From 9eccba776198c210563666787a9b5fe44b386fdc Mon Sep 17 00:00:00 2001
From: Aram Tumanian <aram_tumanian@ukr.net>
Date: Mon, 21 Nov 2016 17:17:47 +0200
Subject: [PATCH] Fix the pawn hash failure when the pawn key is 0

This patch fixed bugs #859 and #882.
At initialization we generate a new random key (Zobrist::noPawns).
It's added to the pawn key of all positions, so that the pawn key
of a pawnless position is no longer 0.

STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 21307 W: 3738 L: 3618 D: 13951

LTC:
LLR: 2.94 (-2.94,2.94) [-3.00,1.00]
Total: 45270 W: 5737 L: 5648 D: 33885

No functional change.
---
 src/position.cpp | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/position.cpp b/src/position.cpp
index fd518488..7c5ffdc5 100644
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -45,7 +45,7 @@ namespace Zobrist {
   Key psq[PIECE_NB][SQUARE_NB];
   Key enpassant[FILE_NB];
   Key castling[CASTLING_RIGHT_NB];
-  Key side;
+  Key side, noPawns;
 }
 
 namespace {
@@ -145,6 +145,7 @@ void Position::init() {
   }
 
   Zobrist::side = rng.rand<Key>();
+  Zobrist::noPawns = rng.rand<Key>();
 }
 
 
@@ -331,7 +332,8 @@ void Position::set_check_info(StateInfo* si) const {
 
 void Position::set_state(StateInfo* si) const {
 
-  si->key = si->pawnKey = si->materialKey = 0;
+  si->key = si->materialKey = 0;
+  si->pawnKey = Zobrist::noPawns;
   si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
   si->psq = SCORE_ZERO;
   si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
-- 
2.39.2