make app ids more unique (#6113)

* make app ids more unique

* in-line the uuid class

packages/flutter_tools/lib/src/base/utils.dart

@@ -5,6 +5,7 @@
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
+import 'dart:math' show Random;
 
 import 'package:crypto/crypto.dart';
 import 'package:path/path.dart' as path;
@@ -161,3 +162,37 @@ class SettingsFile {
     }).join('\n'));
   }
 }
+
+/// A UUID generator. This will generate unique IDs in the format:
+///
+///     f47ac10b-58cc-4372-a567-0e02b2c3d479
+///
+/// The generated uuids are 128 bit numbers encoded in a specific string format.
+///
+/// For more information, see
+/// http://en.wikipedia.org/wiki/Universally_unique_identifier.
+class Uuid {
+  Random _random = new Random();
+
+  /// Generate a version 4 (random) uuid. This is a uuid scheme that only uses
+  /// random numbers as the source of the generated uuid.
+  String generateV4() {
+    // Generate xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx / 8-4-4-4-12.
+    int special = 8 + _random.nextInt(4);
+
+    return
+      '${_bitsDigits(16, 4)}${_bitsDigits(16, 4)}-'
+          '${_bitsDigits(16, 4)}-'
+          '4${_bitsDigits(12, 3)}-'
+          '${_printDigits(special,  1)}${_bitsDigits(12, 3)}-'
+          '${_bitsDigits(16, 4)}${_bitsDigits(16, 4)}${_bitsDigits(16, 4)}';
+  }
+
+  String _bitsDigits(int bitCount, int digitCount) =>
+      _printDigits(_generateBits(bitCount), digitCount);
+
+  int _generateBits(int bitCount) => _random.nextInt(1 << bitCount);
+
+  String _printDigits(int value, int count) =>
+      value.toRadixString(16).padLeft(count, '0');
+}

packages/flutter_tools/lib/src/commands/daemon.dart

@@ -9,6 +9,7 @@ import 'dart:io';
 import '../android/android_device.dart';
 import '../base/context.dart';
 import '../base/logger.dart';
+import '../base/utils.dart';
 import '../build_info.dart';
 import '../cache.dart';
 import '../device.dart';
@@ -280,9 +281,9 @@ class AppDomain extends Domain {
     registerHandler('discover', discover);
   }
 
-  static int _nextAppId = 0;
+  static Uuid _uuidGenerator = new Uuid();
 
-  static String _getNextAppId() => 'app-${_nextAppId++}';
+  static String _getNewAppId() => _uuidGenerator.generateV4();
 
   List<AppInstance> _apps = <AppInstance>[];
 
@@ -341,7 +342,7 @@ class AppDomain extends Domain {
 
     bool supportsRestart = enableHotReload ? device.supportsHotMode : device.supportsRestart;
 
-    AppInstance app = new AppInstance(_getNextAppId(), runner);
+    AppInstance app = new AppInstance(_getNewAppId(), runner);
     _apps.add(app);
     _sendAppEvent(app, 'start', <String, dynamic>{
       'deviceId': deviceId,

packages/flutter_tools/test/utils_test.dart

@@ -18,4 +18,65 @@ baz=qux
       expect(file.values, hasLength(2));
     });
   });
+
+  group('uuid', () {
+    // xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx
+    test('simple', () {
+      Uuid uuid = new Uuid();
+      String result = uuid.generateV4();
+      expect(result.length, 36);
+      expect(result[8], '-');
+      expect(result[13], '-');
+      expect(result[18], '-');
+      expect(result[23], '-');
+    });
+
+    test('can parse', () {
+      Uuid uuid = new Uuid();
+      String result = uuid.generateV4();
+      expect(int.parse(result.substring(0, 8), radix: 16), isNotNull);
+      expect(int.parse(result.substring(9, 13), radix: 16), isNotNull);
+      expect(int.parse(result.substring(14, 18), radix: 16), isNotNull);
+      expect(int.parse(result.substring(19, 23), radix: 16), isNotNull);
+      expect(int.parse(result.substring(24, 36), radix: 16), isNotNull);
+    });
+
+    test('special bits', () {
+      Uuid uuid = new Uuid();
+      String result = uuid.generateV4();
+      expect(result[14], '4');
+      expect(result[19].toLowerCase(), isIn('89ab'));
+
+      result = uuid.generateV4();
+      expect(result[19].toLowerCase(), isIn('89ab'));
+
+      result = uuid.generateV4();
+      expect(result[19].toLowerCase(), isIn('89ab'));
+    });
+
+    test('is pretty random', () {
+      Set<String> set = new Set<String>();
+
+      Uuid uuid = new Uuid();
+      for (int i = 0; i < 64; i++) {
+        String val = uuid.generateV4();
+        expect(set, isNot(contains(val)));
+        set.add(val);
+      }
+
+      uuid = new Uuid();
+      for (int i = 0; i < 64; i++) {
+        String val = uuid.generateV4();
+        expect(set, isNot(contains(val)));
+        set.add(val);
+      }
+
+      uuid = new Uuid();
+      for (int i = 0; i < 64; i++) {
+        String val = uuid.generateV4();
+        expect(set, isNot(contains(val)));
+        set.add(val);
+      }
+    });
+  });
 }