From: Marco Costalba <mcostalba@gmail.com>
Date: Tue, 12 Jan 2010 17:21:15 +0000 (+0100)
Subject: Allow negative history values
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=942c18ef6620967921049ba24f25e08d710c947e

Allow negative history values

Don't clamp to zero if a move continues to fail.

After 946 games at 1+0
Mod vs Orig +208 =562 -176 +12 ELO

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

diff --git a/src/history.cpp b/src/history.cpp
index 3cda3706..22116412 100644
--- a/src/history.cpp
+++ b/src/history.cpp
@@ -75,8 +75,12 @@ void History::failure(Piece p, Square to, Depth d) {
   assert(square_is_ok(to));
 
   history[p][to] -= int(d) * int(d);
-  if (history[p][to] < 0)
-      history[p][to] = 0;
+
+  // Prevent history underflow
+  if (history[p][to] <= -HistoryMax)
+      for (int i = 0; i < 16; i++)
+          for (int j = 0; j < 64; j++)
+              history[i][j] /= 2;
 }