In-Situ implementation of MergeSort

In-SituMergeSort.swift

import Foundation

//Merge Sort

func merge<T: Comparable>(_ lhs: [T],_ rhs: [T]) -> [T] {
    
    switch (lhs,rhs) {
    case (let lhs, let rhs) where lhs.count == 0:
        return rhs
    case (let lhs, let rhs) where rhs.count == 0:
        return lhs
    case (let lhs, let rhs) where lhs.first! < rhs.first!:
        return [lhs.first!] + merge(Array(lhs.dropFirst()), rhs)
    default:
        return [rhs.first!] + merge(lhs, Array(rhs.dropFirst()))
    }
}

func mergesort<T: Comparable>(_ array: [T]) -> [T] {

    guard array.count >= 2 else {
        return array
    }

    return merge(
        mergesort(
            Array(array.dropLast(array.count - Int(array.count/2)))
    ),
        mergesort(
            Array(array.dropFirst(Int(array.count/2)))
    ))
}

let array = [3,4,6,1,7,2]

let a = mergesort(array)