closestPermutation.swift

//: Playground - noun: a place where people can play

import UIKit

func closestPermutation(forNumber number: Int) -> Int {
    guard number.digits().sorted(by: <) != number.digits() else { return number }

    var currentPurmutation = number
    var lastN: Float = 2
    while true {
        for i in 1...Int(powf(2, lastN - 1)) {
            var newPurmutation = currentPurmutation.digits()
            newPurmutation.swapAt(number.digits().count - 1, number.digits().count - i)
            currentPurmutation = newPurmutation.intFromDigits()!
            if currentPurmutation < number {
                return currentPurmutation
            }
        }
        lastN += 1
    }
}

extension Int {
    func digits() -> [Int] {
        return "\(self)".characters.flatMap({ (char) -> Int? in
            return Int(String(char))
        })
    }
}

extension Array where Element == Int {
    func intFromDigits() -> Int? {
        let returnValue = flatMap({ number -> String in return String(number) }).reduce("", +)
        return Int(returnValue)
    }
}

closestPermutation(forNumber: 1234)
closestPermutation(forNumber: 1834)
closestPermutation(forNumber: 1842)
closestPermutation(forNumber: 2381)
closestPermutation(forNumber: 100)