Generalize LRU Cache to any objects.

pkg/front_end/lib/src/byte_store/cache.dart

+// Copyright (c) 2017, 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.
+
+import 'dart:collection';
+
+/**
+ * LRU cache of objects.
+ */
+class Cache<K, V> {
+  final int _maxSizeBytes;
+  final int Function(V) _meter;
+
+  final _map = new LinkedHashMap<K, V>();
+  int _currentSizeBytes = 0;
+
+  Cache(this._maxSizeBytes, this._meter);
+
+  V get(K key, V getNotCached()) {
+    V value = _map.remove(key);
+    if (value == null) {
+      value = getNotCached();
+      if (value != null) {
+        _map[key] = value;
+        _currentSizeBytes += _meter(value);
+        _evict();
+      }
+    } else {
+      _map[key] = value;
+    }
+    return value;
+  }
+
+  void put(K key, V value) {
+    V oldValue = _map[key];
+    if (oldValue != null) {
+      _currentSizeBytes -= _meter(oldValue);
+    }
+    _map[key] = value;
+    _currentSizeBytes += _meter(value);
+    _evict();
+  }
+
+  void _evict() {
+    while (_currentSizeBytes > _maxSizeBytes) {
+      if (_map.isEmpty) {
+        // Should be impossible, since _currentSizeBytes should always match
+        // _map.  But recover anyway.
+        assert(false);
+        _currentSizeBytes = 0;
+        break;
+      }
+      K key = _map.keys.first;
+      V value = _map.remove(key);
+      _currentSizeBytes -= _meter(value);
+    }
+  }
+}