From: Copilot <198982749+Copilot@users.noreply.github.com>
Date: Wed, 10 Sep 2025 20:45:37 +0000 (+0200)
Subject: Fix insufficient material detection for variants with multiple pawn types (#916)
X-Git-Url: http://winboard.nl/cgi-bin?a=commitdiff_plain;h=4aef2cac0d4f570b679920b50a29b448e5863f63;p=fairystockfish.git

Fix insufficient material detection for variants with multiple pawn types (#916)

Closes #915
---

diff --git a/src/apiutil.h b/src/apiutil.h
index 74bdc6d..d0657bc 100644
--- a/src/apiutil.h
+++ b/src/apiutil.h
@@ -372,31 +372,53 @@ inline bool has_insufficient_material(Color c, const Position& pos) {
         || (pos.flag_region(c) && pos.count(c, pos.flag_piece(c))))
         return false;
 
-    // Restricted pieces
-    Bitboard restricted = pos.pieces(~c, KING);
-    // Atomic kings can not help checkmating
-    if (pos.extinction_pseudo_royal() && pos.blast_on_capture() && (pos.extinction_piece_types() & COMMONER))
-        restricted |= pos.pieces(c, COMMONER);
+    // Precalculate if any promotion pawn types have pieces
+    bool hasPromotingPawn = false;
+    for (PieceSet pawnTypes = pos.promotion_pawn_types(c); pawnTypes; )
+    {
+        PieceType pawnType = pop_lsb(pawnTypes);
+        if (pos.count(c, pawnType) > 0)
+        {
+            hasPromotingPawn = true;
+            break;
+        }
+    }
+
+    // Determine checkmating potential of present pieces
+    constexpr PieceSet MAJOR_PIECES = piece_set(ROOK) | QUEEN | ARCHBISHOP | CHANCELLOR
+                                     | SILVER | GOLD | COMMONER | CENTAUR | AMAZON | BERS;
+    constexpr PieceSet COLORBOUND_PIECES = piece_set(BISHOP) | FERS | FERS_ALFIL | ALFIL | ELEPHANT;
+    Bitboard restricted = pos.pieces(KING);
+    Bitboard colorbound = 0;
     for (PieceSet ps = pos.piece_types(); ps;)
     {
         PieceType pt = pop_lsb(ps);
-        if (pt == KING || !(pos.board_bb(c, pt) & pos.board_bb(~c, KING)))
+
+        // Constrained pieces
+        if (pt == KING || !(pos.board_bb(c, pt) & pos.board_bb(~c, KING)) || (pos.extinction_pseudo_royal() && pos.blast_on_capture() && (pos.extinction_piece_types() & pt)))
             restricted |= pos.pieces(c, pt);
-        else if (is_custom(pt) && pos.count(c, pt) > 0)
-            // to be conservative, assume any custom piece has mating potential
-            return false;
+
+        // If piece is a major piece or a custom piece we consider it sufficient for mate.
+        // To avoid false positives, we assume any custom piece has mating potential.
+        else if ((MAJOR_PIECES & pt) || is_custom(pt))
+        {
+            // Check if piece is already on the board
+            if (pos.count(c, pt) > 0)
+                return false;
+
+            // Check if any pawn can promote to this piece type
+            if (hasPromotingPawn && (pos.promotion_piece_types(c) & pt))
+                return false;
+        }
+
+        // Collect color-bound pieces
+        else if (COLORBOUND_PIECES & pt)
+            colorbound |= pos.pieces(pt);
     }
 
-    // Mating pieces
-    for (PieceType pt : { ROOK, QUEEN, ARCHBISHOP, CHANCELLOR, SILVER, GOLD, COMMONER, CENTAUR, AMAZON, BERS })
-        if ((pos.pieces(c, pt) & ~restricted) || (pos.count(c, pos.main_promotion_pawn_type(c)) && (pos.promotion_piece_types(c) & pt)))
-            return false;
+    Bitboard unbound = pos.pieces() ^ restricted ^ colorbound;
 
     // Color-bound pieces
-    Bitboard colorbound = 0, unbound;
-    for (PieceType pt : { BISHOP, FERS, FERS_ALFIL, ALFIL, ELEPHANT })
-        colorbound |= pos.pieces(pt) & ~restricted;
-    unbound = pos.pieces() ^ restricted ^ colorbound;
     if ((colorbound & pos.pieces(c)) && (((DarkSquares & colorbound) && (~DarkSquares & colorbound)) || unbound || pos.stalemate_value() != VALUE_DRAW || pos.check_counting() || pos.makpong()))
         return false;
 
diff --git a/test.py b/test.py
index 8ac27ab..ca5d391 100644
--- a/test.py
+++ b/test.py
@@ -127,6 +127,10 @@ startFen = rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR[Qq] w KQkq - 0 1
 
 [cannonatomic:atomic]
 cannon = c
+
+[multipawn:chess]
+soldier = s
+pawnTypes = ps
 """
 
 sf.load_variant_config(ini_text)
@@ -243,6 +247,10 @@ variant_positions = {
     "wazirking": {
         "7k/6K1/8/8/8/8/8/8 b - - 0 1": (False, False),  # K vs K
     },
+    "multipawn": {
+        "k7/p7/8/8/8/8/8/K7 w - - 0 1": (True, False),  # K vs KP
+        "k7/s7/8/8/8/8/8/K7 w - - 0 1": (True, False),  # K vs KS
+    },
 }
 
 invalid_variant_positions = {