Reverse words in Swift

reversed.swift

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

import UIKit

extension String {
    private func reverseChars(chars: inout [String.Element], range: Range<Int>) {
        guard chars.count > 1 else {
            return
        }

        var down = range.lowerBound
        var up = range.upperBound

        while down < up {
            (chars[down], chars[up]) = (chars[up], chars[down])

            down += 1
            up -= 1
        }
    }

    private func indicesOf(subString: String, searchArray: [String.Element]) -> [Int] {
        // https://en.wikipedia.org/wiki/Knuth–Morris–Pratt_algorithm
        var indices: [Int] = []

        let word = Array(subString)

        var m = 0
        var i = 0
        while m + i < searchArray.count {
            if word[i] == searchArray[m + i] {
                if i == word.count - 1 {
                    indices.append(m)
                    m += i + 1
                    i = 0
                } else {
                    i += 1
                }
            } else {
                m += 1
                i = 0
            }
        }

        return indices
    }

    func reversedWords() -> String {
        var chars = Array(self)
        let range = Range(0..<(chars.count-1))
        self.reverseChars(chars: &chars, range: range)

        let indices = self.indicesOf(subString: " ", searchArray: chars)

        var lower = 0
        for i in indices {
            let range = Range(lower..<(i-1))
            self.reverseChars(chars: &chars, range: range)
            lower = i+1
        }
        let lastRange = Range(((indices.last!)+1)..<(chars.count-1))
        self.reverseChars(chars: &chars, range: lastRange)
        return String(chars)
    }
}

let str = "Stately, plump Buck Mulligan came from the stairhead, bearing a bowl of lather on which a mirror and a razor lay crossed."
let reversed = str.reversedWords()
print(reversed) // crossed. lay razor a and mirror a which on lather of bowl a bearing stairhead, the from came Mulligan Buck plump Stately,