A Quick Sort implementation in Swift

QuickSort.swift

//: Playground - noun: a place where people can play

import UIKit

extension Array where Element: Comparable {
    mutating func quickSort() -> Array<Element> {
        
        // Recursive function that works out where our next pivots will be.
        func qSort(start startIndex: Int, _ pivot: Int) {
            if (startIndex < pivot) {
                
                // Sort value at pivot into place.
                let iPivot = qPartition(start: startIndex, pivot)
                
                qSort(start: startIndex, iPivot - 1)
                qSort(start:iPivot + 1, pivot)
            }
        }
        
        // Start Quick Sort at beginnig of list and set pivot to the end.
        qSort(start: 0, self.endIndex - 1)
    
        return self
    }
    
    // Sort the selected pivot value into the correct index/position
    // Manages the Wall, Current and Pivot index comparisons.
    mutating func qPartition(start startIndex: Int, _ pivot: Int) -> Int {
        var wallIndex: Int = startIndex
        
        // Compare range with pivot, bubble values WRT to the pivot value
        // to below or above that value.
        for currentIndex in wallIndex..<pivot {
            if self[currentIndex] <= self[pivot] {
             
                if wallIndex != currentIndex {
                    self.swapAt(currentIndex, wallIndex)
                }
                
                // Move the wall forward.
                wallIndex += 1
            }
        }
        
        // Finally swap the value at the pivot with the value at the wall
        if wallIndex != pivot {
            self.swapAt(wallIndex, pivot)
        }
        
        return wallIndex
    }
}

var numbers = [7,2,1,6,8,5,3,4]
let results = numbers.quickSort()