Beef up pseudo-legality checking

The routine PseudoLegal, used test killer moves for legality, now checks
if the move is a non-capture, if a dropped piece has the right color,
and accepts Pawn moves (including double-pushes). Castlings are still
rejected.

diff --git a/dropper.c b/dropper.c
index 14a7148..adcd956 100644 (file)
--- a/dropper.c
+++ b/dropper.c
@@ -741,22 +741,28 @@ int
 PseudoLegal (int stm, int move)
 {   // used for testing killers, so we can assume the move must be pseudo-legal for the stm in some position
     int match, from = move >> 8 & 0xFF, to = move & 255;
-    signed char piece = board[from];
-    to = toDecode[to];
+    signed char piece = board[from], mover = move >> 16 & 0xFF;
+    to = toDecode[to]; // could be double-push or castling, though
+    if(board[to]) return 0;
     if(piece < 0) { // drop
        if(piece == -1) return 0; // type not in hand
        piece = dropType[from] - 1;
+       if(!(piece & stm)) return 0; // piece has wrong color
         if((piece & ~COLOR) == 0 && pawnCount[sqr2file[to]] & maxBulk[stm]) return 0; // Pawn drop would over-crowd file
        return (board[to] == 0); // otherwise drop is legal on empty square (assumes drop location always legal for piece type)
     }
     if(!(piece & stm)) return 0; // piece has wrong color
+    if(piece == stm) { /// Pawn
+       if(stm == WHITE) return board[from+22] == 0 && (to - from == 22 || zoneTab[from] & 0x80 && to - from == 44 && board[to] == 0);
+       return board[from-22] == 0 && (to - from == -22 || zoneTab[from] & 0x80 && to - from == -44 && board[to] == 0);
+    }
     match = pieceCode[piece] & captCode[to - from];
     if(!match) return 0; // not aligned
     if(match & C_DISTANT) { // distant alignment
        int step = deltaVec[to - from];
        while(board[to -= step] == 0) {}   // ray scan towards mover
        return (from == to);               // legal if it reaches mover        
-    }
+    } else if((move & 255) > specials) return 0; // must be castling, as Pawns were already taken care of. Forbid for now.
     return 1; // must be leaper alignment, which guarantees hit
 }