From: Miguel Lahoz <miguel_lahoz@protonmail.com>
Date: Fri, 25 Jan 2019 06:37:03 +0000 (+0800)
Subject: Change pinning logic in Static Exchange Evaluation (SEE)
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=242c566c1a80c7f3d95774eac513935fcee0cf0d

Change pinning logic in Static Exchange Evaluation (SEE)

This changes 2 parts with regards to static exchange evaluation.

Currently, we do not allow pinned pieces to recapture if *all* opponent
pinners are still in their starting squares. This changes that to having
a less strict requirement, checking if *any* pinners are still in their
starting square. This makes our SEE give more respect to the pinning
side with regards to exchanges, which makes sense because it helps our
search explore more tactical options.

Furthermore, we change the logic for saving pinners into our state
variable when computing slider_blockers. We will include double pinners,
where two sliders may be looking at the same blocker, a similar concept
to our mobility calculation for sliders in our evaluation section.
Interestingly, I think SEE is the only place where the pinners bitboard
is actually used, so as far as I know there are no other side effects
to this change.

An example and some insights:

White Bf2, Kg1
Black Qe3, Bc5

The move Qg3 will be given the correct value of 0. (Previously < 0)
The move Qd4 will be incorrectly given a value of 0. (Previously < 0)

It seems the tradeoff in search is worth it. Qd4 will likely be pruned
soon by something like probcut anyway, while Qg3 could help us spot
tactics at an earlier depth.

STC:
LLR: 2.96 (-2.94,2.94) [0.50,4.50]
Total: 62162 W: 13879 L: 13408 D: 34875
http://tests.stockfishchess.org/tests/view/5c4ba1a70ebc593af5d49c55

LTC: (Thanks to @alayant)
LLR: 3.40 (-2.94,2.94) [0.00,3.50]
Total: 140285 W: 23416 L: 22825 D: 94044
http://tests.stockfishchess.org/tests/view/5c4bcfba0ebc593af5d49ea8

Bench: 3937213
---

diff --git a/src/position.cpp b/src/position.cpp
index 21eff88c..812eabca 100644
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -498,14 +498,15 @@ Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners
   Bitboard blockers = 0;
   pinners = 0;
 
-  // Snipers are sliders that attack 's' when a piece is removed
+  // Snipers are sliders that attack 's' when a piece and other snipers are removed
   Bitboard snipers = (  (PseudoAttacks[  ROOK][s] & pieces(QUEEN, ROOK))
                       | (PseudoAttacks[BISHOP][s] & pieces(QUEEN, BISHOP))) & sliders;
+  Bitboard occupancy = pieces() & ~snipers;
 
   while (snipers)
   {
     Square sniperSq = pop_lsb(&snipers);
-    Bitboard b = between_bb(s, sniperSq) & pieces();
+    Bitboard b = between_bb(s, sniperSq) & occupancy;
 
     if (b && !more_than_one(b))
     {
@@ -1076,8 +1077,8 @@ bool Position::see_ge(Move m, Value threshold) const {
       stmAttackers = attackers & pieces(stm);
 
       // Don't allow pinned pieces to attack (except the king) as long as
-      // all pinners are on their original square.
-      if (!(st->pinners[~stm] & ~occupied))
+      // any pinners are on their original square.
+      if (st->pinners[~stm] & occupied)
           stmAttackers &= ~st->blockersForKing[stm];
 
       // If stm has no more attackers then give up: stm loses