X-Git-Url: https://git.sesse.net/?p=stockfish;a=blobdiff_plain;f=src%2Fsearch.cpp;h=4ea31a0b36dcde97bda492310b681fcec6025cf5;hp=3dbcb526dbff7187014d5d477df9ae7792f03f04;hb=35ada63174bbec6289d6dea6d3cfc5b5f14d1d27;hpb=55745f410547f22b1edce873b38cc3ce5f3ace5f diff --git a/src/search.cpp b/src/search.cpp index 3dbcb526..4ea31a0b 100644 --- a/src/search.cpp +++ b/src/search.cpp @@ -23,6 +23,7 @@ //// #include +#include #include #include #include @@ -172,9 +173,9 @@ namespace { // best move from the previous iteration, Problem is set back to false. const Value NoProblemMargin = Value(0x14); - // Null move margin. A null move search will not be done if the approximate + // Null move margin. A null move search will not be done if the static // evaluation of the position is more than NullMoveMargin below beta. - const Value NullMoveMargin = Value(0x300); + const Value NullMoveMargin = Value(0x200); // If the TT move is at least SingleReplyMargin better then the // remaining ones we will extend it. @@ -184,24 +185,16 @@ namespace { // and near frontier nodes. const Value FutilityMarginQS = Value(0x80); + Value FutilityMargins[2 * PLY_MAX_PLUS_2]; // Initialized at startup. + // Each move futility margin is decreased const Value IncrementalFutilityMargin = Value(0x8); // Depth limit for razoring const Depth RazorDepth = 4 * OnePly; - // Remaining depth: 1 ply 1.5 ply 2 ply 2.5 ply 3 ply 3.5 ply - const Value RazorMargins[6] = { Value(0x180), Value(0x300), Value(0x300), Value(0x3C0), Value(0x3C0), Value(0x3C0) }; - - // Remaining depth: 1 ply 1.5 ply 2 ply 2.5 ply 3 ply 3.5 ply - const Value RazorApprMargins[6] = { Value(0x520), Value(0x300), Value(0x300), Value(0x300), Value(0x300), Value(0x300) }; - - /// Variables initialized by UCI options - // Minimum number of full depth (i.e. non-reduced) moves at PV and non-PV nodes - int LMRPVMoves, LMRNonPVMoves; - // Depth limit for use of dynamic threat detection Depth ThreatDepth; @@ -223,6 +216,9 @@ namespace { IterationInfoType IterationInfo[PLY_MAX_PLUS_2]; int BestMoveChangesByIteration[PLY_MAX_PLUS_2]; + // Search window management + int AspirationDelta; + // MultiPV mode int MultiPV; @@ -242,6 +238,10 @@ namespace { bool UseLogFile; std::ofstream LogFile; + // Natural logarithmic lookup table and its getter function + float lnArray[512]; + inline float ln(int i) { return lnArray[i]; } + // MP related variables int ActiveThreads = 1; Depth MinimumSplitDepth; @@ -268,11 +268,10 @@ namespace { // History table History H; - /// Functions Value id_loop(const Position& pos, Move searchMoves[]); - Value root_search(Position& pos, SearchStack ss[], RootMoveList& rml, Value alpha, Value beta); + Value root_search(Position& pos, SearchStack ss[], RootMoveList& rml, Value& oldAlpha, Value& beta); Value search_pv(Position& pos, SearchStack ss[], Value alpha, Value beta, Depth depth, int ply, int threadID); Value search(Position& pos, SearchStack ss[], Value beta, Depth depth, int ply, bool allowNullmove, int threadID, Move excludedMove = MOVE_NONE); Value qsearch(Position& pos, SearchStack ss[], Value alpha, Value beta, Depth depth, int ply, int threadID); @@ -289,8 +288,11 @@ namespace { bool ok_to_prune(const Position& pos, Move m, Move threat); bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply); Value refine_eval(const TTEntry* tte, Value defaultEval, int ply); + void reduction_parameters(float base, float Inhibitor, Depth depth, float& logLimit, float& gradient); + Depth reduction(int moveCount, const float LogLimit, const float BaseRed, const float Gradient); void update_history(const Position& pos, Move move, Depth depth, Move movesSearched[], int moveCount); void update_killers(Move m, SearchStack& ss); + void update_gains(const Position& pos, Move move, Value before, Value after); bool fail_high_ply_1(); int current_search_time(); @@ -326,13 +328,6 @@ namespace { //// Functions //// -//FIXME: HACK -static double lnArray[512]; - -inline double ln(int i) -{ - return lnArray[i]; -} /// perft() is our utility to verify move generation is bug free. All the legal /// moves up to given depth are generated and counted and the sum returned. @@ -435,8 +430,6 @@ bool think(const Position& pos, bool infinite, bool ponder, int side_to_move, MateThreatExtension[1] = Depth(get_option_value_int("Mate Threat Extension (PV nodes)")); MateThreatExtension[0] = Depth(get_option_value_int("Mate Threat Extension (non-PV nodes)")); - LMRPVMoves = get_option_value_int("Full Depth Moves (PV nodes)") + 1; - LMRNonPVMoves = get_option_value_int("Full Depth Moves (non-PV nodes)") + 1; ThreatDepth = get_option_value_int("Threat Depth") * OnePly; Chess960 = get_option_value_bool("UCI_Chess960"); @@ -456,6 +449,10 @@ bool think(const Position& pos, bool infinite, bool ponder, int side_to_move, { ActiveThreads = newActiveThreads; init_eval(ActiveThreads); + // HACK: init_eval() destroys the static castleRightsMask[] array in the + // Position class. The below line repairs the damage. + Position p(pos.to_fen()); + assert(pos.is_ok()); } // Wake up sleeping threads @@ -534,7 +531,6 @@ bool think(const Position& pos, bool infinite, bool ponder, int side_to_move, // We're ready to start thinking. Call the iterative deepening loop function Value v = id_loop(pos, searchMoves); - if (UseLSNFiltering) { // Step 1. If this is sudden death game and our position is hopeless, @@ -566,23 +562,30 @@ bool think(const Position& pos, bool infinite, bool ponder, int side_to_move, /// and initializes the split point stack and the global locks and condition /// objects. -#include //FIXME: HACK - void