pub fn riderBlocker(self: @This(), square: Square, piece: Figures) BoardType {
        const rank = @intCast(isize, square.rank());
        const file = @intCast(isize, square.file());
        const moves = switch (piece) {
            .bishop => [_][2]i3{
                .{ -1, -1 },
                .{ -1, 1 },
                .{ 1, -1 },
                .{ 1, 1 },
            },
            .rook => [_][2]i3{
                .{ -1, 0 },
                .{ 1, 0 },
                .{ 0, -1 },
                .{ 0, 1 },
            },
            else => unreachable,
        };
        var blockers: BoardType = 0;
        for (moves) |m| {
            var step: i5 = 1;
            while (step < SIZE - 1) : (step += 1) {
                const dr = rank + m[0] * step;
                const df = file + m[1] * step;
                if (m[0] > 0 and dr > 7) break;
                if (m[0] < 0 and dr < 0) break;
                if (m[1] > 0 and df > 7) break;
                if (m[1] < 0 and df < 0) break;
                const s = @intCast(u6, dr * SIZE + df);
                const mask = @intCast(BoardType, @as(BoardType, 1) << s);
                blockers |= mask;
                if (mask & self.board != 0) break;
            }
        }
        return blockers;
    }

    }
}
test "riderBlocker" {
    var bb: BitBoard = .{};
    { // bishop
        bb.board = 0;
        bb.setSlice(&[_]Square{ .c3, .b6, .f2 });
        const got = bb.riderBlocker(.d4, .bishop);
        bb.board = 0;
        bb.setSlice(&[_]Square{ .c3, .f2, .e3, .c5, .b6, .e5, .f6, .g7, .h8 });
        try std.testing.expectEqual(bb.board, got);
    }
    { // rook
        bb.board = 0;
        bb.setSlice(&[_]Square{ .b4, .d3, .d5 });
        const got = bb.riderBlocker(.d4, .rook);
        bb.board = 0;
        bb.setSlice(&[_]Square{ .c4, .b4, .d3, .d5, .e4, .f4, .g4, .h4 });
        try std.testing.expectEqual(bb.board, got);