index.js

var ObservGrid = require('observ-grid')
var watchGridChanges = require('observ-grid/watch-changes')
var computed = require('observ/computed')

module.exports = function(shape){

  var self = ObservGrid([], shape)

  self.selectedIndexes = computed([self], function(selectionGrid){
    return selectionGrid.data.reduce(function(result, value, i){
      if (value){
        result.push(i)
      }
      return result
    }, [])
  })

  var release = null
  var doneCallback = null
  var down = []

  self.start = function(input, done){
    self.stop()
    release = watchGridChanges(input, handleInput)
    doneCallback = done
  }

  self.clear = function(){
    self.data.set([])
    down.length = 0
  }

  self.stop = function(){
    if (release){
      release()
      release = null
      if (typeof doneCallback == 'function') {
        doneCallback()
        done = null
      }
    }
  }

  return self

  function handleInput(changes){
    changes.forEach(function(change){
      var key = change.slice(0,2).join('/')
      if (change[2]){
        if (down.length){
          var startCoords = down[0].split('/').map(pint)
          selectRange(startCoords, change)
        } else {
          toggleSelected(change[0], change[1])
        }
        down.push(key)
      } else {
        remove(down, key)
      }
    })
  }

  function selectRange(start, end){
    var rowStart = Math.min(start[0], end[0])
    var rowEnd = Math.max(start[0], end[0])
    var colStart = Math.min(start[1], end[1])
    var colEnd = Math.max(start[1], end[1])

    self.transaction(function(raw){
      for (var row=rowStart;row<=rowEnd;row++){
        for (var col=colStart;col<=colEnd;col++){
          raw.set(row, col, true)
        }
      }
    })
  }

  function toggleSelected(row, col){
    if (self.get(row, col)){
      self.set(row, col, null)
    } else {
      self.set(row, col, true)
    }
  }
}

function pint(val){
  return parseInt(val)
}

function remove(array, value){
  var index = array.indexOf(value)
  if (~index){
    array.splice(index, 1)
  }
}