2 files changed, 76 insertions(+), 0 deletions(-)

M src/attack.zig
M test/attack.zig

@@ 94,3 94,47 @@ fn maskKingAttacks(king: Square) u64 {
 pub fn kingAttacks(king: Square) u64 {
     return king_attacks[@enumToInt(king)];
 }
+
+/// Given a square occupied by a bishop, return a bitboard of relevant
+/// occupancy bits.  i.e. squares on which, if other pieces are placed, the
+/// line of attack is blocked and the attack bitboard is altered.
+pub fn maskBishopOccupancy(bishop: Square) u64 {
+    const sq = @enumToInt(bishop);
+    const rank = sq / 8;
+    const file = sq % 8;
+    var bitboard: u64 = 0;
+    var r: u6 = rank + 1;
+    var f: u6 = file + 1;
+    while (r >= 1 and r <= 6 and f >= 1 and f <= 6) : ({
+        r += 1;
+        f += 1;
+    }) {
+        bitboard |= @as(u64, 1) << (8 * r + f);
+    }
+    r = rank - 1;
+    f = file + 1;
+    while (r >= 1 and r <= 6 and f >= 1 and f <= 6) : ({
+        r -= 1;
+        f += 1;
+    }) {
+        bitboard |= @as(u64, 1) << (8 * r + f);
+    }
+    r = rank + 1;
+    f = file - 1;
+    while (r >= 1 and r <= 6 and f >= 1 and f <= 6) : ({
+        r += 1;
+        f -= 1;
+    }) {
+        bitboard |= @as(u64, 1) << (8 * r + f);
+    }
+    r = rank - 1;
+    f = file - 1;
+    while (r >= 1 and r <= 6 and f >= 1 and f <= 6) : ({
+        r -= 1;
+        f -= 1;
+    }) {
+        bitboard |= @as(u64, 1) << (8 * r + f);
+    }
+
+    return bitboard;
+}

@@ 128,3 128,35 @@ test "kings in corners attack only 3 squares" {
     expectEqual(bitboardFromSquares(expected_h1[0..]), kingAttacks(king_h1));
     expectEqual(bitboardFromSquares(expected_a1[0..]), kingAttacks(king_a1));
 }
+
+test "bishop in centre occupancy bits" {
+    const bishop = Square.e4;
+
+    const expected_squares = [_]Square{
+        Square.f5,
+        Square.g6,
+        Square.f3,
+        Square.g2,
+        Square.d3,
+        Square.c2,
+        Square.d5,
+        Square.c6,
+        Square.b7,
+    };
+
+    expectEqual(bitboardFromSquares(expected_squares[0..]), maskBishopOccupancy(bishop));
+}
+
+test "bishop at side occupancy bits" {
+    const bishop = Square.h6;
+
+    const expected_squares = [_]Square{
+        Square.g5,
+        Square.f4,
+        Square.e3,
+        Square.d2,
+        Square.g7,
+    };
+
+    expectEqual(bitboardFromSquares(expected_squares[0..]), maskBishopOccupancy(bishop));
+}