a function to get holes from a 2d boolean array

holes.go

func GetHoles(sh Shape, w, h int) [][]intgeom.Point {

  flooding := make(map[intgeom.Point]bool)

  for x := 0; x < w; x++ {
    for y := 0; y < h; y++ {
      if !sh.At(x, y) {
        flooding[intgeom.Point{x,y}] = true
      }
    }
  }

  border := BorderPoints(w,h)

  for _, p := range border {
    if !sh.At(p.X, p.Y) {
      BFSFlood(flooding, p)
    }
  }


  // flooding is now a map of holes, points which are false
  // but not on the border.

  out := make([][]intgeom.Point)

  for len(flooding) > 0 {
    for k := range flooding {
      out = append(out, BFSFlood(flooding, k))
    }
  }

  return out
}

func BorderPoints(w,h int) []intgeom.Point {
  out := make([]intgeom.Point, (w*2 + h*2) - 4)
  i := 0
  for x := 0; x < w; x++ {
    out[i] = intgeom.Point{x,0}
    out[i+1] = intgeom.Point{x,h-1}
    i+=2
  }
  for y := 1; y < h-1; y++ {
    out[i] = intgeom.Point{0,y}
    out[i+1] = intgeom.Point{w-1,y}
    i+=2
  }
  return out
}

func BFSFlood(m map[intgeom.Point]bool, start intgeom.Point) []intgeom.Point {
  visited := []intgeom.Point{}
  toVisit := []intgeom.Point{start}
  for len(toVisit) > 0 {
    next := toVisit[0]
    delete(m[next])
    toVisit = toVisit[1:]
    visited = append(visited, next)
    
    // literally adjacent points for adjacency
    for x := -1; x <= 1; x++ {
      for y := -1; y <=1; y++ {
        p := intgeom.Point{x+next.X,y+next.Y}
        if _, ok := m[p]; ok {
          toVisit = append(toVisit, p) 
        } 
      }
    }
  }

  return visited
}