From 6d89d0b64a99003576d3e0ed616b43333c9eca01 Mon Sep 17 00:00:00 2001
From: atumanian <1>
Date: Tue, 6 Jun 2017 10:13:10 -0700
Subject: [PATCH] Don't score as an immediate draw 2-fold repetitions of the
 root position

In the current version a search stops when the current position is the same as
any position earlier in the search stack,
including the root position but excluding positions before the root.
The new version makes an exception for repeating the root position.

This gives correct scores for moves in the MultiPV > 1 mode.

Fixes #948 (see it for the detailed description of the bug).

LTC: http://tests.stockfishchess.org/tests/view/587910bc0ebc5915193f754b
ELO: 0.38 +-1.7 (95%) LOS: 66.8%
Total: 40000 W: 5166 L: 5122 D: 29712

STC: http://tests.stockfishchess.org/tests/view/5922e6230ebc59035df34a50
LLR: 2.94 (-2.94,2.94) [-3.00,1.00]
Total: 94622 W: 17059 L: 17064 D: 60499

 LTC: http://tests.stockfishchess.org/tests/view/59273a000ebc59035df34c03
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 61259 W: 7965 L: 7897 D: 45397

Bench: 6599721

Closes #1126
---
 src/position.cpp | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/position.cpp b/src/position.cpp
index 0449222e..4c8145c4 100644
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -1100,10 +1100,10 @@ bool Position::is_draw(int ply) const {
       stp = stp->previous->previous;
 
       // At root position ply is 1, so return a draw score if a position
-      // repeats once earlier but after or at the root, or repeats twice
-      // strictly before the root.
+      // repeats once earlier but strictly after the root, or repeats twice
+      // before or at the root.
       if (   stp->key == st->key
-          && ++cnt + (ply - i > 0) == 2)
+          && ++cnt + (ply - 1 > i) == 2)
           return true;
   }
 
-- 
2.39.2