diff --git a/src/search.cpp b/src/search.cpp index 5445256f..7dc67488 100644 --- a/src/search.cpp +++ b/src/search.cpp @@ -85,6 +85,13 @@ namespace { return d > 17 ? 0 : 29 * d * d + 138 * d - 134; } + // Add a small random component to draw evaluations to keep search dynamic + // and to avoid 3fold-blindness. + Value value_draw(Depth depth, Thread* thisThread) { + return depth < 4 ? VALUE_DRAW + : VALUE_DRAW + Value(2 * (thisThread->nodes.load(std::memory_order_relaxed) % 2) - 1); + } + // Skill structure is used to implement strength limit struct Skill { explicit Skill(int l) : level(l) {} @@ -535,7 +542,7 @@ namespace { && !rootNode && pos.has_game_cycle(ss->ply)) { - alpha = VALUE_DRAW; + alpha = value_draw(depth, pos.this_thread()); if (alpha >= beta) return alpha; } @@ -584,7 +591,8 @@ namespace { if ( Threads.stop.load(std::memory_order_relaxed) || pos.is_draw(ss->ply) || ss->ply >= MAX_PLY) - return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW; + return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) + : value_draw(depth, pos.this_thread()); // Step 3. Mate distance pruning. Even if we mate at the next move our score // would be at best mate_in(ss->ply+1), but if alpha is already bigger because