A simple implementation of Merge Sort in Swift as an extension on Array

MergeSort.swift

// Swift 2.1

extension Array where Element : Comparable {
    
    private func merge(var left: Array<Element>, var _ right: Array<Element>) -> Array<Element> {
        
        var result: Array<Element> = []
        
        while !left.isEmpty && !right.isEmpty {
            
            if left[startIndex] <= right[startIndex] {
                result.append(left.removeFirst())
            } else {
                result.append(right.removeFirst())
            }
            
        }
        
        if !left.isEmpty {
            result.appendContentsOf(left)
        }
        
        if !right.isEmpty {
            result.appendContentsOf(right)
        }
        
        return result
        
    }

    
    public func mergeSort() -> Array<Element> {
        
        guard count > 1 else { return self }
        
        let mid = count / 2
        
        return merge(
            Array(self[startIndex..<mid]).mergeSort(),
            Array(self[mid ..< endIndex]).mergeSort()
        )
    }
    
}


func testMergeSort() {
    
    let integers = [2, 10, 5, 3, 8, 4, 7, 9, 6, 1]
    
    // test that merge sort returns a sorted array.
    assert(integers.mergeSort() == [1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
    
    print("merge sort tests passed")

}