Merge pull request #2670 from apple/stdlib-se-0078-reverse

stdlib: implement reverse() as proposed in SE-0078

Author: gribozavr

stdlib/private/StdlibCollectionUnittest/CheckMutableCollectionType.swift.gyb

@@ -685,6 +685,20 @@ if resiliencyChecks.subscriptOnOutOfBoundsIndicesBehavior != .none {
   }
 }
 
+//===----------------------------------------------------------------------===//
+// reverse()
+//===----------------------------------------------------------------------===//
+
+self.test("\(testNamePrefix).reverse()") {
+  for test in reverseTests {
+    var c = makeWrappedCollection(test.sequence.map(OpaqueValue.init))
+    c.reverse()
+    expectEqual(
+      test.expected, c.map { extractValue($0).value },
+      stackTrace: SourceLocStack().with(test.loc))
+  }
+}
+
 //===----------------------------------------------------------------------===//
 
   } // addMutableBidirectionalCollectionTests

stdlib/public/core/Reverse.swift

@@ -1,4 +1,4 @@
-//===--- Reverse.swift - Lazy sequence reversal ---------------------------===//
+//===--- Reverse.swift - Sequence and collection reversal -----------------===//
 //
 // This source file is part of the Swift.org open source project
 //
@@ -10,6 +10,28 @@
 //
 //===----------------------------------------------------------------------===//
 
+extension MutableCollection where Self : BidirectionalCollection {
+  /// Reverses the elements of the collection in place.
+  ///
+  ///     var characters: [Character] = ["C", "a", "f", "é"]
+  ///     characters.reverse()
+  ///     print(cafe.characters)
+  ///     // Prints "["é", "f", "a", "C"]
+  ///
+  /// - Complexity: O(*n*), where *n* is the number of elements in the
+  ///   collection.
+  public mutating func reverse() {
+    if isEmpty { return }
+    var f = startIndex
+    var l = index(before: endIndex)
+    while f < l {
+      swap(&self[f], &self[l])
+      formIndex(after: &f)
+      formIndex(before: &l)
+    }
+  }
+}
+
 // FIXME(ABI)(compiler limitation): we should have just one type,
 // `ReversedCollection`, that has conditional conformances to
 // `RandomAccessCollection`, and possibly `MutableCollection` and