WithIndex.swift

struct WithIndexGenerator<Base : CollectionType> : GeneratorType {
  
  private let base: Base
  private var index: Base.Index
  
  mutating func next() -> (Base.Index, Base.Generator.Element)? {
    return index == base.endIndex ? nil : (index, base[index++])
  }
}

struct WithIndexSequence<Base : CollectionType> : SequenceType {
  
  private let base: Base
  func generate() -> WithIndexGenerator<Base> {
    return WithIndexGenerator(base: base, index: base.startIndex)
  }
}

extension CollectionType {
  func withIndices() -> WithIndexSequence<Self> {
    return WithIndexSequence(base: self)
  }
}