init_threads() { - // FIXME: HACK!! - for (int i = 0; i < 512; i++) - lnArray[i] = log(double(i)); - volatile int i; + bool ok; #if !defined(_MSC_VER) pthread_t pthread[1]; #endif + // Init our logarithmic lookup table + for (i = 0; i < 512; i++) + lnArray[i] = float(log(double(i))); // log() returns base-e logarithm + for (i = 0; i < THREAD_MAX; i++) Threads[i].activeSplitPoints = 0; + // Init futility margins array + FutilityMargins[0] = FutilityMargins[1] = Value(0); + + for (i = 2; i < 2 * PLY_MAX_PLUS_2; i++) + { + FutilityMargins[i] = Value(112 * bitScanReverse32(i * i / 2)); // FIXME: test using log instead of BSR + } + // Initialize global locks lock_init(&MPLock, NULL); lock_init(&IOLock, NULL); @@ -610,12 +613,18 @@ void init_threads() { for (i = 1; i < THREAD_MAX; i++) { #if !defined(_MSC_VER) - pthread_create(pthread, NULL, init_thread, (void*)(&i)); + ok = (pthread_create(pthread, NULL, init_thread, (void*)(&i)) == 0); #else DWORD iID[1]; - CreateThread(NULL, 0, init_thread, (LPVOID)(&i), 0, iID); + ok = (CreateThread(NULL, 0, init_thread, (LPVOID)(&i), 0, iID) != NULL); #endif + if (!ok) + { + cout << "Failed to create thread number " << i << endl; + Application::exit_with_failure(); + } + // Wait until the thread has finished launching while (!Threads[i].running); } @@ -659,6 +668,8 @@ void SearchStack::init(int ply) { pv[ply] = pv[ply + 1] = MOVE_NONE; currentMove = threatMove = MOVE_NONE; reduction = Depth(0); + eval = VALUE_NONE; + evalInfo = NULL; } void SearchStack::initKillers() { @@ -683,6 +694,7 @@ namespace { // searchMoves are verified, copied, scored and sorted RootMoveList rml(p, searchMoves); + // Handle special case of searching on a mate/stale position if (rml.move_count() == 0) { if (PonderSearch) @@ -733,10 +745,11 @@ namespace { int prevDelta1 = IterationInfo[Iteration - 1].speculatedValue - IterationInfo[Iteration - 2].speculatedValue; int prevDelta2 = IterationInfo[Iteration - 2].speculatedValue - IterationInfo[Iteration - 3].speculatedValue; - int delta = Max(2 * abs(prevDelta1) + abs(prevDelta2), ProblemMargin); + AspirationDelta = Max(abs(prevDelta1) + abs(prevDelta2) / 2, 16); + AspirationDelta = (AspirationDelta + 7) / 8 * 8; // Round to match grainSize - alpha = Max(IterationInfo[Iteration - 1].value - delta, -VALUE_INFINITE); - beta = Min(IterationInfo[Iteration - 1].value + delta, VALUE_INFINITE); + alpha = Max(IterationInfo[Iteration - 1].value - AspirationDelta, -VALUE_INFINITE); + beta = Min(IterationInfo[Iteration - 1].value + AspirationDelta, VALUE_INFINITE); } else { @@ -844,7 +857,7 @@ namespace { // If we are pondering or in infinite search, we shouldn't print the // best move before we are told to do so. - if (PonderSearch || InfiniteSearch) + if (!AbortSearch && (PonderSearch || InfiniteSearch)) wait_for_stop_or_ponderhit(); else // Print final search statistics @@ -889,11 +902,25 @@ namespace { // similar to search_pv except that it uses a different move ordering // scheme and prints some information to the standard output. - Value root_search(Position& pos, SearchStack ss[], RootMoveList& rml, Value alpha, Value beta) { + Value root_search(Position& pos, SearchStack ss[], RootMoveList& rml, Value& oldAlpha, Value& beta) { - Value oldAlpha = alpha; + int64_t nodes; + Move move; + StateInfo st; + Depth depth, ext, newDepth; Value value; CheckInfo ci(pos); + int researchCount = 0; + bool moveIsCheck, captureOrPromotion, dangerous; + Value alpha = oldAlpha; + bool isCheck = pos.is_check(); + + // Evaluate the position statically + EvalInfo ei; + ss[0].eval = !isCheck ? evaluate(pos, ei, 0) : VALUE_NONE; + + while (1) // Fail low loop + { // Loop through all the moves in the root move list for (int i = 0; i < rml.move_count() && !AbortSearch; i++) @@ -906,10 +933,6 @@ namespace { rml.set_move_score(i, -VALUE_INFINITE); continue; } - int64_t nodes; - Move move; - StateInfo st; - Depth depth, ext, newDepth; RootMoveNumber = i + 1; FailHigh = false; @@ -929,17 +952,25 @@ namespace { << " currmovenumber " << RootMoveNumber << endl; // Decide search depth for this move - bool moveIsCheck = pos.move_is_check(move); - bool captureOrPromotion = pos.move_is_capture_or_promotion(move); - bool dangerous; - depth = (Iteration - 2) * OnePly + InitialDepth; + moveIsCheck = pos.move_is_check(move); + captureOrPromotion = pos.move_is_capture_or_promotion(move); + depth = (Iteration - 2) * OnePly + InitialDepth; ext = extension(pos, move, true, captureOrPromotion, moveIsCheck, false, false, &dangerous); newDepth = depth + ext; + value = - VALUE_INFINITE; + + // Precalculate reduction parameters + float LogLimit, Gradient, BaseReduction = 0.5; + reduction_parameters(BaseReduction, 6.0, depth, LogLimit, Gradient); + + while (1) // Fail high loop + { + // Make the move, and search it pos.do_move(move, st, ci, moveIsCheck); - if (i < MultiPV) + if (i < MultiPV || value > alpha) { // Aspiration window is disabled in multi-pv case if (MultiPV > 1) @@ -961,24 +992,24 @@ namespace { { // Try to reduce non-pv search depth by one ply if move seems not problematic, // if the move fails high will be re-searched at full depth. + bool doFullDepthSearch = true; + if ( depth >= 3*OnePly // FIXME was newDepth && !dangerous && !captureOrPromotion && !move_is_castle(move)) { - double red = 0.5 + ln(RootMoveNumber - MultiPV + 1) * ln(depth / 2) / 6.0; - if (red >= 1.0) + ss[0].reduction = reduction(RootMoveNumber - MultiPV + 1, LogLimit, BaseReduction, Gradient); + if (ss[0].reduction) { - ss[0].reduction = Depth(int(floor(red * int(OnePly)))); value = -search(pos, ss, -alpha, newDepth-ss[0].reduction, 