// Specifically, return the y component of a line with a length of 1 extending
// in the given direction from origin.
int DirY(int dir) {
    // Is a diagonal.
    if (dir & 100b) {
        return Cond(dir & 010b, 0.7071, -0.7071);
    }
    // Is vertical.
    if (!(dir & 010b)) {
        return Cond(dir & 001b, 1, -1);
    }
    return 0;
}