From ae4967744686ae6e95ca0cdb9e0d788de05d4e03 Mon Sep 17 00:00:00 2001
From: Tord Romstad <tord@glaurungchess.com>
Date: Thu, 6 Aug 2009 18:07:32 +0200
Subject: [PATCH] Fixed a bug in PV extraction from the transposition table:
 The previous used move_is_legal to verify that the move from the TT was
 legal, and the old version of move_is_legal only works when the side to move
 is not in check. Fixed this by adding a separate, slower version of
 move_is_legal which works even when the side to move is in check.

---
 src/movegen.cpp | 23 +++++++++++++++++++++++
 src/movegen.h   |  1 +
 src/tt.cpp      |  2 +-
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/src/movegen.cpp b/src/movegen.cpp
index 079918e9..eb65e7b9 100644
--- a/src/movegen.cpp
+++ b/src/movegen.cpp
@@ -567,6 +567,29 @@ bool move_is_legal(const Position& pos, const Move m, Bitboard pinned) {
 }
 
 
+/// Another version of move_is_legal(), which takes only a position and a move
+/// as input. This function does not require that the side to move is not in
+/// check. It is not optimized for speed, and is only used for verifying move
+/// legality when building a PV from the transposition table.
+
+bool move_is_legal(const Position& pos, const Move m) {
+
+  Bitboard pinned = pos.pinned_pieces(pos.side_to_move());
+  if (!pos.is_check())
+      return move_is_legal(pos, m, pinned);
+  else
+  {
+      Position p(pos);
+      MoveStack moves[64];
+      int n = generate_evasions(p, moves, pinned);
+      for (int i = 0; i < n; i++)
+          if (moves[i].move == m)
+              return true;
+      return false;
+  }
+}
+
+
 namespace {
 
   template<PieceType Piece>
diff --git a/src/movegen.h b/src/movegen.h
index e16a8ab5..991b0de3 100644
--- a/src/movegen.h
+++ b/src/movegen.h
@@ -38,6 +38,7 @@ extern int generate_non_capture_checks(const Position& pos, MoveStack* mlist, Bi
 extern int generate_evasions(const Position& pos, MoveStack* mlist, Bitboard pinned);
 extern int generate_legal_moves(const Position& pos, MoveStack* mlist);
 extern bool move_is_legal(const Position& pos, const Move m, Bitboard pinned);
+extern bool move_is_legal(const Position& pos, const Move m);
 
 
 #endif // !defined(MOVEGEN_H_INCLUDED)
diff --git a/src/tt.cpp b/src/tt.cpp
index c1a8b1b9..1e98aa24 100644
--- a/src/tt.cpp
+++ b/src/tt.cpp
@@ -207,7 +207,7 @@ void TranspositionTable::extract_pv(const Position& pos, Move pv[]) {
        tte && tte->move() != MOVE_NONE && !stop;
        tte = retrieve(p.get_key()), ply++)
   {
-      if (!move_is_legal(p, tte->move(), p.pinned_pieces(p.side_to_move())))
+      if (!move_is_legal(p, tte->move()))
           break;
       pv[ply] = tte->move();
       p.do_move(pv[ply], st[ply]);
-- 
2.39.2