From 2a461b4b745b2542f6e13bab8c60abdb366bc128 Mon Sep 17 00:00:00 2001
From: Marco Costalba <mcostalba@gmail.com>
Date: Sat, 11 Jul 2009 09:54:30 +0100
Subject: [PATCH] Introduce see_sign() and use it to shortcut full see()

Mostly of times we are interested only in the sign of SEE,
namely if a capture is negative or not.

If the capturing piece is smaller then the captured one we
already know SEE cannot be negative and this information
is enough most of the times. And of course it is much
faster to detect then a full SEE.

Note that in case see_sign() is negative then the returned
value is exactly the see() value, this is very important,
especially for ordering capturing moves.

With this patch the calls to the costly see() are reduced
of almost 30%.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
---
 src/movepick.cpp |  2 +-
 src/position.cpp | 16 ++++++++++++++++
 src/position.h   |  1 +
 src/search.cpp   |  8 +++-----
 4 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/src/movepick.cpp b/src/movepick.cpp
index 22845ba2..25cbdefd 100644
--- a/src/movepick.cpp
+++ b/src/movepick.cpp
@@ -244,7 +244,7 @@ void MovePicker::score_captures() {
   for (int i = 0; i < numOfMoves; i++)
   {
       m = moves[i].move;
-      seeValue = pos.see(m);
+      seeValue = pos.see_sign(m);
       if (seeValue >= 0)
       {
           if (move_is_promotion(m))
diff --git a/src/position.cpp b/src/position.cpp
index 1179f6f9..8099764c 100644
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -1474,6 +1474,22 @@ int Position::see(Move m) const {
   return see(move_from(m), move_to(m));
 }
 
+int Position::see_sign(Move m) const {
+
+  assert(move_is_ok(m));
+
+  Square from = move_from(m);
+  Square to = move_to(m);
+
+  // Early return if SEE cannot be negative because capturing piece value
+  // is not bigger then captured one.
+  if (   midgame_value_of_piece_on(from) <= midgame_value_of_piece_on(to)
+      && type_of_piece_on(from) != KING)
+         return 1;
+
+  return see(from, to);
+}
+
 int Position::see(Square from, Square to) const {
 
   // Material values
diff --git a/src/position.h b/src/position.h
index 5fe1a8fd..97166360 100644
--- a/src/position.h
+++ b/src/position.h
@@ -267,6 +267,7 @@ public:
   int see(Square from, Square to) const;
   int see(Move m) const;
   int see(Square to) const;
+  int see_sign(Move m) const;
 
   // Accessing hash keys
   Key get_key() const;
diff --git a/src/search.cpp b/src/search.cpp
index 7e2f9727..e6495028 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1561,9 +1561,7 @@ namespace {
       // Don't search captures and checks with negative SEE values
       if (   !isCheck
           && !move_is_promotion(move)
-          && (pos.midgame_value_of_piece_on(move_from(move)) >
-              pos.midgame_value_of_piece_on(move_to(move)))
-          &&  pos.see(move) < 0)
+          &&  pos.see_sign(move) < 0)
           continue;
 
       // Make and search the move.
@@ -2240,7 +2238,7 @@ namespace {
     if (   pvNode
         && capture
         && pos.type_of_piece_on(move_to(m)) != PAWN
-        && pos.see(m) >= 0)
+        && pos.see_sign(m) >= 0)
     {
         result += OnePly/2;
         *dangerous = true;
@@ -2313,7 +2311,7 @@ namespace {
         && threat != MOVE_NONE
         && piece_is_slider(pos.piece_on(tfrom))
         && bit_is_set(squares_between(tfrom, tto), mto)
-        && pos.see(m) >= 0)
+        && pos.see_sign(m) >= 0)
         return false;
 
     return true;
-- 
2.39.2