This code complements https://diningwithrobots.com/2020/09/04/quick-sort-in-swift/

QuickSort.swift

import Foundation

func quickSort(array: inout [Int], low: Int, high: Int) {
    // if the high index value is greater than the low index value we need to partition and sort
    if high > low {
        // determine our pivot index by generating partitions
        let pi = partition(array: &array, low: low, high: high)

        // recursively pass our upper and lower partitions back into quickSort using the new pivot index
        quickSort(array: &array, low: low, high: pi - 1)
        quickSort(array: &array, low: pi + 1, high: high)
    }

    // the array is in order the func will cease
}

func partition(array: inout [Int], low: Int, high: Int) -> Int {
    var i = low
    // loop through the partition of the array
    for j in (low + 1)..<(high + 1) {
        // if the value at index j is less than the value at the lower end of our partition
        // we increment our pivot index and swap the values
        if array[j] < array[low] {
            i += 1
            array.swapAt(i, j)
        }
    }

    // once the loop is done, place pivot value at the first position in the array
    array.swapAt(i, low)

    // return the pivot index
    return i
}

var numbers = [Int]()
for _ in 0..<20 {
    numbers.append(Int(arc4random_uniform(UInt32(1000))))
}

print(numbers) // [508, 581, 858, 660, 971, 158, 247, 363, 562, 87, 498, 925, 766, 165, 279, 195, 302, 516, 939, 4]

quickSort(array: &numbers, low: 0, high: numbers.count - 1)

print(numbers) // [4, 87, 158, 165, 195, 247, 279, 302, 363, 498, 508, 516, 562, 581, 660, 766, 858, 925, 939, 971]