var CELLS = Symbol()

var key = function key(x,y) { return `${x},${y}` }

function grid(w, h)
{
  this[CELLS] = new Map()
  this.width = w;
  this.height = h;
}

grid.prototype = {
  cell(x,y) {
    var k = key(x,y)
    if (!this[CELLS].has(k)) this[CELLS].set(k,[])
    return this[CELLS].get(k)
  },
  
  add(entity, pos) {
    this.cell(pos.x, pos.y).push(entity);
    entity.coord = pos.slice();
  },
  
  remove(entity, pos) {
    var c = this.cell(pos.x, pos.y);
    c.splice(c.indexOf(entity), 1);
  },
  
  at(pos) {
    return this.cell(pos.x, pos.y);
  },
  
  inBounds(pos) {
    return pos.x >= 0 && pos.x < this.width && pos.y >= 0 && pos.y < this.height;
  },
  
  each(fn) {
    for (var [k, list] of this[CELLS])
      for (var p of list) fn(p, p.coord);
  },
  
  toString() {
    var out = `grid [${this.width}x${this.height}]
`
    for (var y = 0; y < this.height; y++) {
      for (var x = 0; x < this.width; x++) {
        var cell = this.at([x,y]);
        out += cell.length
      }
      if (y !== this.height - 1) out += "\n"
    }
      
    return out
  },
}

return grid