Do not claim repetition after null move

Null moves can artificially create a repetition
draw where instead there is no one.

So use a second counter to reset history after
a null move.

Idea from Joona.

After 999 games at 1+0

Mod vs Orig +238 =553 -208 51.50%  514.5/999  +10 ELO

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

diff --git a/src/position.cpp b/src/position.cpp
index e0e9aa3a9b37b316dc1b55f02b00755a3305c42b..4e9a6dc7b02388ddae1c8224a9bfd8eeedea2368 100644 (file)
--- 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;
   // pointer to point to the new, ready to be updated, state.
   struct ReducedStateInfo {
     Key key, pawnKey, materialKey;

-    int castleRights, rule50;
+    int castleRights, rule50, pliesFromNull;

     Square epSquare;
     Value mgValue, egValue;
     Value npMaterial[2];
     Square epSquare;
     Value mgValue, egValue;
     Value npMaterial[2];


@@ -724,6 +724,7 @@ 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++;
   // 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))
   {
 
   if (move_is_castle(m))
   {


@@ -1242,6 +1243,7 @@ void Position::do_null_move(StateInfo& backupSt) {
   backupSt.mgValue  = st->mgValue;
   backupSt.egValue  = st->egValue;
   backupSt.previous = st->previous;
   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
   st->previous = &backupSt;
 
   // Save the current key to the history[] array, in order to be able to


@@ -1258,6 +1260,7 @@ void Position::do_null_move(StateInfo& backupSt) {
   sideToMove = opposite_color(sideToMove);
   st->epSquare = SQ_NONE;
   st->rule50++;
   sideToMove = opposite_color(sideToMove);
   st->epSquare = SQ_NONE;
   st->rule50++;

+  st->pliesFromNull = 0;

   gamePly++;
 
   st->mgValue += (sideToMove == WHITE)? TempoValueMidgame : -TempoValueMidgame;
   gamePly++;
 
   st->mgValue += (sideToMove == WHITE)? TempoValueMidgame : -TempoValueMidgame;


@@ -1279,6 +1282,7 @@ void Position::undo_null_move() {
   st->mgValue  = backupSt->mgValue;
   st->egValue  = backupSt->egValue;
   st->previous = backupSt->previous;
   st->mgValue  = backupSt->mgValue;
   st->egValue  = backupSt->egValue;
   st->previous = backupSt->previous;

+  st->pliesFromNull = backupSt->pliesFromNull;

 
   // Update the necessary information
   sideToMove = opposite_color(sideToMove);
 
   // Update the necessary information
   sideToMove = opposite_color(sideToMove);


@@ -1683,7 +1687,7 @@ bool Position::is_draw() const {
       return true;
 
   // Draw by repetition?
       return true;
 
   // Draw by repetition?

-  for (int i = 2; i < Min(gamePly, st->rule50); i += 2)
+  for (int i = 2; i < Min(Min(gamePly, st->rule50), st->pliesFromNull); i += 2)

       if (history[gamePly - i] == st->key)
           return true;
 
       if (history[gamePly - i] == st->key)
           return true;
 

diff --git a/src/position.h b/src/position.h
index 370bb1d45e0476a9618c42725e350b7dc0e018e2..606c7486fc8563db19d155541435f236de61415c 100644 (file)
--- a/src/position.h
+++ b/src/position.h
@@ -88,7 +88,7 @@ enum Phase {
 
 struct StateInfo {
   Key key, pawnKey, materialKey;
 
 struct StateInfo {
   Key key, pawnKey, materialKey;

-  int castleRights, rule50;
+  int castleRights, rule50, pliesFromNull;

   Square epSquare;
   Value mgValue, egValue;
   Value npMaterial[2];
   Square epSquare;
   Value mgValue, egValue;
   Value npMaterial[2];