Swift: Heap's algorithm with Generics

permutations.swift

 public extension CollectionType {
    var permutations: [[Generator.Element]] {
        func permute<C: CollectionType>(items: C) -> [[C.Generator.Element]] {
            var scratch = Array(items) // This is a scratch space for Heap's algorithm
            var result: [[C.Generator.Element]] = [] // This will accumulate our result
            // Heap's algorithm
            func heap(n: Int) {
                if n == 1 {
                    result.append(scratch)
                    return
                }
                for i in 0..<n-1 {
                    heap(n-1)
                    let j = (n%2 == 1) ? 0 : i
                    swap(&scratch[j], &scratch[n-1])
                }
                heap(n-1)
            }
            // Let's get started
            heap(scratch.count)
            // And return the result we built up
            return result
        }
        // Return all permutations
        return permute(self)
    }
}

public extension String {
    var permutations: [String] {
        return self.characters.permutations.map { String($0) }
    }
}

print([1,2,3,4].permutations)

print("ERIC".permutations)

print("ERIC".characters.permutations)