From d892063cd34f0e1ca9f36f0514ebe87767120356 Mon Sep 17 00:00:00 2001
From: Marco Costalba <mcostalba@gmail.com>
Date: Fri, 9 Oct 2009 04:35:11 +0100
Subject: [PATCH] Rewrite previous patch using only one counter

Use only rule50 and retire pliesFromNull.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
---
 src/position.cpp | 13 +++++--------
 src/position.h   |  2 +-
 2 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/src/position.cpp b/src/position.cpp
index 4e9a6dc7..e3062918 100644
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -703,7 +703,7 @@ void Position::do_move(Move m, StateInfo& newSt, Bitboard dcCandidates) {
   // pointer to point to the new, ready to be updated, state.
   struct ReducedStateInfo {
     Key key, pawnKey, materialKey;
-    int castleRights, rule50, pliesFromNull;
+    int castleRights, rule50;
     Square epSquare;
     Value mgValue, egValue;
     Value npMaterial[2];
@@ -724,7 +724,6 @@ void Position::do_move(Move m, StateInfo& newSt, Bitboard dcCandidates) {
   // Increment the 50 moves rule draw counter. Resetting it to zero in the
   // case of non-reversible moves is taken care of later.
   st->rule50++;
-  st->pliesFromNull++;
 
   if (move_is_castle(m))
   {
@@ -1239,11 +1238,11 @@ void Position::do_null_move(StateInfo& backupSt) {
   // Note that differently from normal case here backupSt is actually used as
   // a backup storage not as a new state to be used.
   backupSt.key      = st->key;
+  backupSt.rule50   = st->rule50;
   backupSt.epSquare = st->epSquare;
   backupSt.mgValue  = st->mgValue;
   backupSt.egValue  = st->egValue;
   backupSt.previous = st->previous;
-  backupSt.pliesFromNull = st->pliesFromNull;
   st->previous = &backupSt;
 
   // Save the current key to the history[] array, in order to be able to
@@ -1259,8 +1258,7 @@ void Position::do_null_move(StateInfo& backupSt) {
 
   sideToMove = opposite_color(sideToMove);
   st->epSquare = SQ_NONE;
-  st->rule50++;
-  st->pliesFromNull = 0;
+  st->rule50 = 0;
   gamePly++;
 
   st->mgValue += (sideToMove == WHITE)? TempoValueMidgame : -TempoValueMidgame;
@@ -1278,15 +1276,14 @@ void Position::undo_null_move() {
   // Restore information from the our backup StateInfo object
   StateInfo* backupSt = st->previous;
   st->key      = backupSt->key;
+  st->rule50   = backupSt->rule50;
   st->epSquare = backupSt->epSquare;
   st->mgValue  = backupSt->mgValue;
   st->egValue  = backupSt->egValue;
   st->previous = backupSt->previous;
-  st->pliesFromNull = backupSt->pliesFromNull;
 
   // Update the necessary information
   sideToMove = opposite_color(sideToMove);
-  st->rule50--;
   gamePly--;
 }
 
@@ -1687,7 +1684,7 @@ bool Position::is_draw() const {
       return true;
 
   // Draw by repetition?
-  for (int i = 2; i < Min(Min(gamePly, st->rule50), st->pliesFromNull); i += 2)
+  for (int i = 2; i < Min(gamePly, st->rule50); i += 2)
       if (history[gamePly - i] == st->key)
           return true;
 
diff --git a/src/position.h b/src/position.h
index 606c7486..370bb1d4 100644
--- a/src/position.h
+++ b/src/position.h
@@ -88,7 +88,7 @@ enum Phase {
 
 struct StateInfo {
   Key key, pawnKey, materialKey;
-  int castleRights, rule50, pliesFromNull;
+  int castleRights, rule50;
   Square epSquare;
   Value mgValue, egValue;
   Value npMaterial[2];
-- 
2.39.2