Magic. Prim's Algorithm

            // Magic. Don't touch.
            if (x % 2 + y % 2 == 0) {
                if (Wall.has(result, new Wall(x / 2 - 1, y / 2 - 1, x / 2, y / 2 - 1)))
                    dirs |= UP
                if (Wall.has(result, new Wall(x / 2 - 1, y / 2, x / 2, y / 2)))
                    dirs |= DOWN
                if (Wall.has(result, new Wall(x / 2 - 1, y / 2 - 1, x / 2 - 1, y / 2)))
                    dirs |= LEFT
                if (Wall.has(result, new Wall(x / 2, y / 2 - 1, x / 2, y / 2)))
                    dirs |= RIGHT
            }

// ...

            // Magic. Don't touch.
            if (Walls.has(result, new Wall(x, y, x + 1, y))
                grid[2 * x + 2][2 * y + 1] = BOXCHARS[UP | DOWN];
            if (Walls.has(result, new Wall(x, y, x, y + 1))
                grid[2 * x + 1][2 * y + 2] = BOXCHARS[LEFT | RIGHT];