroke-miterlimit:4;stroke-dasharray:none;stroke-opacity:1%22/></svg>',
};

function svg_piece_theme(piece) {
        return svg_pieces[piece];
}

function init() {
        unique = get_unique();

        // Load settings from HTML5 local storage if available.
        if (supports_html5_storage() && window['localStorage']['enable_sound']) {
                set_sound(parseInt(window['localStorage']['enable_sound']));
        } else {
                set_sound(false);
        }

        let admin_match = window.location.href.match(/\?password=([a-zA-Z0-9_-]+)/);
        if (admin_match !== null) {
                admin_password = admin_match[1];
        }

        // Create board.
        board = new window.ChessBoard('board', {
                onMoveEnd: function() { board_is_animating = false; },

                draggable: true,
                pieceTheme: svg_piece_theme,
                onDragStart: onDragStart,
                onDrop: onDrop,
                onSnapEnd: onSnapEnd
        });
        document.getElementById("board").addEventListener('mousedown', mousedownSquare);
        document.getElementById("board").addEventListener('mouseup', mouseupSquare);

        if (window['inline_json']) {
                let j = window['inline_json'];
                process_update_response(j['data'], { 'get': (h) => j['headers'][h] });
                delete window['inline_json'];
        }
        request_update();
        window.addEventListener('resize', function() {
                board.resize();
                update_sparkline(displayed_analysis_data || current_analysis_data);
                update_board_highlight();
                redraw_arrows();
        });
        new ResizeObserver(() => update_sparkline(displayed_analysis_data || current_analysis_data)).observe(document.getElementById('scoresparkcontainer'));
        window.addEventListener('keyup', function(event) {
                if (event.which == 39) {  // Left arrow.
                        next_move();
                } else if (event.which == 37) {  // Right arrow.
                        prev_move();
                } else if (event.which >= 49 && event.which <= 57) {  // 1-9.
                        let num = event.which - 49;
                        if (current_games && current_games.length >= num) {
                                switch_backend(current_games[num]);
                        }
                } else if (event.which == 78) {  // N.
                        next_game();
                }
        });
        window.addEventListener('hashchange', possibly_switch_game_from_hash, false);
        possibly_switch_game_from_hash();
};
document.addEventListener('DOMContentLoaded', init);

})();