Implement move borrowing in XBetza

The XBetza 'x' modifier can now be used in final legs to indicate move
borrowing from a friendly target. This only works when the target's move
had an XBetza definition (i.e. was defined by a 'piece' command).
Although borrowing from equal type is suppressed, there is a danger
of infinite recursion when two different move borrowers occur in one game.
XBoard tries to suppress recursive move borrowing in general, by not
executing borrowing specs in a borrowed move, but a preceding leg can
hide this.

diff --git a/moves.c b/moves.c
index f0fa2db..4bcdaaa 100644 (file)
--- a/moves.c
+++ b/moves.c
@@ -414,6 +414,7 @@ MovesFromString (Board board, int flags, int f, int r, int tx, int ty, int angle
        if(isdigit(*++p)) expo = atoi(p++);           // read exponent
        if(expo > 9) p++;                             // allow double-digit
        desc = p;                                     // this is start of next move
+       if(tx == -2 && mode & 1<<12) continue;        // prevent recursive move borrowing (flaky)
        epFlag = // flags initial orthogonal and diagonal pawn non-capture multi-pushes (which have legacy meaning)
                (initial && promo != NormalMove && !cont && mode == 4 && (!dx || dx == dy) && (dy > 1 ? !jump : expo > 1));
        if(initial == 2) { if(board[r][f] != initialPosition[r-2*his+3][f]) continue; initial = 0; } else
@@ -532,7 +533,11 @@ MovesFromString (Board board, int flags, int f, int r, int tx, int ty, int angle
                }
                if(mode & 16 && (board[y][x] == WhiteKing || board[y][x] == BlackKing)) break; // tame piece, cannot capture royal
                if(occup & mode) {
-                 cb(board, flags, y == promoRank ? promo : put ? Swap : NormalMove, r, f, y, x, cl); // allowed, generate
+                 if(mode & 1<<12) {
+                   ChessSquare neighbor = board[y][x];
+                   char *borrow = (neighbor == pc ? NULL : pieceDesc[neighbor]); // do not borrow from equal type
+                   if(borrow) MovesFromString(board, flags, f, r, -2, -2, dir, range, borrow, cb, cl);      // borrow moves from neighbor
+                 } else cb(board, flags, y == promoRank ? promo : put ? Swap : NormalMove, r, f, y, x, cl); // allowed, generate
                }
                if(occup != 4) break; // not valid transit square
            } while(--i);