Demonstrates rot13 in Swift with non-[A-Z][a-z] pass-through.

rot13.swift

import Cocoa

var input = "This is a test"
var expected = "Guvf vf n grfg"

/// Advances `i` by `by` through `string`, wrapping to start on hitting
/// end.
/// NOTE: `by` must be non-negative.
func wrappingAdvance(string: String, i: String.Index, by: Int)
    -> String.Index {
    assert(by > 0)
    var left = by
    var j = i
    let s = string.startIndex
    let e = string.endIndex
    while left > 0 {
        j = j.successor()
        if j == e {
            j = s
        }
        left -= 1
    }
    return j
}

/// Returns the string with A-Za-z rotated.
func rot13(text: String) -> String {
    let uppercase = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    let lowercase = uppercase.lowercaseString
    let count: Int = countElements(uppercase)
    let rotation = count / 2
    assert(countElements(lowercase) == count)

    var output = ""
    for ch in text {
        if let i = find(uppercase, ch) {
            let rotated = wrappingAdvance(uppercase, i, rotation)
            output += uppercase[rotated]
        } else if let i = find(lowercase, ch) {
            let rotated = wrappingAdvance(lowercase, i, rotation)
            output += lowercase[rotated]
        } else {
            output += ch
        }
    }
    return output
}

rot13(input) == expected
"été" == rot13(rot13("été"))

Here's another go, a little shorter but less elegant: https://gist.github.com/shepting/bdeab98418410fdeedc9

Pic: https://twitter.com/stevenhepting/status/498020537885417472/photo/1