Document asymmetric SEE pruning trick

[stockfish] / src / position.cpp

diff --git a/src/position.cpp b/src/position.cpp
index 310e46e36190704121571efff452399bf53911a9..1b2defa54662a34718bb46c79e40ae9abffbe595 100644 (file)
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -1,7 +1,7 @@
 /*
   Stockfish, a UCI chess playing engine derived from Glaurung 2.1
   Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
-  Copyright (C) 2008-2012 Marco Costalba, Joona Kiiski, Tord Romstad
+  Copyright (C) 2008-2013 Marco Costalba, Joona Kiiski, Tord Romstad
 
   Stockfish is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
@@ -19,6 +19,7 @@
 
 #include <cassert>
 #include <cstring>
+#include <iomanip>
 #include <iostream>
 #include <sstream>
 #include <algorithm>
@@ -278,11 +279,11 @@ void Position::set(const string& fenStr, bool isChess960, Thread* th) {
   }
 
   // 5-6. Halfmove clock and fullmove number
-  ss >> std::skipws >> st->rule50 >> startPosPly;
+  ss >> std::skipws >> st->rule50 >> gamePly;
 
   // Convert from fullmove starting from 1 to ply starting from 0,
   // handle also common incorrect FEN with fullmove = 0.
-  startPosPly = std::max(2 * (startPosPly - 1), 0) + int(sideToMove == BLACK);
+  gamePly = std::max(2 * (gamePly - 1), 0) + int(sideToMove == BLACK);
 
   st->key = compute_key();
   st->pawnKey = compute_pawn_key();
@@ -373,7 +374,7 @@ const string Position::fen() const {
       ss << '-';
 
   ss << (ep_square() == SQ_NONE ? " - " : " " + square_to_string(ep_square()) + " ")
-      << st->rule50 << " " << 1 + (startPosPly - int(sideToMove == BLACK)) / 2;
+      << st->rule50 << " " << 1 + (gamePly - int(sideToMove == BLACK)) / 2;
 
   return ss.str();
 }
@@ -400,7 +401,8 @@ const string Position::pretty(Move move) const {
       if (piece_on(sq) != NO_PIECE)
           brd[513 - 68*rank_of(sq) + 4*file_of(sq)] = PieceToChar[piece_on(sq)];
 
-  ss << brd << "\nFen: " << fen() << "\nKey: " << st->key << "\nCheckers: ";
+  ss << brd << "\nFen: " << fen() << "\nKey: " << std::hex << std::uppercase
+     << std::setfill('0') << std::setw(16) << st->key << "\nCheckers: ";
 
   for (Bitboard b = checkers(); b; )
       ss << square_to_string(pop_lsb(&b)) << " ";
@@ -735,8 +737,9 @@ void Position::do_move(Move m, StateInfo& newSt, const CheckInfo& ci, bool moveI
   // Update side to move
   k ^= Zobrist::side;
 
-  // Increment the 50 moves rule draw counter. Resetting it to zero in the
-  // case of a capture or a pawn move is taken care of later.
+  // Increment ply counters.In particular rule50 will be later reset it to zero
+  // in case of a capture or a pawn move.
+  gamePly++;
   st->rule50++;
   st->pliesFromNull++;
 
@@ -802,7 +805,7 @@ void Position::do_move(Move m, StateInfo& newSt, const CheckInfo& ci, bool moveI
       // Update piece list, move the last piece at index[capsq] position and
       // shrink the list.
       //
-      // WARNING: This is a not revresible operation. When we will reinsert the
+      // WARNING: This is a not reversible operation. When we will reinsert the
       // captured piece in undo_move() we will put it at the end of the list and
       // not in its original place, it means index[] and pieceList[] are not
       // guaranteed to be invariant to a do_move() + undo_move() sequence.
@@ -1054,6 +1057,7 @@ void Position::undo_move(Move m) {
 
   // Finally point our state pointer back to the previous state
   st = st->previous;
+  gamePly--;
 
   assert(pos_is_ok());
 }
@@ -1144,6 +1148,21 @@ int Position::see_sign(Move m) const {
 }
 
 int Position::see(Move m) const {
+  return do_see<false>(m, 0);
+}
+
+/// Position::see_asymm() takes tempi into account.
+/// If the side who initiated the capturing sequence does the last capture,
+/// he loses a tempo. In this case if the result is below asymmThreshold 
+/// the capturing sequence is considered bad.
+
+int Position::see_asymm(Move m, int asymmThreshold) const
+{
+  return do_see<true>(m, asymmThreshold);
+}
+
+template <bool Asymmetric>
+int Position::do_see(Move m, int asymmThreshold) const {
 
   Square from, to;
   Bitboard occupied, attackers, stmAttackers;
@@ -1220,6 +1239,18 @@ int Position::see(Move m) const {
 
   } while (stmAttackers);
 
+  // If we are doing asymmetric SEE evaluation and the same side does the first
+  // and the last capture, he loses a tempo and gain must be at least worth "asymmThreshold".
+  // If not, we replace the score with a very low value, before negamaxing.
+  if (Asymmetric)
+  {
+      for (int i = 0; i < slIndex ; i += 2)
+      {
+          if (swapList[i] < asymmThreshold)
+               swapList[i] = - QueenValueMg * 16;
+      }
+  }
+
   // Having built the swap list, we negamax through it to find the best
   // achievable score from the point of view of the side to move.
   while (--slIndex)
@@ -1417,7 +1448,7 @@ void Position::flip() {
   thisThread = pos.this_thread();
   nodes = pos.nodes_searched();
   chess960 = pos.is_chess960();
-  startPosPly = pos.startpos_ply_counter();
+  gamePly = pos.game_ply();
 
   for (Square s = SQ_A1; s <= SQ_H8; s++)
       if (!pos.is_empty(s))