Merge sort implementation on Swift

MergeSort.swift

import Foundation

/**
 Sorts an array using the merge arrangement, which relays in order the left & right half arrays in a major set of elements. giving a complexity of O(n log n).
 - Returns: An ordered array.
 */
func mergeSort(arr:[Int]) -> [Int] {
    guard arr.count > 1 else {return arr}
    let middle: Int = Int(ceil(Double(arr.count/2)))
    let left: [Int] = mergeSort(Array(arr[0..<arr.count/2]))
    let right: [Int] = mergeSort(Array(arr[middle..<arr.count]))
    return merge(left, right)
}

/**
 Traverses both arrays and merge the result in an orderer array, supposing left & right are ordered.
 - Parameter left: An ordered array to be merged.
 - Parameter right: An ordered array to be merged.
 - Returns: An ordered array with complexity O(n).
 */
func merge(left: [Int], _ right: [Int]) -> [Int]{
    var leftIndex = 0
    var rightIndex = 0
    var result: [Int] = []
    //is append available on right or left
    while leftIndex<left.count || rightIndex<right.count {
        //apennd can happen on left side
        if leftIndex<left.count &&
            (rightIndex>=right.count || left[leftIndex] < right[rightIndex]) {
            result.append(left[leftIndex])
            leftIndex += 1
        } else {
            result.append(right[rightIndex])
            rightIndex += 1
        }
    }
    return result
}


let array = [5,6,4,3,2,1]
print(mergeSort(array))