Lesson 02 · Walls and walkable cells

Walls and walkable cells

A map is only interesting once some cells are blocked. Today you let a cell be marked as a wall and ask whether a cell is walkable, which is the single distinction every search in this project will branch on.

The goal

Mark a cell as a wall and report whether a given cell is walkable.

Start here - the target

TO DO

Scenario: A cell can be a wall, and walkability reflects it

Givena new 3 by 3 grid where every cell starts open

Whenthe cell at (1, 1) is marked a wall with SetWall

ThenWall(1, 1) is true and Walkable(1, 1) is false

Andan untouched cell (0, 0) reports Wall false and Walkable true

Background

A grid of open cells is just an empty field. What turns it into a map worth searching is obstacles: cells you cannot enter. We represent that with a single bit per cell, a wall flag, stored in a flat slice indexed by y*W + x. Every cell starts open, and marking a wall sets its bit.

The one question every search asks about a cell is walkable: can I stand here? For now that is simply the opposite of being a wall. Next lesson we fold in bounds checking so that stepping off the edge of the grid counts as not walkable too, but the shape is already here: walls carve the searchable space, and Walkable is the gate every neighbor step will pass through.

Make it work

// name the (x,y) pair now; it will be a map key and path element later
type Coord struct{ X, Y int }
// store one bool per cell; index it as Y*W + X
type Grid struct {
  W, H  int
  walls []bool
}
func NewGrid(w, h int) *Grid { return &Grid{W: w, H: h, walls: make([]bool, w*h)} }
func (g *Grid) SetWall(c Coord)    { g.walls[c.Y*g.W+c.X] = true }
func (g *Grid) Wall(c Coord) bool  { return g.walls[c.Y*g.W+c.X] }
// Walkable is just: not a wall (bounds come next lesson)

CheckpointDONE

Cells can be walls, and you can ask whether a cell is walkable. Commit and stop here.