Merge pull request TheAlgorithms#24 from codersanjeev/master

DIRECTORY.md

@@ -29,6 +29,7 @@
   * [Insertionsort](https://github.com/TheAlgorithms/Swift/blob/master/sorts/InsertionSort.swift)
   * [Quicksort](https://github.com/TheAlgorithms/Swift/blob/master/sorts/QuickSort.swift)
   * [Selectionsort](https://github.com/TheAlgorithms/Swift/blob/master/sorts/SelectionSort.swift)
+  * [MergeSort](https://github.com/TheAlgorithms/Swift/blob/master/sorts/MergeSort.swift)
 
 ## Trees
   * [Tree](https://github.com/TheAlgorithms/Swift/blob/master/trees/tree.swift)

sorts/MergeSort.swift

@@ -0,0 +1,69 @@
+
+import Foundation
+
+extension Array where Element: Comparable {
+
+	mutating func mergeSort(by comparison: (Element, Element) -> Bool) {
+		guard self.count > 1 else {
+			return
+		}
+		_mergeSort(from: 0, to: count - 1, by: comparison)
+	}
+
+	mutating private func _mergeSort(
+		from left: Int,
+		to right: Int,
+		by comparison: (Element, Element) -> Bool
+	) {
+		if left < right {
+			let mid = left + (right - left) / 2
+			_mergeSort(from: 0, to: mid, by: comparison)
+			_mergeSort(from: mid + 1, to: right, by: comparison)
+			_merge(from: left, mid: mid, to: right, by: comparison)
+		}
+	}
+
+	mutating private func _merge(
+		from left: Int,
+		mid: Int,
+		to right: Int,
+		by comparison: (Element, Element) -> Bool
+	) {
+		var copy = [Element](repeating: self[left], count: right - left + 1)
+		var (leftStartIndex, rightStartIndex, currentIndex) = (left, mid + 1, 0)
+		for _ in left ... right {
+			if leftStartIndex > mid {
+				copy[currentIndex] = self[rightStartIndex]
+				rightStartIndex += 1
+			} else if rightStartIndex > right {
+				copy[currentIndex] = self[leftStartIndex]
+				leftStartIndex += 1
+			} else if comparison(self[leftStartIndex], self[rightStartIndex]) {
+				copy[currentIndex] = self[leftStartIndex]
+				leftStartIndex += 1
+			} else {
+				copy[currentIndex] = self[rightStartIndex]
+				rightStartIndex += 1
+			}
+			currentIndex += 1
+		}
+		leftStartIndex = left
+		for i in copy.indices {
+			self[leftStartIndex] = copy[i]
+			leftStartIndex += 1
+		}
+	}
+
+	func mergeSorted(by comparison: (Element, Element) -> Bool) -> Array {
+		var copy = self
+		copy.mergeSort(by: comparison)
+		return copy
+	}
+
+}
+
+// The code below can be used for testing
+// var numberList = [15, 2, 23, 11, 3, 9]
+// debugPrint(numberList.mergeSorted(by: >))
+// numberList.mergeSort(by: <)
+// debugPrint(numberList)