Heap's Algorithm for permutations in Swift

HeapsAlgorithm.swift

// https://en.wikipedia.org/wiki/Heap%27s_algorithm

import Foundation

/**
 Generates all possible permutations of `data` using Heap's Algorithm

 - parameters:
   - data: The data to permute
   - output: A closure called with each permutation of the data
 */
func heapPermutation<T>(data: inout Array<T>, output: (Array<T>) -> Void) {
    generate(n: data.count, data: &data, output: output)
}

func generate<T>(n: Int, data: inout Array<T>, output: (Array<T>) -> Void) {
    if n == 1 {
        output(data)
    } else {
        for i in 0 ..< n {
            generate(n: n - 1, data: &data, output: output)
            if n % 2 == 0 {
                data.swapAt(i, n - 1)
            } else {
                data.swapAt(0, n - 1)
            }
        }
    }
}

var data = ["a", "b", "c"] // This can be any array
heapPermutation(data: &data) { result in print(result) }