Reverse array by group size

The following solution is based on a combo of stride+map:

let groupSize = 3
let array = [1, 2, 3, 4, 5, 6]
let reversedArray = Array(array.reversed())
let result = stride(from: 0, to: reversedArray.count, by: groupSize).map {
  reversedArray[$0 ..< min($0 + groupSize, reversedArray.count)].reversed()
}.reduce([Int](), +)
print(result) //  [4, 5, 6, 1, 2, 3]