Base64: Speedup decoding fix 3

Sources/Foundation/NSData.swift

@@ -206,9 +206,22 @@ open class NSData : NSObject, NSCopying, NSMutableCopying, NSSecureCoding {
 
     /// Initializes a data object with the given Base64 encoded string.
     public init?(base64Encoded base64String: String, options: Base64DecodingOptions = []) {
-        guard var decodedBytes = NSData.base64DecodeBytes(base64String.utf8, options: options) else {
-            return nil
+
+        let result: UnsafeMutableRawBufferPointer?
+        if let _result = base64String.utf8.withContiguousStorageIfAvailable({ buffer -> UnsafeMutableRawBufferPointer? in
+            let rawBuffer = UnsafeRawBufferPointer(start: buffer.baseAddress!, count: buffer.count)
+            return NSData.base64DecodeBytes(rawBuffer, options: options)
+        }) {
+            result = _result
+        } else {
+            // Slow path, unlikely that withContiguousStorageIfAvailable will fail but if it does, fall back to .utf8CString.
+            // This will allocate and copy but it is the simplest way to get a contiguous buffer.
+            result = base64String.utf8CString.withUnsafeBufferPointer { buffer -> UnsafeMutableRawBufferPointer? in
+                let rawBuffer = UnsafeRawBufferPointer(start: buffer.baseAddress!, count: buffer.count - 1) // -1 to ignore the terminating NUL
+                return NSData.base64DecodeBytes(rawBuffer, options: options)
+            }
         }
+        guard let decodedBytes = result else { return nil }
         super.init()
         _init(bytes: decodedBytes.baseAddress!, length: decodedBytes.count, copy: false, deallocator: { (ptr, length) in
             ptr.deallocate()
@@ -217,7 +230,9 @@ open class NSData : NSObject, NSCopying, NSMutableCopying, NSSecureCoding {
 
     /// Initializes a data object with the given Base64 encoded data.
     public init?(base64Encoded base64Data: Data, options: Base64DecodingOptions = []) {
-        guard var decodedBytes = NSData.base64DecodeBytes(base64Data, options: options) else {
+        guard let decodedBytes = base64Data.withUnsafeBytes({ rawBuffer in
+                NSData.base64DecodeBytes(rawBuffer, options: options)
+        }) else {
             return nil
         }
         super.init()
@@ -638,7 +653,7 @@ open class NSData : NSObject, NSCopying, NSMutableCopying, NSSecureCoding {
      - parameter options:    Options for handling invalid input
      - returns:              The decoded bytes.
      */
-    private static func base64DecodeBytes<T: Collection>(_ bytes: T, options: Base64DecodingOptions = []) -> UnsafeMutableRawBufferPointer? where T.Element == UInt8 {
+    private static func base64DecodeBytes(_ bytes: UnsafeRawBufferPointer, options: Base64DecodingOptions = []) -> UnsafeMutableRawBufferPointer? {
 
         // This table maps byte values 0-127, input bytes >127 are always invalid.
         // Map the ASCII characters "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/" -> 0...63