/// Reorder elements in-place into the next permutation, returning false when
/// the collection has been permuted into lexicographical order. To cycle
/// through all permutations, start with a lexicographically-sorted collection
/// and permute until it returns false.
extension MutableCollection where Self: BidirectionalCollection {
  mutating func permute(
    by areInIncreasingOrder: (Element,Element)->Bool
  ) -> Bool {
    guard !isEmpty else { return false }
    var i = index(before: endIndex)

    while i != startIndex {
      let j = i
      formIndex(before: &i)
      let x = self[i]
      if areInIncreasingOrder(x, self[j]) {
        guard let k = lastIndex(where: { areInIncreasingOrder(x, $0)})
          else { preconditionFailure("Nodetermistic comparison behavior")}
        swapAt(i, k)
        self[j...].reverse()
        return true
      }
    }
    reverse()
    return false
  }
}

var a = Array((1...4).reversed())
repeat {
  print(a)
} while a.permute(by: >)