Functional Luhn algorithm

gistfile1.swift

func isLuhnValid(number: String) -> Bool {
    let asciiOffset: UInt8 = 48
    let convert: (UInt8) -> (UInt8) = {
        let n = $0 * 2
        return n > 9 ? n - 9 : n
    }
    let digits = Array(number.utf8).reverse().map{$0 - asciiOffset}
    let indices = Array(0..digits.count)
    let evens = indices.filter{$0 & 1 == 0}.map{digits[$0]}
    let odds = indices.filter{$0 & 1 == 1}.map{digits[$0]}.map(convert)
    let sum = (evens + odds).reduce(0, +)
    return sum % 10 == 0
}