Dart - optimize writeString for ASCII (#6736)

* Dart - optimize writeString for ASCII

* Dart - writeString() ASCII optimization optional

dart/lib/flat_buffers.dart

@@ -668,29 +668,68 @@ class Builder {
     return result;
   }
 
-  /// Write the given string [value] and return its offset.
-  int writeString(String value) {
+  /// Write the given string [value] and return its offset
+  ///
+  /// Dart strings are UTF-16 but must be stored as UTF-8 in FlatBuffers.
+  /// If the given string consists only of ASCII characters, you can indicate
+  /// enable [asciiOptimization]. In this mode, [writeString()] first tries to
+  /// copy the ASCII string directly to the output buffer and if that fails
+  /// (because there are no-ASCII characters in the string) it falls back and to
+  /// the default UTF-16 -> UTF-8 conversion (with slight performance penalty).
+  int? writeString(String value, {bool asciiOptimization = false}) {
     _ensureNoVTable();
     if (_strings != null) {
-      return _strings!.putIfAbsent(value, () => _writeString(value));
+      return _strings!
+          .putIfAbsent(value, () => _writeString(value, asciiOptimization));
     } else {
-      return _writeString(value);
+      return _writeString(value, asciiOptimization);
     }
   }
 
-  int _writeString(String value) {
-    // TODO(scheglov) optimize for ASCII strings
-    List<int> bytes = utf8.encode(value);
-    int length = bytes.length;
+  int _writeString(String value, bool asciiOptimization) {
+    if (asciiOptimization) {
+      // [utf8.encode()] is slow (up to at least Dart SDK 2.13). If the given
+      // string is ASCII we can just write it directly, without any conversion.
+      final originalTail = _tail;
+      if (_tryWriteASCIIString(value)) return _tail;
+      // if non-ASCII: reset the output buffer position for [_writeUTFString()]
+      _tail = originalTail;
+    }
+    _writeUTFString(value);
+    return _tail;
+  }
+
+  // Try to write the string as ASCII, return false if there's a non-ascii char.
+  @pragma('vm:prefer-inline')
+  bool _tryWriteASCIIString(String value) {
+    _prepare(4, 1, additionalBytes: value.length + 1);
+    final length = value.length;
+    var offset = _buf.lengthInBytes - _tail + 4;
+    for (var i = 0; i < length; i++) {
+      // utf16 code unit, e.g. for '†' it's [0x20 0x20], which is 8224 decimal.
+      // ASCII characters go from 0x00 to 0x7F (which is 0 to 127 decimal).
+      final char = value.codeUnitAt(i);
+      if ((char & ~0x7F) != 0) {
+        return false;
+      }
+      _buf.setUint8(offset++, char);
+    }
+    _buf.setUint8(offset, 0); // trailing zero
+    _setUint32AtTail(_buf, _tail, value.length);
+    return true;
+  }
+
+  @pragma('vm:prefer-inline')
+  void _writeUTFString(String value) {
+    final bytes = utf8.encode(value) as Uint8List;
+    final length = bytes.length;
     _prepare(4, 1, additionalBytes: length + 1);
-    final int result = _tail;
     _setUint32AtTail(_buf, _tail, length);
-    int offset = _buf.lengthInBytes - _tail + 4;
+    var offset = _buf.lengthInBytes - _tail + 4;
     for (int i = 0; i < length; i++) {
       _buf.setUint8(offset++, bytes[i]);
     }
     _buf.setUint8(offset, 0); // trailing zero
-    return result;
   }
 
   /// Throw an exception if there is not currently a vtable.

dart/test/flat_buffers_test.dart

@@ -160,7 +160,7 @@ class BuilderTest {
     final str = fbBuilder.writeString('MyMonster');
 
     fbBuilder.writeString('test1');
-    fbBuilder.writeString('test2');
+    fbBuilder.writeString('test2', asciiOptimization: true);
     final testArrayOfString = fbBuilder.endStructVector(2);
 
     final fred = fbBuilder.writeString('Fred');
@@ -360,8 +360,10 @@ class BuilderTest {
     List<int> byteList;
     {
       Builder builder = new Builder(initialSize: 0);
-      int? latinStringOffset = builder.writeString(latinString);
-      int? unicodeStringOffset = builder.writeString(unicodeString);
+      int? latinStringOffset =
+          builder.writeString(latinString, asciiOptimization: true);
+      int? unicodeStringOffset =
+          builder.writeString(unicodeString, asciiOptimization: true);
       builder.startTable(2);
       builder.addOffset(0, latinStringOffset);
       builder.addOffset(1, unicodeStringOffset);