Matrix2D.swift

struct Matrix<Element> {
  private var storage: [[Element]]
  
  init(_ storage: [[Element]]) {
    self.storage = storage
  }
}

extension Matrix: Collection {
  struct Index: Comparable {
    static func < (lhs: Matrix<Element>.Index, rhs: Matrix<Element>.Index) -> Bool {
      return lhs.x < rhs.x && lhs.y < rhs.y
    }
    
    let x: Int
    let y: Int
  }
  
  func index(after i: Index) -> Index {
    if (i.x == storage.count - 1) {
      return Index(x: 0, y: i.y + 1)
    } else {
      return Index(x: i.x + 1, y: i.y)
    }
  }
  
  subscript(position: Index) -> Element {
    get {
      storage[position.y][position.x]
    }
    set {
      storage[position.y][position.x] = newValue
    }
  }
  
  var startIndex: Index {
    Index(x: 0, y: 0)
  }
  
  var endIndex: Index {
    Index(x: 0, y: storage[0].count)
  }
}

let m = Matrix([
  [0, 1],
  [2, 3]
])

for e in m {
  print(e)
}