1, true, 0); + doFullDepthSearch = (value > alpha); } - else - value = alpha + 1; // Just to trigger next condition - } else - value = alpha + 1; // Just to trigger next condition + } - if (value > alpha) + if (doFullDepthSearch) { + ss[0].reduction = Depth(0); value = -search(pos, ss, -alpha, newDepth, 1, true, 0); if (value > alpha) @@ -995,6 +1026,47 @@ namespace { pos.undo_move(move); + // Can we exit fail high loop ? + if (AbortSearch || value < beta) + break; + + // We are failing high and going to do a research. It's important to update score + // before research in case we run out of time while researching. + rml.set_move_score(i, value); + update_pv(ss, 0); + TT.extract_pv(pos, ss[0].pv, PLY_MAX); + rml.set_move_pv(i, ss[0].pv); + + // Print search information to the standard output + cout << "info depth " << Iteration + << " score " << value_to_string(value) + << ((value >= beta) ? " lowerbound" : + ((value <= alpha)? " upperbound" : "")) + << " time " << current_search_time() + << " nodes " << nodes_searched() + << " nps " << nps() + << " pv "; + + for (int j = 0; ss[0].pv[j] != MOVE_NONE && j < PLY_MAX; j++) + cout << ss[0].pv[j] << " "; + + cout << endl; + + if (UseLogFile) + { + ValueType type = (value >= beta ? VALUE_TYPE_LOWER + : (value <= alpha ? VALUE_TYPE_UPPER : VALUE_TYPE_EXACT)); + + LogFile << pretty_pv(pos, current_search_time(), Iteration, + nodes_searched(), value, type, ss[0].pv) << endl; + } + + // Prepare for a research after a fail high, each time with a wider window + researchCount++; + beta = Min(beta + AspirationDelta * (1 << researchCount), VALUE_INFINITE); + + } // End of fail high loop + // Finished searching the move. If AbortSearch is true, the search // was aborted because the user interrupted the search or because we // ran out of time. In this case, the return value of the search cannot @@ -1089,6 +1161,18 @@ namespace { FailLow = (alpha == oldAlpha); } + + // Can we exit fail low loop ? + if (AbortSearch || alpha > oldAlpha) + break; + + // Prepare for a research after a fail low, each time with a wider window + researchCount++; + alpha = Max(alpha - AspirationDelta * (1 << researchCount), -VALUE_INFINITE); + oldAlpha = alpha; + + } // Fail low loop + return alpha; } @@ -1104,7 +1188,6 @@ namespace { assert(threadID >= 0 && threadID < ActiveThreads); Move movesSearched[256]; - EvalInfo ei; StateInfo st; const TTEntry* tte; Move ttMove, move; @@ -1112,7 +1195,7 @@ namespace { Value oldAlpha, value; bool isCheck, mateThreat, singleEvasion, moveIsCheck, captureOrPromotion, dangerous; int moveCount = 0; - Value bestValue = -VALUE_INFINITE; + Value bestValue = value = -VALUE_INFINITE; if (depth < OnePly) return qsearch(pos, ss, alpha, beta, Depth(0), ply, threadID); @@ -1125,12 +1208,9 @@ namespace { if (AbortSearch || thread_should_stop(threadID)) return Value(0); - if (pos.is_draw()) + if (pos.is_draw() || ply >= PLY_MAX - 1) return VALUE_DRAW; - if (ply >= PLY_MAX - 1) - return pos.is_check() ? quick_evaluate(pos) : evaluate(pos, ei, threadID); - // Mate distance pruning oldAlpha = alpha; alpha = Max(value_mated_in(ply), alpha); @@ -1160,13 +1240,26 @@ namespace { tte = TT.retrieve(pos.get_key()); } + isCheck = pos.is_check(); + if (!isCheck) + { + // Update gain statistics of the previous move that lead + // us in this position. + EvalInfo ei; + ss[ply].eval = evaluate(pos, ei, threadID); + update_gains(pos, ss[ply - 1].currentMove, ss[ply - 1].eval, ss[ply].eval); + } + // Initialize a MovePicker object for the current position, and prepare // to search all moves - isCheck = pos.is_check(); mateThreat = pos.has_mate_threat(opposite_color(pos.side_to_move())); CheckInfo ci(pos); MovePicker mp = MovePicker(pos, ttMove, depth, H, &ss[ply]); + // Precalculate reduction parameters + float LogLimit, Gradient, BaseReduction = 0.5; + reduction_parameters(BaseReduction, 6.0, depth, LogLimit, Gradient); + // Loop through all legal moves until no moves remain or a beta cutoff // occurs. while ( alpha < beta @@ -1217,25 +1310,23 @@ namespace { { // Try to reduce non-pv search depth by one ply if move seems not problematic, // if the move fails high will be re-searched at full depth. + bool doFullDepthSearch = true; + if ( depth >= 3*OnePly && !dangerous && !captureOrPromotion && !move_is_castle(move) && !move_is_killer(move, ss[ply])) { - double red = 0.5 + ln(moveCount) * ln(depth / 2) / 6.0; - if (red >= 1.0) - { - ss[ply].reduction = Depth(int(floor(red * int(OnePly)))); - value = -search(pos, ss, -alpha, newDepth-ss[ply].reduction, ply+1, true, threadID); - } - else - value = alpha + 1; // Just to trigger next condition + ss[ply].reduction = reduction(moveCount, LogLimit, BaseReduction, Gradient); + if (ss[ply].reduction) + { + value = -search(pos, ss, -alpha, newDepth-ss[ply].reduction, ply+1, true, threadID); + doFullDepthSearch = (value > alpha); + } } - else - value = alpha + 1; // Just to trigger next condition - if (value > alpha) // Go with full depth non-pv search + if (doFullDepthSearch) // Go with full depth non-pv search { ss[ply].reduction = Depth(0); value = -search(pos, ss, -alpha, newDepth, ply+1, true, threadID); @@ -1338,11 +1429,11 @@ namespace { const TTEntry* tte; Move ttMove, move; Depth ext, newDepth; - Value staticValue, nullValue, value, futilityValue, futilityValueScaled; - bool isCheck, useFutilityPruning, singleEvasion, moveIsCheck, captureOrPromotion, dangerous; + Value bestValue, staticValue, nullValue, value, futilityValue, futilityValueScaled; + bool isCheck, singleEvasion, moveIsCheck, captureOrPromotion, dangerous; bool