[Foundation] Correct sequence initializers for Data when repeating:count: is called and add a memset fast-path

stdlib/public/SDK/Foundation/Data.swift

@@ -1010,6 +1010,17 @@ public struct Data : ReferenceConvertible, Equatable, Hashable, RandomAccessColl
             return _DataStorage(bytes: $0.baseAddress, length: $0.count)
         }
     }
+
+    /// Initialze a `Data` with a repeating byte pattern
+    ///
+    /// - parameter repeatedValue: A byte to initialze the pattern
+    /// - parameter count: The number of bytes the data initially contains initialzed to the repeatedValue
+    public init(repeating repeatedValue: UInt8, count: Int) {
+        self.init(count: count)
+        withUnsafeMutableBytes { (bytes: UnsafeMutablePointer<UInt8>) -> Void in
+            memset(bytes, Int32(repeatedValue), count)
+        }
+    }
 
     /// Initialize a `Data` with the specified size.
     ///
@@ -1330,9 +1341,12 @@ public struct Data : ReferenceConvertible, Equatable, Hashable, RandomAccessColl
         }
         count += estimatedCount
         for byte in newElements {
+            let newIndex = idx + 1
+            if newIndex > count {
+                count = newIndex
+            }
             self[idx] = byte
-            idx += 1
-            count = idx
+            idx = newIndex
         }
     }
 

test/stdlib/TestData.swift

@@ -907,6 +907,20 @@ class TestData : TestDataSuper {
         }
         expectEqual(len, 1)
     }
+
+    func test_repeatingValueInitialization() {
+        var d = Data(repeating: 0x01, count: 3)
+        let elements = repeatElement(UInt8(0x02), count: 3) // ensure we fall into the sequence case
+        d.append(contentsOf: elements)
+
+        expectEqual(d[0], 0x01)
+        expectEqual(d[1], 0x01)
+        expectEqual(d[2], 0x01)
+
+        expectEqual(d[3], 0x02)
+        expectEqual(d[4], 0x02)
+        expectEqual(d[5], 0x02)
+    }
 }
 
 #if !FOUNDATION_XCTEST
@@ -951,6 +965,7 @@ DataTests.test("test_AnyHashableContainingData") { TestData().test_AnyHashableCo
 DataTests.test("test_AnyHashableCreatedFromNSData") { TestData().test_AnyHashableCreatedFromNSData() }
 DataTests.test("test_noCopyBehavior") { TestData().test_noCopyBehavior() }
 DataTests.test("test_doubleDeallocation") { TestData().test_doubleDeallocation() }
+DataTests.test("test_repeatingValueInitialization") { TestData().test_repeatingValueInitialization() }
 
 // XCTest does not have a crash detection, whereas lit does
 DataTests.test("bounding failure subdata") {