From: Lolligerhans <lolligerhans@gmx.de>
Date: Tue, 12 Jan 2021 13:30:25 +0000 (+0100)
Subject: Add penalty for doubled pawns in agile structure
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=77eeea407c9d10cf6dbca6f4e07efbbe836c4fb0;hp=6dddcecb09df268d93810a1a38deb116f97672af

Add penalty for doubled pawns in agile structure

Give an additional penalty of S(20, 10) for any doubled pawn if none of
the opponent's pawns is facing any of our
 - pawns or
 - pawn attacks;
that means, each of their pawns can push at least one square without
being captured.
This ignores their non-pawns pieces and attacks.

One possible justification: Their pawns' ability to push freely provides
options to react to our threats by changing their pawn structure. Our
doubled pawns however will likely lead to an exploitable weakness, even
if the pawn structure is not yet fixed.

Note that the notion of "their pawns not being fixed" is symmetric for
both players: If all of their pawns can push freely so can ours. All
pawns being freely pushable might just be an early-game-indicator.
However, it can trigger during endgame pawns races, where doubled pawns
are especially hindering, too.

LTC
LLR: 2.94 (-2.94,2.94) {0.25,1.25}
Total: 134976 W: 17964 L: 17415 D: 99597
Ptnml(0-2): 998, 12702, 39619, 13091, 1078
https://tests.stockfishchess.org/tests/view/5ffdd5316019e097de3ef281

STC
LLR: 2.94 (-2.94,2.94) {-0.25,1.25}
Total: 35640 W: 7219 L: 6904 D: 21517
Ptnml(0-2): 645, 4096, 8084, 4289, 706
https://tests.stockfishchess.org/tests/view/5ffda4a16019e097de3ef265

closes https://github.com/official-stockfish/Stockfish/pull/3302

Bench: 4363873
---

diff --git a/src/pawns.cpp b/src/pawns.cpp
index de47570e..5d6770ed 100644
--- a/src/pawns.cpp
+++ b/src/pawns.cpp
@@ -32,6 +32,7 @@ namespace {
   // Pawn penalties
   constexpr Score Backward      = S( 6, 23);
   constexpr Score Doubled       = S(13, 53);
+  constexpr Score DoubledEarly  = S(20, 10);
   constexpr Score Isolated      = S( 2, 15);
   constexpr Score WeakLever     = S( 5, 57);
   constexpr Score WeakUnopposed = S(16, 22);
@@ -86,6 +87,7 @@ namespace {
 
     constexpr Color     Them = ~Us;
     constexpr Direction Up   = pawn_push(Us);
+    constexpr Direction Down = -Up;
 
     Bitboard neighbours, stoppers, support, phalanx, opposed;
     Bitboard lever, leverPush, blocked;
@@ -123,6 +125,13 @@ namespace {
         phalanx    = neighbours & rank_bb(s);
         support    = neighbours & rank_bb(s - Up);
 
+        if (doubled)
+        {
+            // Additional doubled penalty if none of their pawns is fixed
+            if (!(ourPawns & shift<Down>(theirPawns | pawn_attacks_bb<Them>(theirPawns))))
+                score -= DoubledEarly;
+        }
+
         // A pawn is backward when it is behind all pawns of the same color on
         // the adjacent files and cannot safely advance.
         backward =  !(neighbours & forward_ranks_bb(Them, s + Up))