From 3cb1c6c3c6206c3c2a0d78ce1cb9820256efc96e Mon Sep 17 00:00:00 2001
From: protonspring <mike@whiteley.org>
Date: Tue, 31 Mar 2020 15:08:55 -0600
Subject: [PATCH] remove KNPK endgame code

In more than 100k local KNPK games, there is no discernible difference between master and master with this endgame removed: master:42971, patch:42973, draws: 3969. Removal does not seem to regress in normal games.

STC
LLR: 2.94 (-2.94,2.94) {-1.50,0.50}
Total: 46390 W: 8998 L: 8884 D: 28508
Ptnml(0-2): 707, 5274, 11163, 5300, 751
https://tests.stockfishchess.org/tests/view/5e83b18ee42a5c3b3ca2ef02

LTC
LLR: 2.94 (-2.94,2.94) {-1.50,0.50}
Total: 44768 W: 5863 L: 5814 D: 33091
Ptnml(0-2): 251, 3918, 14028, 3905, 282
https://tests.stockfishchess.org/tests/view/5e84a82a4411759d9d0984f4

In tests with a book of endgames that can convert into KNPK, no significant difference can be seen either

```
TC 1.0+0.01
Score of patch vs master: 6131 - 6188 - 27681  [0.499] 40000
Elo difference: -0.5 +/- 1.9, LOS: 30.4 %, DrawRatio: 69.2 %

TC 2.0+0.02
Score of patch vs master: 5740 - 5741 - 28519 [0.500] 40000
Elo difference: -0.0 +/- 1.8, LOS: 49.6 %, DrawRatio: 71.3 %
``

closes https://github.com/official-stockfish/Stockfish/pull/2611

Bench 4512059
---
 src/endgame.cpp | 20 --------------------
 src/endgame.h   |  1 -
 2 files changed, 21 deletions(-)

diff --git a/src/endgame.cpp b/src/endgame.cpp
index 1a5959e5..e232da62 100644
--- a/src/endgame.cpp
+++ b/src/endgame.cpp
@@ -79,7 +79,6 @@ namespace Endgames {
     add<KQKR>("KQKR");
     add<KNNKP>("KNNKP");
 
-    add<KNPK>("KNPK");
     add<KRPKR>("KRPKR");
     add<KRPKB>("KRPKB");
     add<KBPKB>("KBPKB");
@@ -713,25 +712,6 @@ ScaleFactor Endgame<KBPKN>::operator()(const Position& pos) const {
 }
 
 
-/// KNP vs K. There is a single rule: if the pawn is a rook pawn on the 7th rank
-/// and the defending king prevents the pawn from advancing, the position is drawn.
-template<>
-ScaleFactor Endgame<KNPK>::operator()(const Position& pos) const {
-
-  assert(verify_material(pos, strongSide, KnightValueMg, 1));
-  assert(verify_material(pos, weakSide, VALUE_ZERO, 0));
-
-  // Assume strongSide is white and the pawn is on files A-D
-  Square pawnSq     = normalize(pos, strongSide, pos.square<PAWN>(strongSide));
-  Square weakKingSq = normalize(pos, strongSide, pos.square<KING>(weakSide));
-
-  if (pawnSq == SQ_A7 && distance(SQ_A8, weakKingSq) <= 1)
-      return SCALE_FACTOR_DRAW;
-
-  return SCALE_FACTOR_NONE;
-}
-
-
 /// KP vs KP. This is done by removing the weakest side's pawn and probing the
 /// KP vs K bitbase: If the weakest side has a draw without the pawn, it probably
 /// has at least a draw with the pawn as well. The exception is when the stronger
diff --git a/src/endgame.h b/src/endgame.h
index 49ebb603..fd1aba2d 100644
--- a/src/endgame.h
+++ b/src/endgame.h
@@ -57,7 +57,6 @@ enum EndgameCode {
   KBPKB,   // KBP vs KB
   KBPPKB,  // KBPP vs KB
   KBPKN,   // KBP vs KN
-  KNPK,    // KNP vs K
   KPKP     // KP vs KP
 };
 
-- 
2.39.2