Transposition.swift

struct Transposition<Base: Collection>: RandomAccessCollection
  where Base.Element: RandomAccessCollection
{
  typealias Index = Int
  typealias Indices = Range<Int>
  
  var base: Base
  
  struct Transposed: Collection {
    typealias Index = Base.Index
    
    var base: Base
    var offset: Int
    
    var startIndex: Base.Index { base.startIndex }
    var endIndex: Base.Index { base.endIndex }
    
    subscript(position: Base.Index) -> Base.Element.Element {
      let subCollection = base[position]
      let subIndex = subCollection.index(subCollection.startIndex, offsetBy: offset)
      return subCollection[subIndex]
    }
    
    func index(after i: Base.Index) -> Base.Index {
      base.index(after: i)
    }
  }
  
  var startIndex: Int { 0 }
  var endIndex: Int { base.first?.count ?? 0 }
  
  subscript(i: Int) -> Transposed {
    Transposed(base: base, offset: i)
  }
}

extension Collection where Element: RandomAccessCollection {
  func transposed() -> Transposition<Self> {
    Transposition(base: self)
  }
}