Workshop › Pac-Man-style Maze Chase › Step 7 of 42

Where can you walk?

What we are doing

Before anything moves, it must know where the walls are. isWall reads the map (the ghost house counts as a wall), and canGo asks whether the tile one step in a direction is free. A direction is a pair [dx, dy]. Columns are wrapped, so stepping off the left end of the tunnel lands on the right end.

```js
const wrap = (col) => (col + COLS) % COLS

function isWall(col, row) {
  const ch = MAZE[row][wrap(col)]
  return ch === '#' || ch === '-'
}

function canGo(e, dir) {
  return !isWall(e.col + dir[0], e.row + dir[1])
}
```

What it means

% is the remainder: (-1 + 19) % 19 is 18 and (19 + 19) % 19 is 0, so any column one step outside comes back in on the other side.
isWall reads the character of the tile; || means "or".
canGo adds the direction to the position and asks isWall; ! turns the answer around.

Your turn

Under key, write wrap, isWall and canGo.

Skills: Collision detection

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