Implement a custom Data.Iterator

This iterator uses an inline 32-byte buffer so it doesn't have to call
copyBytes(to:count:) for every single byte. It results in an approximate
6x speedup on my computer.

Author: Kevin Ballard

stdlib/public/SDK/Foundation/Data.swift

@@ -649,7 +649,40 @@ public struct Data : ReferenceConvertible, Equatable, Hashable, RandomAccessColl
     ///
     /// The iterator will increment byte-by-byte.
     public func makeIterator() -> Data.Iterator {
-        return IndexingIterator(_elements: self)
+        return Iterator(_data: self)
+    }
+
+    public struct Iterator : IteratorProtocol {
+        private let _data: Data
+        private var _buffer: (
+            UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8,
+            UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8,
+            UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8,
+            UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8, UInt8)
+        private var _idx: Data.Index
+        private let _endIdx: Data.Index
+
+        private init(_data: Data) {
+            self._data = _data
+            _buffer = (0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0)
+            _idx = 0
+            _endIdx = _data.endIndex
+        }
+
+        public mutating func next() -> UInt8? {
+            guard _idx < _endIdx else { return nil }
+            defer { _idx += 1 }
+            let bufferSize = sizeofValue(_buffer)
+            return withUnsafeMutablePointer(to: &_buffer) { ptr_ in
+                let ptr = UnsafeMutableRawPointer(ptr_).assumingMemoryBound(to: UInt8.self)
+                let bufferIdx = _idx % bufferSize
+                if bufferIdx == 0 {
+                    // populate the buffer
+                    _data.copyBytes(to: ptr, from: _idx..<(_endIdx - _idx > bufferSize ? _idx + bufferSize : _endIdx))
+                }
+                return ptr[bufferIdx]
+            }
+        }
     }
 
     // MARK: -

validation-test/stdlib/Data.swift

@@ -0,0 +1,32 @@
+// RUN: %target-run-simple-swift
+// REQUIRES: executable_test
+// REQUIRES: objc_interop
+
+import StdlibUnittest
+import StdlibCollectionUnittest
+import Foundation
+
+var DataTestSuite = TestSuite("Data")
+DataTestSuite.test("Data.Iterator semantics") {
+  // Empty data
+  checkSequence([], Data())
+
+  // Small data
+  checkSequence([1,2,4,8,16], Data(bytes: [1,2,4,8,16]))
+
+  // Boundary conditions
+  checkSequence([5], Data(bytes: [5]))
+  checkSequence(1...31, Data(bytes: Array(1...31)))
+  checkSequence(1...32, Data(bytes: Array(1...32)))
+  checkSequence(1...33, Data(bytes: Array(1...33)))
+
+  // Large data
+  var data = Data(count: 65535)
+  data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) -> () in
+    for i in 0..<data.count {
+      ptr[i] = UInt8(i % 23)
+    }
+  }
+  checkSequence((0..<65535).lazy.map({ UInt8($0 % 23) }), data)
+}
+runAllTests()