mateThreat = false; int moveCount = 0; - Value bestValue = -VALUE_INFINITE; + futilityValue = staticValue = bestValue = value = -VALUE_INFINITE; if (depth < OnePly) return qsearch(pos, ss, beta-1, beta, Depth(0), ply, threadID); @@ -1355,12 +1446,9 @@ namespace { if (AbortSearch || thread_should_stop(threadID)) return Value(0); - if (pos.is_draw()) + if (pos.is_draw() || ply >= PLY_MAX - 1) return VALUE_DRAW; - if (ply >= PLY_MAX - 1) - return pos.is_check() ? quick_evaluate(pos) : evaluate(pos, ei, threadID); - // Mate distance pruning if (value_mated_in(ply) >= beta) return beta; @@ -1383,23 +1471,34 @@ namespace { } isCheck = pos.is_check(); - ei.futilityMargin = Value(0); // Manually initialize futilityMargin - - // Evaluate the position statically - if (isCheck) - staticValue = quick_evaluate(pos); - else if (tte && (tte->type() & VALUE_TYPE_EVAL)) - staticValue = value_from_tt(tte->value(), ply); - else - staticValue = evaluate(pos, ei, threadID); // Calculate depth dependant futility pruning parameters const int FutilityMoveCountMargin = 3 + (1 << (3 * int(depth) / 8)); - const int FutilityValueMargin = 112 * bitScanReverse32(int(depth) * int(depth) / 2); - // Enhance score accuracy with TT value if possible - futilityValue = staticValue + FutilityValueMargin; - staticValue = refine_eval(tte, staticValue, ply); + // Evaluate the position statically + if (!isCheck) + { + if (tte && (tte->type() & VALUE_TYPE_EVAL)) + staticValue = value_from_tt(tte->value(), ply); + else + { + staticValue = evaluate(pos, ei, threadID); + ss[ply].evalInfo = &ei; + } + + ss[ply].eval = staticValue; + futilityValue = staticValue + FutilityMargins[int(depth)]; //FIXME: Remove me, only for split + staticValue = refine_eval(tte, staticValue, ply); // Enhance accuracy with TT value if possible + update_gains(pos, ss[ply - 1].currentMove, ss[ply - 1].eval, ss[ply].eval); + } + + // Do a "stand pat". If we are above beta by a good margin then + // return immediately. + if ( !isCheck + && allowNullmove + && depth < RazorDepth + && staticValue - FutilityMargins[int(depth)] >= beta) + return staticValue - FutilityMargins[int(depth)]; // Null move search if ( allowNullmove @@ -1452,13 +1551,14 @@ namespace { } // Null move search not allowed, try razoring else if ( !value_is_mate(beta) + && !isCheck && depth < RazorDepth - && staticValue < beta - RazorApprMargins[int(depth) - 2] + && staticValue < beta - (NullMoveMargin + 16 * depth) && ss[ply - 1].currentMove != MOVE_NULL && ttMove == MOVE_NONE && !pos.has_pawn_on_7th(pos.side_to_move())) { - Value rbeta = beta - RazorMargins[int(depth) - 2]; + Value rbeta = beta - (NullMoveMargin + 16 * depth); Value v = qsearch(pos, ss, rbeta-1, rbeta, Depth(0), ply, threadID); if (v < rbeta) return v; @@ -1466,7 +1566,7 @@ namespace { // Go with internal iterative deepening if we don't have a TT move if (UseIIDAtNonPVNodes && ttMove == MOVE_NONE && depth >= 8*OnePly && - !isCheck && evaluate(pos, ei, threadID) >= beta - IIDMargin) + !isCheck && ss[ply].eval >= beta - IIDMargin) { search(pos, ss, beta, Min(depth/2, depth-2*OnePly), ply, false, threadID); ttMove = ss[ply].pv[ply]; @@ -1477,7 +1577,10 @@ namespace { // to search all moves. MovePicker mp = MovePicker(pos, ttMove, depth, H, &ss[ply]); CheckInfo ci(pos); - useFutilityPruning = depth < SelectiveDepth && !isCheck; + + // Precalculate reduction parameters + float LogLimit, Gradient, BaseReduction = 0.5; + reduction_parameters(BaseReduction, 3.0, depth, LogLimit, Gradient); // Loop through all legal moves until no moves remain or a beta cutoff occurs while ( bestValue < beta @@ -1524,9 +1627,10 @@ namespace { movesSearched[moveCount++] = ss[ply].currentMove = move; // Futility pruning - if ( useFutilityPruning + if ( !isCheck && !dangerous && !captureOrPromotion + && !move_is_castle(move) && move != ttMove) { // Move count based pruning @@ -1536,13 +1640,29 @@ namespace { continue; // Value based pruning - futilityValueScaled = futilityValue - moveCount * IncrementalFutilityMargin; + Depth predictedDepth = newDepth; + + //FIXME: We are ignoring condition: depth >= 3*OnePly, BUG?? + ss[ply].reduction = reduction(moveCount, LogLimit, BaseReduction, Gradient); + if (ss[ply].reduction) + predictedDepth -= ss[ply].reduction; - if (futilityValueScaled < beta) + if (predictedDepth < SelectiveDepth) { - if (futilityValueScaled > bestValue) - bestValue = futilityValueScaled; - continue; + int preFutilityValueMargin = 0; + if (predictedDepth >= OnePly) + preFutilityValueMargin = FutilityMargins[int(predictedDepth)]; + + preFutilityValueMargin += H.gain(pos.piece_on(move_from(move)), move_from(move), move_to(move)) + 45; + + futilityValueScaled = ss[ply].eval + preFutilityValueMargin - moveCount * IncrementalFutilityMargin; + + if (futilityValueScaled < beta) + { + if (futilityValueScaled > bestValue) + bestValue = futilityValueScaled; + continue; + } } } @@ -1551,26 +1671,23 @@ namespace { // Try to reduce non-pv search depth by one ply if move seems not problematic, // if the move fails high will be re-searched at full depth. + bool doFullDepthSearch = true; + if ( depth >= 3*OnePly && !dangerous && !captureOrPromotion && !move_is_castle(move) - && !move_is_killer(move, ss[ply]) - /* && move != ttMove*/) + && !move_is_killer(move, ss[ply])) { - double red = 0.5 + ln(moveCount) * ln(depth / 2) / 3.0; - if (red >= 1.0) + ss[ply].reduction = reduction(moveCount, LogLimit, BaseReduction, Gradient); + if (ss[ply].reduction) { - ss[ply].reduction = Depth(int(floor(red * int(OnePly)))); value = -search(pos, ss, -(beta-1), newDepth-ss[ply].reduction, ply+1, true, threadID); + doFullDepthSearch = (value >= beta); } - else - value = beta; // Just to trigger next condition } - else - value = beta; // Just to trigger next condition - if (value >= beta) // Go with full depth non-pv search + if (doFullDepthSearch) // Go with full depth non-pv search { ss[ply].reduction = Depth(0); value = -search(pos, ss, -(beta-1), newDepth, ply+1, true, threadID); @@ -1598,7 +1715,7 @@ namespace { && idle_thread_exists(threadID) && !AbortSearch && !thread_should_stop(threadID) - && split(pos, ss, ply, &beta, &beta, &bestValue, futilityValue, + && split(pos, ss, ply, &beta, &beta, &bestValue, futilityValue, //FIXME: SMP & futilityValue depth, &moveCount, &mp, threadID, false)) break; } @@ -1651,10 +1768,11 @@ namespace { StateInfo st; Move ttMove, move; Value staticValue, bestValue, value, futilityBase, futilityValue; - bool isCheck, enoughMaterial, moveIsCheck; + bool isCheck, enoughMaterial, moveIsCheck, evasionPrunable; const TTEntry* tte = NULL; int moveCount = 0; bool pvNode = (beta - alpha != 1); + Value oldAlpha = alpha; // Initialize, and make an early exit in case of an aborted search, // an instant draw, maximum ply reached, etc. @@ -1664,12 +1782,9 @@ namespace { if (AbortSearch || thread_should_stop(threadID)) return Value(0); - if (pos.is_draw()) + if (pos.is_draw() || ply >= PLY_MAX - 1) return VALUE_DRAW; - if (ply >= PLY_MAX - 1) - return pos.is_check() ? quick_evaluate(pos) : evaluate(pos, ei, threadID); - // Transposition table lookup. At PV nodes, we don't use the TT for // pruning, but only for move ordering. tte = TT.retrieve(pos.get_key()); @@ -1684,7 +1799,6 @@ namespace { } isCheck = pos.is_check(); - ei.futilityMargin = Value(0); // Manually initialize futilityMargin // Evaluate the position statically if (isCheck) @@ -1694,6 +1808,12 @@ namespace { else staticValue = evaluate(pos, ei, threadID); + if (!isCheck) + { + ss[ply].eval = staticValue; + update_gains(pos, ss[ply - 1].currentMove, ss[ply - 1].eval, ss[ply].eval); + } + // Initialize "stand pat score", and return it immediately if it is // at least beta. bestValue = staticValue; @@ -1701,7 +1821,7 @@ namespace { if (bestValue >= beta) { // Store the score to avoid a future costly evaluation() call - if (!isCheck && !tte && ei.futilityMargin == 0) + if (!isCheck && !tte && ei.futilityMargin[pos.side_to_move()] == 0) TT.store(pos.get_key(), value_to_tt(bestValue, ply), VALUE_TYPE_EV_LO, Depth(-127*OnePly), MOVE_NONE); return bestValue; @@ -1710,13 +1830,17 @@ namespace { if (bestValue > alpha) alpha = bestValue; + // If we are near beta then try to get a cutoff pushing checks a bit further + bool deepChecks = depth == -OnePly && staticValue >= beta - PawnValueMidgame / 8; + // Initialize a MovePicker object for the current position, and prepare - // to search the moves. Because the depth is <= 0 here, only captures, - // queen promotions and checks (only if depth == 0) will be generated. - MovePicker mp = MovePicker(pos, ttMove, depth, H); + // to search the moves. Because the depth is <= 0 here, only captures, + // queen promotions and checks (only if depth == 0 or depth == -OnePly + // and we are near beta) will be generated. + MovePicker mp = MovePicker(pos, ttMove, deepChecks ? Depth(0) : depth, H); CheckInfo ci(pos); enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMidgame; - futilityBase = staticValue + FutilityMarginQS + ei.futilityMargin; + futilityBase = staticValue + FutilityMarginQS + ei.futilityMargin[pos.side_to_move()]; // Loop through the moves until no moves remain or a beta cutoff // occurs. @@ -1752,8 +1876,15 @@ namespace { } } - // Don't search captures and checks with negative SEE values - if ( !isCheck + // Detect blocking evasions that are candidate to be pruned + evasionPrunable = isCheck + && bestValue != -VALUE_INFINITE + && !pos.move_is_capture(move) + && pos.type_of_piece_on(move_from(move)) != KING + && !pos.can_castle(pos.side_to_move()); + + // Don't search moves with negative SEE values + if ( (!isCheck || evasionPrunable) && move != ttMove && !move_is_promotion(move) && pos.see_sign(move) < 0) @@ -1785,14 +1916,14 @@ namespace { // Update transposition table Depth d = (depth == Depth(0) ? Depth(0) : Depth(-1)); - if (bestValue < beta) + if (bestValue <= oldAlpha) { // If bestValue isn't changed it means it is still the static evaluation // of the node, so keep this info to avoid a future evaluation() call. - ValueType type = (bestValue == staticValue && !ei.futilityMargin ? VALUE_TYPE_EV_UP : VALUE_TYPE_UPPER); + ValueType type = (bestValue == staticValue && !ei.futilityMargin[pos.side_to_move()] ? VALUE_TYPE_EV_UP : VALUE_TYPE_UPPER); TT.store(pos.get_key(), value_to_tt(bestValue, ply), type, d, MOVE_NONE); } - else + else if (bestValue >= beta) { move = ss[ply].pv[ply]; TT.store(pos.get_key(), value_to_tt(bestValue, ply), VALUE_TYPE_LOWER, d, move); @@ -1801,6 +1932,8 @@ namespace { if (!pos.move_is_capture_or_promotion(move)) update_killers(move, ss[ply]); } + else + TT.store(pos.get_key(), value_to_tt(bestValue, ply), VALUE_TYPE_EXACT, d, ss[ply].pv[ply]); assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE); @@ -1821,34 +1954,38 @@ namespace { assert(threadID >= 0 && threadID < ActiveThreads); assert(ActiveThreads > 1); - Position pos = Position(sp->pos); + Position pos(*sp->pos); CheckInfo ci(pos); SearchStack* ss = sp->sstack[threadID]; - Value value; + Value value = -VALUE_INFINITE; Move move; + int moveCount; bool isCheck = pos.is_check(); bool useFutilityPruning = sp->depth < SelectiveDepth && !isCheck; const