Be more optimistic in aspiration window

Be more optimistic wrt search instability, and set the unviolated bound
half window.

STC
LLR: 2.96 (-2.94,2.94) [-1.00,4.00]
Total: 16362 W: 3371 L: 3197 D: 9794

LTC
LLR: 2.94 (-2.94,2.94) [0.00,5.00]
Total: 21666 W: 3780 L: 3569 D: 14317

Bench: 6807896

Resolves #105

diff --git a/src/search.cpp b/src/search.cpp
index 44eb4ccc39d6168e3835601e6579fb4901641828..2b58eda81891fb2c2709e5096fdd1e4889adae91 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -322,18 +322,22 @@ namespace {
                 // re-search, otherwise exit the loop.
                 if (bestValue <= alpha)
                 {
+                    beta = (alpha + beta) / 2;
                     alpha = std::max(bestValue - delta, -VALUE_INFINITE);
 
                     Signals.failedLowAtRoot = true;
                     Signals.stopOnPonderhit = false;
                 }
                 else if (bestValue >= beta)
+                {
+                    alpha = (alpha + beta) / 2;
                     beta = std::min(bestValue + delta, VALUE_INFINITE);
+                }
 
                 else
                     break;
 
-                delta += 3 * delta / 8;
+                delta += delta / 2;
 
                 assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
             }