From: Marco Costalba <mcostalba@gmail.com>
Date: Sat, 20 Jul 2013 11:03:45 +0000 (+0200)
Subject: Fix critical SEE bug
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=a6c5b4c6fbd1db45377f9dfecd361cebffe27a8d

Fix critical SEE bug

It is somewhat unbilievable but our SEE is broken !

If the first SEE move is a king capture and square is
defended then SEE continues instead of breaking.

The bug shows only on normal SEE, not see_sign() so
probing with a:

dbg_hit_on_c(slIndex==1, captured == KING);

reports just a tiny:

Total 3465656 Hits 6646 hit rate (%) 0

Bug was there since 351ef5c85b6d4b9c71e9da of 26/6/2011 (!)
although for some reason didn't show immediately, indeed the
bougous patch was a "No functional change" (!!)

bench: 4793754
---

diff --git a/src/position.cpp b/src/position.cpp
index 0c817631..3526a3d0 100644
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -1136,7 +1136,7 @@ int Position::see_sign(Move m) const {
   // Early return if SEE cannot be negative because captured piece value
   // is not less then capturing one. Note that king moves always return
   // here because king midgame value is set to 0.
-  if (PieceValue[MG][piece_on(to_sq(m))] >= PieceValue[MG][piece_moved(m)])
+  if (PieceValue[MG][piece_moved(m)] <= PieceValue[MG][piece_on(to_sq(m))])
       return 1;
 
   return see(m);
@@ -1197,6 +1197,12 @@ int Position::see(Move m, int asymmThreshold) const {
   do {
       assert(slIndex < 32);
 
+      if (captured == KING) // Stop before processing a king capture
+      {
+          swapList[slIndex++] = QueenValueMg * 16;
+          break;
+      }
+
       // Add the new entry to the swap list
       swapList[slIndex] = -swapList[slIndex - 1] + PieceValue[MG][captured];
       slIndex++;
@@ -1206,15 +1212,6 @@ int Position::see(Move m, int asymmThreshold) const {
       stm = ~stm;
       stmAttackers = attackers & pieces(stm);
 
-      if (captured == KING)
-      {
-          // Stop before processing a king capture
-          if (stmAttackers)
-              swapList[slIndex++] = QueenValueMg * 16;
-
-          break;
-      }
-
   } while (stmAttackers);
 
   // If we are doing asymmetric SEE evaluation and the same side does the first