Add unit tests for documented examples of rotate functions

Author: mdznr

Tests/SwiftAlgorithmsTests/RotateTests.swift

@@ -13,6 +13,49 @@ import XCTest
 import Algorithms
 
 final class RotateTests: XCTestCase {
+  /// Tests `rotate(subrange:toStartAt:)` with an empty subrange
+  /// The order of elements are unchanged
+  func testRotateEmptySubrange() {
+    var numbers = [10, 20, 30, 40, 50, 60, 70, 80]
+    let oldStart = numbers.rotate(subrange: 3..<3, toStartAt: 3)
+    XCTAssertEqual(numbers, [10, 20, 30, 40, 50, 60, 70, 80])
+    XCTAssertEqual(numbers[oldStart], 40)
+  }
+  
+  /// Tests `rotate(subrange:toStartAt:)` with an empty collection
+  func testRotateSubrangeOnEmptyCollection() {
+    var numbers = [Int]()
+    let oldStart = numbers.rotate(subrange: 0..<0, toStartAt: 0)
+    XCTAssertEqual(numbers, [])
+    XCTAssertEqual(oldStart, numbers.startIndex)
+  }
+  
+  /// Tests `rotate(subrange:toStartAt:)` with the full range of the collection
+  func testRotateFullRange() {
+    var numbers = [10, 20, 30, 40, 50, 60, 70, 80]
+    let oldStart = numbers.rotate(subrange: 0..<8, toStartAt: 1)
+    XCTAssertEqual(numbers, [20, 30, 40, 50, 60, 70, 80, 10])
+    XCTAssertEqual(numbers[oldStart], 10)
+  }
+  
+  /// Tests the example given in `rotate(subrange:toStartAt:)`’s documentation
+  func testRotateSubrange() {
+    var numbers = [10, 20, 30, 40, 50, 60, 70, 80]
+    let oldStart = numbers.rotate(subrange: 0..<4, toStartAt: 2)
+    XCTAssertEqual(numbers, [30, 40, 10, 20, 50, 60, 70, 80])
+    XCTAssertEqual(numbers[oldStart], 10)
+  }
+  
+  /// Tests the example given in `rotate(toStartAt:)`’s documentation
+  func testRotateExample() {
+    var numbers = [10, 20, 30, 40, 50, 60, 70, 80]
+    let oldStart = numbers.rotate(toStartAt: 3)
+    XCTAssertEqual(numbers, [40, 50, 60, 70, 80, 10, 20, 30])
+    XCTAssertEqual(numbers[oldStart], 10)
+  }
+  
+  /// Tests `rotate(toStartAt:)` on collections of varying lengths, at different
+  /// starting points
   func testRotate() {
     for length in 0...15 {
       let a = Array(0..<length)