Add KNNvKP Endgame Heuristic

This is a somewhat different patch. It fixes blindspots for
 two knights vs pawn endgame.

With local testing starting from random KNNvKP positions where the
pawn has not advanced beyond the 4th rank (thanks @protonspring !)
at 15+0.15 (4 cores), this went +105=868-27 against master. All except
two losses were won in reverse.

The heuristic is simple but effective - the strategy in these endgames
is to push the opposing king to the corner, then move the knight that's
blocking the pawn in for the checkmate while the pawn is free to move
and prevents stalemate. This patch gives SF the little boost it needs
to search the relevant king-cornering mating lines.

See the discussion in pull request 1939 for some more good results for
this test in independant tests:
https://github.com/official-stockfish/Stockfish/pull/1939

Bench: 3310239

diff --git a/src/endgame.cpp b/src/endgame.cpp
index 835173c..3e57220 100644 (file)
--- a/src/endgame.cpp
+++ b/src/endgame.cpp
@@ -286,6 +286,21 @@ Value Endgame<KQKR>::operator()(const Position& pos) const {
 }
 
 
+/// KNN vs KP. Simply push the opposing king to the corner.
+template<>
+Value Endgame<KNNKP>::operator()(const Position& pos) const {
+
+    assert(verify_material(pos, strongSide, 2 * KnightValueMg, 0));
+    assert(verify_material(pos, weakSide, VALUE_ZERO, 1));
+
+    Value result =  2 * KnightValueEg
+                  - PawnValueEg
+                  + PushToEdges[pos.square<KING>(weakSide)];
+
+    return strongSide == pos.side_to_move() ? result : -result;
+}
+
+
 /// Some cases of trivial draws
 template<> Value Endgame<KNNK>::operator()(const Position&) const { return VALUE_DRAW; }
 

diff --git a/src/endgame.h b/src/endgame.h
index 941cb31..2a48488 100644 (file)
--- a/src/endgame.h
+++ b/src/endgame.h
@@ -37,6 +37,7 @@ enum EndgameCode {
 
   EVALUATION_FUNCTIONS,
   KNNK,  // KNN vs K
+  KNNKP, // KNN vs KP
   KXK,   // Generic "mate lone king" eval
   KBNK,  // KBN vs K
   KPK,   // KP vs K
@@ -125,6 +126,7 @@ public:
     add<KRKN>("KRKN");
     add<KQKP>("KQKP");
     add<KQKR>("KQKR");
+    add<KNNKP>("KNNKP");
 
     add<KNPK>("KNPK");
     add<KNPKB>("KNPKB");

diff --git a/src/search.cpp b/src/search.cpp
index 6bb1e1d..2e7dd69 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -830,7 +830,7 @@ namespace {
         int probCutCount = 0;
 
         while (  (move = mp.next_move()) != MOVE_NONE
-                && probCutCount < 2 + 2 * cutNode)
+               && probCutCount < 2 + 2 * cutNode)
             if (move != excludedMove && pos.legal(move))
             {
                 probCutCount++;