Run-length encoding in Swift.

rle.swift

/**
http://rosettacode.org/wiki/Run-length_encoding

Given a string containing uppercase characters (A-Z), 
compress repeated 'runs' of the same character by storing the length of that run, 
and provide a function to reverse the compression. 
The output can be anything, as long as you can recreate the input with it.

*/

import Foundation

// "WWWBWW" -> [(3, W), (1, B), (2, W)]
func encode(input: String) -> [(Int, Character)] {
    return input.characters.reduce([(Int, Character)]()) {
        if $0.last?.1 == $1 { var r = $0; r[r.count - 1].0++; return r; }
        return $0 + [(1, $1)]
    }
}

// [(3, W), (1, B), (2, W)] -> "WWWBWW"
func decode(encoded: [(Int, Character)]) -> String {
    return encoded.reduce("") { $0 + String(count: $1.0, repeatedValue: $1.1) }
}