Quicksort - the only use for partition(by:) I could find

Quicksort.swift

import Foundation

extension Array where Element: Comparable {
    mutating func quickSort() {
        self[self.indices].quickSort()
    }
}

extension ArraySlice where Element: Comparable {
    mutating func quickSort() {
        guard self.count > 1 else { return }
        let pivotValue = self[startIndex]
        let pivotIndex = self.partition(by: { $0 >= pivotValue })

        if (pivotIndex == startIndex) {
            self[index(after: startIndex)..<self.endIndex].quickSort()
        } else {
            self[self.startIndex..<pivotIndex].quickSort()
            self[pivotIndex..<self.endIndex].quickSort()
        }
    }
}

for _ in 0..<100 {
    var a = (0..<10).map({ _ in arc4random_uniform(100) })

    var copy = a

    a.quickSort()

    if a != copy.sorted() {
        print("failed!")
        print(a)
        break
    }
}