[cloud_firestore] Update autoId generation to match native implementation more closely (flutter#1907)

Author: KevinTheGray

packages/cloud_firestore/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.12.8
+
+* Updated how document ids are generated to more closely match native implementations.
+
 ## 0.12.7+1
 
 * Update google-services Android gradle plugin to 4.3.0 in documentation and examples.

packages/cloud_firestore/lib/cloud_firestore.dart

@@ -15,7 +15,7 @@ import 'package:flutter/foundation.dart' show ReadBuffer, WriteBuffer;
 import 'package:flutter/services.dart';
 import 'package:meta/meta.dart';
 
-import 'src/utils/push_id_generator.dart';
+import 'src/utils/auto_id_generator.dart';
 
 part 'src/blob.dart';
 part 'src/collection_reference.dart';

packages/cloud_firestore/lib/src/collection_reference.dart

@@ -40,7 +40,7 @@ class CollectionReference extends Query {
   DocumentReference document([String path]) {
     List<String> childPath;
     if (path == null) {
-      final String key = PushIdGenerator.generatePushChildName();
+      final String key = AutoIdGenerator.autoId();
       childPath = List<String>.from(_pathComponents)..add(key);
     } else {
       childPath = List<String>.from(_pathComponents)..addAll(path.split(('/')));

packages/cloud_firestore/lib/src/utils/auto_id_generator.dart

@@ -0,0 +1,34 @@
+// Copyright 2017, the Chromium project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:math';
+
+/// Utility class for generating Firebase child node keys.
+///
+/// Since the Flutter plugin API is asynchronous, there's no way for us
+/// to use the native SDK to generate the node key synchronously and we
+/// have to do it ourselves if we want to be able to reference the
+/// newly-created node synchronously.
+///
+/// This code is based largely on the Android implementation and ported to Dart.
+
+class AutoIdGenerator {
+  static const int _AUTO_ID_LENGTH = 20;
+
+  static const String _AUTO_ID_ALPHABET =
+      'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
+
+  static final Random _random = Random();
+
+  static String autoId() {
+    final StringBuffer stringBuffer = StringBuffer();
+    final int maxRandom = _AUTO_ID_ALPHABET.length;
+
+    for (int i = 0; i < _AUTO_ID_LENGTH; ++i) {
+      stringBuffer.write(_AUTO_ID_ALPHABET[_random.nextInt(maxRandom)]);
+    }
+
+    return stringBuffer.toString();
+  }
+}