Extends Array<Int> to include Radix Sort. Uses Counting Sort as a subroutine. This implementation is for nonnegative, base 10 integers.

RadixSort.swift

extension Array where Element == Int {
    mutating func radixSort() {
        let radix = 10
        var digit = 1
        var done = false
        var unsortedArray = self
        while !done {
            done = true
            /// Counting Sort: Step 1
            /// - Tally each number in count array.
            var countArray = Array<Int>(repeating: 0, count: radix)
            for number in unsortedArray {
                guard number >= 0 else {
                    return
                }
                let index = number / digit
                countArray[index % radix] += 1
                if index > 0 && done {
                    done = false
                }
            }
            /// Counting Sort: Step 2
            /// - Modify count array to apply proper indices.
            for i in 1..<radix {
                countArray[i] += countArray[i - 1]
            }
            /// Counting Sort: Step 3
            /// - Build sorted array from indices in count array.
            var sortedArray = Array<Int>(repeating: 0, count: unsortedArray.count)
            /// Iterate over elements in reverse for stable sort.
            for number in unsortedArray.reversed() {
                let index = (number / digit) % radix
                countArray[index] -= 1
                sortedArray[countArray[index]] = number
            }
            unsortedArray = sortedArray
            digit *= radix
        }
        self = unsortedArray
    }
}