From 644db060aea9512830b8573c180296390a35927a Mon Sep 17 00:00:00 2001
From: Marco Costalba <mcostalba@gmail.com>
Date: Wed, 15 Oct 2008 07:18:05 +0100
Subject: [PATCH] Add psqt ordering when there is no history

This seems to increase strenght (about 15 ELO),
still to test some variations on this theme that
could increase ELO even more.

Idea from Rebel (http://members.home.nl/matador/chess840.htm)

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
---
 src/movepick.cpp | 37 +++++++++++++++++++++++++++++--------
 src/position.h   |  6 ++++++
 2 files changed, 35 insertions(+), 8 deletions(-)

diff --git a/src/movepick.cpp b/src/movepick.cpp
index 4d2d3116..d0ceaf1c 100644
--- a/src/movepick.cpp
+++ b/src/movepick.cpp
@@ -246,15 +246,36 @@ void MovePicker::score_captures() {
 }
 
 void MovePicker::score_noncaptures() {
-  for(int i = 0; i < numOfMoves; i++) {
-    Move m = moves[i].move;
-    if(m == killer1)
-      moves[i].score = HistoryMax + 2;
-    else if(m == killer2)
-      moves[i].score = HistoryMax + 1;
-    else
-      moves[i].score = H.move_ordering_score(pos->piece_on(move_from(m)), m);
+
+  bool all_zero = true;
+  for (int i = 0; i < numOfMoves; i++)
+  {
+      Move m = moves[i].move;
+      if (m == killer1)
+      {
+          moves[i].score = HistoryMax + 2;
+          all_zero = false;
+      }
+      else if (m == killer2)
+      {
+          moves[i].score = HistoryMax + 1;
+          all_zero = false;
+      }
+      else
+      {
+          moves[i].score = H.move_ordering_score(pos->piece_on(move_from(m)), m);
+          if (all_zero && moves[i].score != 0)
+              all_zero = false;
+      }
   }
+  if (!all_zero)
+      return;
+
+  // If we don't have at least one history score then
+  // try to order using psq tables difference between 
+  // from square and to square.
+  for (int i = 0; i < numOfMoves; i++)
+      moves[i].score = pos->mg_pst_delta(moves[i].move);
 }
 
 void MovePicker::score_evasions() {
diff --git a/src/position.h b/src/position.h
index 9acb2966..25596bf6 100644
--- a/src/position.h
+++ b/src/position.h
@@ -278,6 +278,7 @@ public:
   Value eg_value() const;
   Value non_pawn_material(Color c) const;
   Phase game_phase() const;
+  Value mg_pst_delta(Move m) const;
 
   // Game termination checks
   bool is_mate();
@@ -681,6 +682,11 @@ inline Value Position::mg_pst(Color c, PieceType pt, Square s) const {
   return MgPieceSquareTable[piece_of_color_and_type(c, pt)][s];
 }
 
+inline Value Position::mg_pst_delta(Move m) const {
+  return MgPieceSquareTable[piece_on(move_from(m))][move_to(m)]
+        -MgPieceSquareTable[piece_on(move_from(m))][move_from(m)];
+}
+
 inline Value Position::eg_pst(Color c, PieceType pt, Square s) const {
   return EgPieceSquareTable[piece_of_color_and_type(c, pt)][s];
 }
-- 
2.39.2