string-permuter.swift

//
//  main.swift
//  string-permuter
//
//  Created by Michael Rockhold on 8/22/20.
//  Copyright © 2020 Michael Rockhold. All rights reserved.
//

import Foundation

func permuter(s: String, feedback: (String)->Void) {
    func swapChars(_ s: String, i1: String.Index, i2: String.Index) -> String {
        let c1 = s[i1]
        let c2 = s[i2]
        var ss = s
        ss.replaceSubrange(i1...i1, with: [c2])
        ss.replaceSubrange(i2...i2, with: [c1])
        return ss
    }
    
    func permute(_ s: String, left: String.Index, right: String.Index) {
          // Base case
        if left == right {
            feedback(s)
        }
        else {
            var idx = left
            repeat {
                let swapped = swapChars(s, i1: left, i2: idx)
                var nextLeft = left
                s.formIndex(after: &nextLeft)
                permute(swapped, left: nextLeft, right: right)
                s.formIndex(after: &idx)
            } while idx != right
        }
    }
    
    permute(s, left: s.startIndex, right: s.endIndex)
}

permuter(s: "ABC") { ss in
    print(ss)
}