CollectionType+indexBy.swift

extension CollectionType {
  func indexBy<Key: Hashable>(keyFunction: (Generator.Element) -> Key) -> [Key: [Generator.Element]] {
    var map: [Key: [Generator.Element]] = [:]
    for element in self {
      let key = keyFunction(element)
      var elements = map[key] ?? []
      elements.append(element)
      map[key] = elements
    }
    return map
  }

  func indexByAll<Key: Hashable, Keys: SequenceType where Keys.Generator.Element == Key>(keyFunction: (Generator.Element) -> Keys) -> [Key: [Generator.Element]] {
    var map: [Key: [Generator.Element]] = [:]
    for element in self {
      for key in keyFunction(element) {
        var elements = map[key] ?? []
        elements.append(element)
        map[key] = elements
      }
    }
    return map
  }
}