[stdlib] Add & implement Random._fill(bytes:) requirement

(cherry picked from commit 12a2b32)

Author: lorentey

stdlib/public/core/Random.swift

@@ -54,6 +54,26 @@ public protocol RandomNumberGenerator {
   ///
   /// - Returns: An unsigned 64-bit random value.
   mutating func next() -> UInt64
+
+  // FIXME: De-underscore after swift-evolution amendment
+  mutating func _fill(bytes buffer: UnsafeMutableRawBufferPointer)
+}
+
+extension RandomNumberGenerator {
+  public mutating func _fill(bytes buffer: UnsafeMutableRawBufferPointer) {
+    // FIXME: Optimize
+    var chunk: UInt64 = 0
+    var chunkBytes = 0
+    for i in 0..<buffer.count {
+      if chunkBytes == 0 {
+        chunk = next()
+        chunkBytes = UInt64.bitWidth / 8
+      }
+      buffer[i] = UInt8(truncatingIfNeeded: chunk)
+      chunk >>= UInt8.bitWidth
+      chunkBytes -= 1
+    }
+  }
 }
 
 extension RandomNumberGenerator {
@@ -157,11 +177,10 @@ public struct Random : RandomNumberGenerator {
     _stdlib_random(&random, MemoryLayout<T>.size)
     return random
   }
-}
 
-@usableFromInline internal// @testable
-func _stdlib_random(_ bytes: UnsafeMutableRawBufferPointer) {
-  if !bytes.isEmpty {
-    _stdlib_random(bytes.baseAddress!, bytes.count)
+  public mutating func _fill(bytes buffer: UnsafeMutableRawBufferPointer) {
+    if !buffer.isEmpty {
+      _stdlib_random(buffer.baseAddress!, buffer.count)
+    }
   }
 }

test/stdlib/Random.swift

@@ -6,9 +6,9 @@ import StdlibCollectionUnittest
 
 let RandomTests = TestSuite("Random")
 
-// _stdlib_random
+// _fill(bytes:)
 
-RandomTests.test("_stdlib_random") {
+RandomTests.test("_fill(bytes:)") {
   for count in [100, 1000] {
     var bytes1 = [UInt8](repeating: 0, count: count)
     var bytes2 = [UInt8](repeating: 0, count: count)
@@ -17,11 +17,11 @@ RandomTests.test("_stdlib_random") {
     expectEqual(bytes1, zeros)
     expectEqual(bytes2, zeros)
 
-    bytes1.withUnsafeMutableBytes { _stdlib_random($0) }
+    bytes1.withUnsafeMutableBytes { Random.default._fill(bytes: $0) }
     expectNotEqual(bytes1, bytes2)
     expectNotEqual(bytes1, zeros)
 
-    bytes2.withUnsafeMutableBytes { _stdlib_random($0) }
+    bytes2.withUnsafeMutableBytes { Random.default._fill(bytes: $0) }
     expectNotEqual(bytes1, bytes2)
     expectNotEqual(bytes2, zeros)
   }