Lesson 06 · Printing the grid - Build a Pathfinder

Printing the grid

To see what the pathfinder is doing you need to look at the map. Today you render a grid back to ASCII, the mirror of parsing, giving the chapter its first demoable deliverable, a grid that round-trips text to grid and back.

The goal

Render a grid to a multi-line ASCII string that round-trips with the parser.

Start here - the target

TO DO

Scenario: Printing a parsed grid reproduces the original drawing

Givena grid parsed from the three rows '...' , '.#.' , '...' joined by newlines

Whenthe grid is rendered with String

Thenthe result equals the original string exactly, '...' then '.#.' then '...' joined by newlines

Andwalls render as # and open cells as ., with no trailing newline after the last row

Background

Being able to see the map is what makes everything after this debuggable. If a search returns a strange path, you print the grid, and later you print the path overlaid on it. Rendering is the exact mirror of parsing: walk the cells row by row, emit # for a wall and . for an open cell, and join rows with newlines.

The one detail that matters is the round trip. Parsing an ASCII map and printing it again should give back the identical string, which means no stray trailing newline after the last row. That property is a tiny but real correctness check on both directions at once, and it closes out the chapter: you now have a grid you can build from text, mark with walls, ask for neighbors, and print back out, which is the entire world the searches in the next chapters will explore.

Make it work

func (g *Grid) String() string {
  var b strings.Builder
  for y := 0; y < g.H; y++ {
    for x := 0; x < g.W; x++ {
      if g.Wall(Coord{x, y}) { b.WriteByte('#') } else { b.WriteByte('.') }
    }
    if y < g.H-1 { b.WriteByte('\n') }
  }
  return b.String()
}

CheckpointDONE

A grid prints back to the same ASCII you parsed, closing the round trip. Commit and stop here.