[PermutationIterator] Use of Heap's Algorithm to create permutations

PermutationIterator.swift

public struct PermutationIterator<T>: IteratorProtocol {
    private var hasReturnedInitial = false
    private var a: [T]
    private var c: [Int]
    private let n: Int
    private var i = 0

    public init<C: Collection>(_ values: C) where C.Element == T {
        a = Array(values)
        n = a.count
        c = Array(repeating: 0, count: n)
    }

    public mutating func next() -> [T]? {
        if hasReturnedInitial == false {
            hasReturnedInitial = true
            return a
        }

        // this uses a non-recursive version of Heap's Algorithm
        // https://en.wikipedia.org/wiki/Heap%27s_algorithm
        while i < n {
            if c[i] < i {
                if i % 2 == 0 {
                    a.swapAt(0, i)
                } else {
                    a.swapAt(c[i], i)
                }

                c[i] += 1
                i = 0

                return a
            } else {
                c[i] = 0
                i += 1
            }
        }

        return nil
    }
}