int FutilityMoveCountMargin = 3 + (1 << (3 * int(sp->depth) / 8)); - const int FutilityValueMargin = 112 * bitScanReverse32(int(sp->depth) * int(sp->depth) / 2); - while ( sp->bestValue < sp->beta + // Precalculate reduction parameters + float LogLimit, Gradient, BaseReduction = 0.5; + reduction_parameters(BaseReduction, 3.0, sp->depth, LogLimit, Gradient); + + while ( lock_grab_bool(&(sp->lock)) + && sp->bestValue < sp->beta && !thread_should_stop(threadID) - && (move = sp->mp->get_next_move(sp->lock)) != MOVE_NONE) + && (move = sp->mp->get_next_move()) != MOVE_NONE) { + moveCount = ++sp->moves; + lock_release(&(sp->lock)); + assert(move_is_ok(move)); bool moveIsCheck = pos.move_is_check(move, ci); bool captureOrPromotion = pos.move_is_capture_or_promotion(move); - lock_grab(&(sp->lock)); - int moveCount = ++sp->moves; - lock_release(&(sp->lock)); - ss[sp->ply].currentMove = move; - // Decide the new search depth. + // Decide the new search depth bool dangerous; Depth ext = extension(pos, move, false, captureOrPromotion, moveIsCheck, false, false, &dangerous); Depth newDepth = sp->depth - OnePly + ext; @@ -1865,12 +2002,6 @@ namespace { continue; // Value based pruning - if (sp->futilityValue == VALUE_NONE) - { - EvalInfo ei; - sp->futilityValue = evaluate(pos, ei, threadID) + FutilityValueMargin; - } - Value futilityValueScaled = sp->futilityValue - moveCount * IncrementalFutilityMargin; if (futilityValueScaled < sp->beta) @@ -1892,24 +2023,22 @@ namespace { // Try to reduce non-pv search depth by one ply if move seems not problematic, // if the move fails high will be re-searched at full depth. + bool doFullDepthSearch = true; + if ( !dangerous && !captureOrPromotion && !move_is_castle(move) && !move_is_killer(move, ss[sp->ply])) { - double red = 0.5 + ln(moveCount) * ln(sp->depth / 2) / 3.0; - if (red >= 1.0) + ss[sp->ply].reduction = reduction(moveCount, LogLimit, BaseReduction, Gradient); + if (ss[sp->ply].reduction) { - ss[sp->ply].reduction = Depth(int(floor(red * int(OnePly)))); value = -search(pos, ss, -(sp->beta-1), newDepth-ss[sp->ply].reduction, sp->ply+1, true, threadID); + doFullDepthSearch = (value >= sp->beta); } - else - value = sp->beta; // Just to trigger next condition } - else - value = sp->beta; // Just to trigger next condition - if (value >= sp->beta) // Go with full depth non-pv search + if (doFullDepthSearch) // Go with full depth non-pv search { ss[sp->ply].reduction = Depth(0); value = -search(pos, ss, -(sp->beta - 1), newDepth, sp->ply+1, true, threadID); @@ -1919,7 +2048,10 @@ namespace { assert(value > -VALUE_INFINITE && value < VALUE_INFINITE); if (thread_should_stop(threadID)) + { + lock_grab(&(sp->lock)); break; + } // New best move? if (value > sp->bestValue) // Less then 2% of cases @@ -1942,7 +2074,7 @@ namespace { } } - lock_grab(&(sp->lock)); + /* Here we have the lock still grabbed */ // If this is the master thread and we have been asked to stop because of // a beta cutoff higher up in the tree, stop all slave threads. @@ -1971,28 +2103,33 @@ namespace { assert(threadID >= 0 && threadID < ActiveThreads); assert(ActiveThreads > 1); - Position pos = Position(sp->pos); + Position pos(*sp->pos); CheckInfo ci(pos); SearchStack* ss = sp->sstack[threadID]; - Value value; + Value value = -VALUE_INFINITE; + int moveCount; Move move; - while ( sp->alpha < sp->beta + // Precalculate reduction parameters + float LogLimit, Gradient, BaseReduction = 0.5; + reduction_parameters(BaseReduction, 6.0, sp->depth, LogLimit, Gradient); + + while ( lock_grab_bool(&(sp->lock)) + && sp->alpha < sp->beta && !thread_should_stop(threadID) - && (move = sp->mp->get_next_move(sp->lock)) != MOVE_NONE) + && (move = sp->mp->get_next_move()) != MOVE_NONE) { - bool moveIsCheck = pos.move_is_check(move, ci); - bool captureOrPromotion = pos.move_is_capture_or_promotion(move); + moveCount = ++sp->moves; + lock_release(&(sp->lock)); assert(move_is_ok(move)); - lock_grab(&(sp->lock)); - int moveCount = ++sp->moves; - lock_release(&(sp->lock)); + bool moveIsCheck = pos.move_is_check(move, ci); + bool captureOrPromotion = pos.move_is_capture_or_promotion(move); ss[sp->ply].currentMove = move; - // Decide the new search depth. + // Decide the new search depth bool dangerous; Depth ext = extension(pos, move, true, captureOrPromotion, moveIsCheck, false, false, &dangerous); Depth newDepth = sp->depth - OnePly + ext; @@ -2003,39 +2140,46 @@ namespace { // Try to reduce non-pv search depth by one ply if move seems not problematic, // if the move fails high will be re-searched at full depth. + bool doFullDepthSearch = true; + if ( !dangerous && !captureOrPromotion && !move_is_castle(move) && !move_is_killer(move, ss[sp->ply])) { - double red = 0.5 + ln(moveCount) * ln(sp->depth / 2) / 6.0; - if (red >= 1.0) + ss[sp->ply].reduction = reduction(moveCount, LogLimit, BaseReduction, Gradient); + if (ss[sp->ply].reduction) { - ss[sp->ply].reduction = Depth(int(floor(red * int(OnePly)))); - value = -search(pos, ss, -sp->alpha, newDepth-ss[sp->ply].reduction, sp->ply+1, true, threadID); + Value localAlpha = sp->alpha; + value = -search(pos, ss, -localAlpha, newDepth-ss[sp->ply].reduction, sp->ply+1, true, threadID); + doFullDepthSearch = (value > localAlpha); } - else - value = sp->alpha + 1; // Just to trigger next condition } - else - value = sp->alpha + 1; // Just to trigger next condition - if (value > sp->alpha) // Go with full depth non-pv search + if (doFullDepthSearch) // Go with full depth non-pv search { + Value localAlpha = sp->alpha; ss[sp->ply].reduction = Depth(0); - value = -search(pos, ss, -sp->alpha, newDepth, sp->ply+1, true, threadID); + value = -search(pos, ss, -localAlpha, newDepth, sp->ply+1, true, threadID); - if (value > sp->alpha && value < sp->beta) + if (value > localAlpha && value < sp->beta) { // When the search fails high at ply 1 while searching the first - // move at the root, set the flag failHighPly1. This is used for + // move at the root, set the flag failHighPly1. This is used for // time managment: We don't want to stop the search early in // such cases, because resolving the fail high at ply 1 could // result in a big drop in score at the root. if (sp->ply == 1 && RootMoveNumber == 1) Threads[threadID].failHighPly1 = true; - value = -search_pv(pos, ss, -sp->beta, -sp->alpha, newDepth, sp->ply+1, threadID); + // If another thread has failed high then sp->alpha has been increased + // to be higher or equal then beta, if so, avoid to start a PV search. + localAlpha = sp->alpha; + if (localAlpha < sp->beta) + value = -search_pv(pos, ss, -sp->beta, -localAlpha, newDepth, sp->ply+1, threadID); + else + assert(thread_should_stop(threadID)); + Threads[threadID].failHighPly1 = false; } } @@ -2044,41 +2188,49 @@ namespace { assert(value > -VALUE_INFINITE && value < VALUE_INFINITE); if (thread_should_stop(threadID)) + { + lock_grab(&(sp->lock)); break; + } // New best move? - lock_grab(&(sp->lock)); - if (value > sp->bestValue && !thread_should_stop(threadID)) + if (value > sp->bestValue) // Less then 2% of cases { - sp->bestValue = value; - if (value > sp->alpha) + lock_grab(&(sp->lock)); + if (value > sp->bestValue && !thread_should_stop(threadID)) { - sp->alpha = value; - sp_update_pv(sp->parentSstack, ss, sp->ply); - if (value == value_mate_in(sp->ply + 1)) - ss[sp->ply].mateKiller = move; - - if (value >= sp->beta) + sp->bestValue = value; + if (value > sp->alpha) { - for (int i = 0; i < ActiveThreads; i++) - if (i != threadID && (i == sp->master || sp->slaves[i])) - Threads[i].stop = true; + // Ask threads to stop before to modify sp->alpha + if (value >= sp->beta) + { + for (int i = 0; i < ActiveThreads; i++) + if (i != threadID && (i == sp->master || sp->slaves[i])) + Threads[i].stop = true; - sp->finished = true; + sp->finished = true; + } + + sp->alpha = value; + + sp_update_pv(sp->parentSstack, ss, sp->ply); + if (value == value_mate_in(sp->ply + 1)) + ss[sp->ply].mateKiller = move; } - } - // If we are at ply 1, and we are searching the first root move at - // ply 0, set the 'Problem' variable if the score has dropped a lot - // (from the computer's point of view) since the previous iteration. - if ( sp->ply == 1 - && Iteration >= 2 - && -value <= IterationInfo[Iteration-1].value - ProblemMargin) - Problem = true; + // If we are at ply 1, and we are searching the first root move at + // ply 0, set the 'Problem' variable if the score has dropped a lot + // (from the computer's point of view) since the previous iteration. + if ( sp->ply == 1 + && Iteration >= 2 + && -value <= IterationInfo[Iteration-1].value - ProblemMargin) + Problem = true; + } + lock_release(&(sp->lock)); } - lock_release(&(sp->lock)); } - lock_grab(&(sp->lock)); + /* Here we have the lock still grabbed */ // If this is the master thread and we have been asked to stop because of // a beta cutoff higher up in the tree, stop all slave threads. @@ -2126,7 +2278,9 @@ namespace { RootMoveList::RootMoveList(Position& pos, Move searchMoves[]) : count(0) { + SearchStack ss[PLY_MAX_PLUS_2]; MoveStack mlist[MaxRootMoves]; + StateInfo st; bool includeAllMoves = (searchMoves[0] == MOVE_NONE); // Generate all legal moves @@ -2144,16 +2298,13 @@ namespace { continue; // Find a quick score for the move - StateInfo st; - SearchStack ss[PLY_MAX_PLUS_2]; init_ss_array(ss); - + pos.do_move(cur->move, st); moves[count].move = cur->move; - pos.do_move(moves[count].move, st); moves[count].score = -qsearch(pos, ss, -VALUE_INFINITE, VALUE_INFINITE, Depth(0), 1, 0); - pos.undo_move(moves[count].move); - moves[count].pv[0] = moves[count].move; + moves[count].pv[0] = cur->move; moves[count].pv[1] = MOVE_NONE; + pos.undo_move(cur->move); count++; } sort(); @@ -2457,9 +2608,8 @@ namespace { Square mfrom, mto, tfrom, tto; - // Prune if there isn't any threat move and - // is not a castling move (common case). - if (threat == MOVE_NONE && !move_is_castle(m)) + // Prune if there isn't any threat move + if (threat == MOVE_NONE) return true; mfrom = move_from(m); @@ -2467,15 +2617,11 @@ namespace { tfrom = move_from(threat); tto = move_to(threat); - // Case 1: Castling moves are never pruned - if (move_is_castle(m)) - return false; - - // Case 2: Don't prune moves which move the threatened piece + // Case 1: Don't prune moves which move the threatened piece if (mfrom == tto) return false; - // Case 3: If the threatened piece has value less than or equal to the + // Case 2: If the threatened piece has value less than or equal to the // value of the threatening piece, don't prune move which defend it. if ( pos.move_is_capture(threat) && ( pos.midgame_value_of_piece_on(tfrom) >= pos.midgame_value_of_piece_on(tto) @@ -2483,7 +2629,7 @@ namespace { && pos.move_attacks_square(m, tto)) return false; - // Case 4: If the moving piece in the threatened move is a slider, don't + // Case 3: If the moving piece in the threatened move is a slider, don't // prune safe moves which block its ray. if ( piece_is_slider(pos.piece_on(tfrom)) && bit_is_set(squares_between(tfrom, tto), mto) @@ -2527,6 +2673,35 @@ namespace { return defaultEval; } + + // reduction_parameters() precalculates some parameters used later by reduction. Becasue + // floating point operations are involved we try to recalculate reduction at each move, but + // we do the most consuming computation only once per node. + + void reduction_parameters(float baseReduction, float reductionInhibitor, Depth depth, float& logLimit, float& gradient) + { + // Precalculate some parameters to avoid to calculate the following formula for each move: + // + // red = baseReduction + ln(moveCount) * ln(depth / 2) / reductionInhibitor; + // + logLimit = depth > OnePly ? (1 - baseReduction) * reductionInhibitor / ln(depth / 2) : 1000; + gradient = depth > OnePly ? ln(depth / 2) / reductionInhibitor : 0; + } + + + // reduction() returns reduction in plies based on moveCount and depth. + // Reduction is always at least one ply. + + Depth reduction(int moveCount, float logLimit, float baseReduction, float gradient) { + + if (ln(moveCount) < logLimit) + return Depth(0); + + float red = baseReduction + ln(moveCount) * gradient; + return Depth(int(floor(red * int(OnePly)))); + } + + // update_history() registers a good move that produced a beta-cutoff // in history and marks as failures all the other moves of that ply. @@ -2564,6 +2739,21 @@ namespace { } + // update_gains() updates the gains table of a non-capture move given + // the static position evaluation before and after the move. + + void update_gains(const Position& pos, Move m, Value before, Value after) { + + if ( m != MOVE_NULL + && before != VALUE_NONE + && after != VALUE_NONE + && pos.captured_piece() == NO_PIECE_TYPE + && !move_is_castle(m) + && !move_is_promotion(m)) + H.set_gain(pos.piece_on(move_to(m)), move_from(m), move_to(m), -(before + after)); + } + + // fail_high_ply_1() checks if some thread is currently resolving a fail // high at ply 1 at the node below the first root node. This information // is used for time management. @@ -2809,6 +2999,8 @@ namespace { // If this thread has been assigned work, launch a search if (Threads[threadID].workIsWaiting) { + assert(!Threads[threadID].idle); + Threads[threadID].workIsWaiting = false; if (Threads[threadID].splitPoint->pvNode) sp_search_pv(Threads[threadID].splitPoint, threadID); @@ -2895,7 +3087,10 @@ namespace { if (!Threads[slave].idle || slave == master) return false; - if (Threads[slave].activeSplitPoints == 0) + // Make a local copy to be sure doesn't change under our feet + int localActiveSplitPoints = Threads[slave].activeSplitPoints; + + if (localActiveSplitPoints == 0) // No active split points means that the thread is available as // a slave for any other thread. return true; @@ -2903,8 +3098,10 @@ namespace { if (ActiveThreads == 2) return true; - // Apply the "helpful master" concept if possible - if (SplitPointStack[slave][Threads[slave].activeSplitPoints - 1].slaves[master]) + // Apply the "helpful master" concept if possible. Use localActiveSplitPoints + // that is known to be > 0, instead of Threads[slave].activeSplitPoints that + // could have been set to 0 by another thread leading to an out of bound access. + if (SplitPointStack[slave][localActiveSplitPoints - 1].slaves[master]) return true; return false; @@ -2954,7 +3151,6 @@ namespace { assert(ActiveThreads > 1); SplitPoint* splitPoint; - int i; lock_grab(&MPLock); @@ -2971,7 +3167,7 @@ namespace { splitPoint = SplitPointStack[master] + Threads[master].activeSplitPoints; Threads[master].activeSplitPoints++; - // Initialize the split point object and copy current position + // Initialize the split point object splitPoint->parent = Threads[master].splitPoint; splitPoint->finished = false; splitPoint->ply = ply; @@ -2985,37 +3181,40 @@ namespace { splitPoint->mp = mp; splitPoint->moves = *moves; splitPoint->cpus = 1; - splitPoint->pos.copy(p); + splitPoint->pos = &p; splitPoint->parentSstack = sstck; - for (i = 0; i < ActiveThreads; i++) + for (int i = 0; i < ActiveThreads; i++) splitPoint->slaves[i] = 0; - // Copy the current search stack to the master thread - memcpy(splitPoint->sstack[master], sstck, (ply+1) * sizeof(SearchStack)); + Threads[master].idle = false; + Threads[master].stop = false; Threads[master].splitPoint = splitPoint; - // Make copies of the current position and search stack for each thread - for (i = 0; i < ActiveThreads && splitPoint->cpus < MaxThreadsPerSplitPoint; i++) + // Allocate available threads setting idle flag to false + for (int i = 0; i < ActiveThreads && splitPoint->cpus < MaxThreadsPerSplitPoint; i++) if (thread_is_available(i, master)) { - memcpy(splitPoint->sstack[i], sstck, (ply+1) * sizeof(SearchStack)); + Threads[i].idle = false; + Threads[i].stop = false; Threads[i].splitPoint = splitPoint; splitPoint->slaves[i] = 1; splitPoint->cpus++; } + assert(splitPoint->cpus > 1); + + // We can release the lock because master and slave threads are already booked + lock_release(&MPLock); + // Tell the threads that they have work to do. This will make them leave - // their idle loop. - for (i = 0; i < ActiveThreads; i++) + // their idle loop. But before copy search stack tail for each thread. + for (int i = 0; i < ActiveThreads; i++) if (i == master || splitPoint->slaves[i]) { - Threads[i].workIsWaiting = true; - Threads[i].idle = false; - Threads[i].stop = false; + memcpy(splitPoint->sstack[i] + ply - 1, sstck + ply - 1, 3 * sizeof(SearchStack)); + Threads[i].workIsWaiting = true; // This makes the slave to exit from idle_loop() } - lock_release(&MPLock); - // Everything is set up. The master thread enters the idle loop, from // which it will instantly launch a search, because its workIsWaiting // slot is 'true'. We send the split point as a second parameter to the