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Simplify casting extension

On November 16th, before the removal of the depth condition, I tried
revising castling extensions to only handle castling moves, rather than
moves that change castling rights generally. It appeared to be a slight
Elo gain at STC but insufficient to pass [0, 4] (+0.5 Elo), but what I
overlooked was that it made pos.can_castle(us) irrelevant and should
have been a simplification. Recent discussion with @Chess13234 and
Michael Chaly (@Vizvezdenec) inspired me to take a second look, and
the simplification continues to pass when rebased on the current master.

This replaces two conditions with one, because type_of(move) == CASTLING
implies pos.can_castle(Us), allowing us to remove the latter condition.

STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 110948 W: 24209 L: 24263 D: 62476
http://tests.stockfishchess.org/tests/view/5bf8f65c0ebc5902bced3a63

LTC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 88283 W: 14681 L: 14668 D: 58934
http://tests.stockfishchess.org/tests/view/5bf994a60ebc5902bced4349

Bench: 3939338
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31m059 authored and snicolet committed Nov 24, 2018
1 parent de7182f commit 7b6fa353a3858b092e1a43ef69b3035cb5d3b5c0
Showing with 2 additions and 2 deletions.
  1. +2 −2 src/search.cpp
@@ -948,8 +948,8 @@ namespace {
&& pos.see_ge(move))
extension = ONE_PLY;

else if ( pos.can_castle(us) // Extension for king moves that change castling rights
&& type_of(movedPiece) == KING)
// Extension if castling
else if (type_of(move) == CASTLING)
extension = ONE_PLY;

// Calculate new depth for this move

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