mergeSort in swift

swift/12_sorts/SortsTests.swift

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+//
+//  SortsTests.swift
+//  SortsTests
+//
+//  Created by Wenru Dong on 2018/10/14.
+//  Copyright © 2018年 Wenru Dong. All rights reserved.
+//
+
+import XCTest
+
+class SortsTests: XCTestCase {
+    
+    override func setUp() {
+        super.setUp()
+        // Put setup code here. This method is called before the invocation of each test method in the class.
+    }
+    
+    override func tearDown() {
+        // Put teardown code here. This method is called after the invocation of each test method in the class.
+        super.tearDown()
+    }
+    
+    func testMergeSort() {
+        var a1 = [1, 1, 1, 1]
+        mergeSort(&a1)
+        XCTAssertEqual(a1, [1, 1, 1, 1])
+        
+        var a2 = [4, 3, 2, 1]
+        mergeSort(&a2)
+        XCTAssertEqual(a2, [1, 2, 3, 4])
+        
+        var a3 = [3, 6, 9, 7, 8, -1, 9, 3, -2, 0]
+        mergeSort(&a3)
+        XCTAssertEqual(a3, [-2, -1, 0, 3, 3, 6, 7, 8, 9, 9])
+    }
+    
+    func testPerformanceExample() {
+        // This is an example of a performance test case.
+        self.measure {
+            // Put the code you want to measure the time of here.
+        }
+    }
+    
+}

swift/12_sorts/mergeSort.swift

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+//
+//  sorts.swift
+//  algo
+//
+//  Created by Wenru Dong on 2018/10/14.
+//  Copyright © 2018年 Wenru Dong. All rights reserved.
+//
+
+import Foundation
+
+public func mergeSort<T>(_ a: inout T) where T: RandomAccessCollection, T: MutableCollection, T.Element: Comparable {
+    mergeSort(&a, from: a.startIndex, to: a.index(before: a.endIndex))
+}
+
+fileprivate func mergeSort<T>(_ a: inout T, from low: T.Index, to high: T.Index) where T: RandomAccessCollection, T: MutableCollection, T.Element: Comparable {
+    if low >= high { return }
+    let dist = a.distance(from: low, to: high)
+    let mid = a.index(low, offsetBy: dist/2)
+    mergeSort(&a, from: low, to: mid)
+    mergeSort(&a, from: a.index(mid, offsetBy: 1), to: high)
+    print(a)
+    merge(&a, from: low, through: mid, to: high)
+}
+
+fileprivate func merge<T>(_ a: inout T, from low: T.Index, through mid: T.Index, to high: T.Index) where T: RandomAccessCollection, T: MutableCollection, T.Element: Comparable {
+    var i = low
+    var j = a.index(mid, offsetBy: 1)
+    var tmp = Array<T.Element>()
+    tmp.reserveCapacity(a.distance(from: low, to: high) + 1)
+    while i <= mid && j <= high {
+        if a[i] <= a[j] {
+            tmp.append(a[i])
+            a.formIndex(after: &i)
+        } else {
+            tmp.append(a[j])
+            a.formIndex(after: &j)
+        }
+    }
+    
+    var start = i
+    var end = mid
+    if j <= high {
+        start = j
+        end = high
+    }
+    tmp.append(contentsOf: a[start...end])
+    
+    var current = low
+    for element in tmp {
+        a[current] = element
+        print(element)
+        a.formIndex(after: &current)
+    }
+}