Diagnostic status reporting (#24932).

pkg/analysis_server/lib/src/utilities/average.dart

+// Copyright (c) 2015, the Dart 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.
+
+library analysis_server.src.utilities.average;
+
+/// Simple rolling average sample counter.
+class Average {
+  num _val;
+  final int _sampleCount;
+
+  /// Create an average with the given (optional) sample count size.
+  Average([this._sampleCount = 20]);
+
+  /// The current average.
+  num get value => _val ?? 0;
+
+  /// Add the given [sample].
+  void addSample(num sample) {
+    if (_val == null) {
+      _val = sample;
+    } else {
+      _val = _val * ((_sampleCount - 1) / _sampleCount) +
+          sample * (1 / _sampleCount);
+    }
+  }
+
+  @override
+  String toString() => 'average: ${value}';
+}