sorting.coffee

Move =
  move: (modelOrIndex, endIndex) ->
    if typeof modelOrIndex == "number"
      this.__moveIndexTo__(modelOrIndex, endIndex)
    else
      this.__moveModelTo__(modelOrIndex, endIndex)

  __moveModelTo__: (model, endIndex) ->
    this.__moveIndexTo__ _.indexOf(this.models, model), endIndex

  __moveIndexTo__: (startIndex, endIndex) ->
    movedUp   = startIndex > endIndex
    movedDown = startIndex < endIndex
    _.each this.models, (model, i) ->
      model._index = if i == startIndex
        endIndex
      else if movedDown && i <= endIndex
        i - 1
      else if movedUp   && i >= endIndex
        i + 1
      else
        i

    # Save old comparator
    c = this.comparator

    # Build comparator on _index just set
    # and sort on this index.
    this.comparator = (model) -> model._index
    this.sort()

    # Return comparator to old value
    this.comparator = c

_.extend Backbone.Collection.prototype, Move