Lesson 03 · Coordinates and staying in bounds

Coordinates and staying in bounds

Search steps constantly ask "is this cell even on the map?" Today you add an in-bounds check to the grid and fold it into walkable, so a step off any edge is rejected cleanly.

The goal

Add an InBounds test and make walkable respect the edges as well as walls.

Start here - the target

TO DO

Scenario: Coordinates outside the grid are not in bounds or walkable

Givena 3 by 3 grid with all cells open

WhenInBounds is asked about several coordinates

ThenInBounds of (0, 0) and (2, 2) is true, while InBounds of (3, 0), (0, 3), and (-1, 0) is false

AndWalkable of (-1, 0) is false (out of bounds), and Walkable of (2, 2) is true

Background

You already have a Coord to name a cell and a wall flag. The gap is the edge of the map: a coordinate like (-1, 0) or (3, 0) on a 3 by 3 grid names no real cell at all, and indexing the wall slice with it would be wrong or crash.

The work today is bounds. A search repeatedly looks at a cell’s neighbors, and near an edge some of those neighbors fall off the grid. Rather than guard for that everywhere, we make it part of Walkable: a coordinate is walkable only if it is in bounds and not a wall. That one predicate now safely answers “can the search step here?” for any coordinate at all, on the map or not.

Make it work

func (g *Grid) InBounds(c Coord) bool {
  return c.X >= 0 && c.Y >= 0 && c.X < g.W && c.Y < g.H
}
// Walkable now: in bounds AND not a wall
func (g *Grid) Walkable(c Coord) bool {
  return g.InBounds(c) && !g.Wall(c)
}

CheckpointDONE

Walkable now rejects anything off the edges as well as walls. Commit and stop here.