From 57bb4d1aec903b5336aa06d1b0de4d016a403043 Mon Sep 17 00:00:00 2001
From: Peter Elliott <pelliott@ualberta.ca>
Date: Sat, 19 Jun 2021 14:49:53 -0600
Subject: [PATCH] LibChess: Only save hash of board state for repetition
 checking

This more than doubles the number of nodes that MCTS can search for a
given time limit, and greatly reduces memory usage.
---
 Userland/Libraries/LibChess/Chess.cpp | 12 +++++-------
 Userland/Libraries/LibChess/Chess.h   |  7 ++++---
 2 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/Userland/Libraries/LibChess/Chess.cpp b/Userland/Libraries/LibChess/Chess.cpp
index 981db187159..d173b78c830 100644
--- a/Userland/Libraries/LibChess/Chess.cpp
+++ b/Userland/Libraries/LibChess/Chess.cpp
@@ -579,14 +579,12 @@ bool Board::apply_move(const Move& move, Color color)
 
 bool Board::apply_illegal_move(const Move& move, Color color)
 {
-    Board clone = *this;
-    clone.m_previous_states = {};
-    clone.m_moves = {};
+    auto state = Traits<Board>::hash(*this);
     auto state_count = 0;
-    if (m_previous_states.contains(clone))
-        state_count = m_previous_states.get(clone).value();
+    if (m_previous_states.contains(state))
+        state_count = m_previous_states.get(state).value();
 
-    m_previous_states.set(clone, state_count + 1);
+    m_previous_states.set(state, state_count + 1);
     m_moves.append(move);
 
     m_turn = opposing_color(color);
@@ -758,7 +756,7 @@ Board::Result Board::game_result() const
         if (m_moves_since_capture == 50 * 2)
             return Result::FiftyMoveRule;
 
-        auto repeats = m_previous_states.get(*this);
+        auto repeats = m_previous_states.get(Traits<Board>::hash(*this));
         if (repeats.has_value()) {
             if (repeats.value() == 3)
                 return Result::ThreeFoldRepetition;
diff --git a/Userland/Libraries/LibChess/Chess.h b/Userland/Libraries/LibChess/Chess.h
index ac227ea2878..9ad01408aee 100644
--- a/Userland/Libraries/LibChess/Chess.h
+++ b/Userland/Libraries/LibChess/Chess.h
@@ -172,7 +172,8 @@ private:
     bool m_black_can_castle_kingside { true };
     bool m_black_can_castle_queenside { true };
 
-    HashMap<Board, int> m_previous_states;
+    // We trust that hash collisions will not happen to save lots of memory and time.
+    HashMap<unsigned, int> m_previous_states;
     Vector<Move> m_moves;
     friend struct Traits<Board>;
 };
@@ -276,7 +277,7 @@ void Board::generate_moves(Callback callback, Color color) const
 
 template<>
 struct AK::Traits<Chess::Piece> : public GenericTraits<Chess::Piece> {
-    static unsigned hash(Chess::Piece piece)
+    static unsigned hash(const Chess::Piece& piece)
     {
         return pair_int_hash(static_cast<u32>(piece.color), static_cast<u32>(piece.type));
     }
@@ -284,7 +285,7 @@ struct AK::Traits<Chess::Piece> : public GenericTraits<Chess::Piece> {
 
 template<>
 struct AK::Traits<Chess::Board> : public GenericTraits<Chess::Board> {
-    static unsigned hash(Chess::Board chess)
+    static unsigned hash(const Chess::Board chess)
     {
         unsigned hash = 0;
         hash = pair_int_hash(hash, static_cast<u32>(chess.m_white_can_castle_queenside));