Map merging.

pkg/analyzer/lib/src/util/yaml.dart

Index: pkg/analyzer/lib/src/util/yaml.dart
diff --git a/pkg/analyzer/lib/src/util/yaml.dart b/pkg/analyzer/lib/src/util/yaml.dart
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+++ b/pkg/analyzer/lib/src/util/yaml.dart
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+// 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 analyzer.src.util.yaml;
+
+import 'dart:collection';
+
+/// Merges two maps (of yaml) with simple override semantics, suitable for
+/// merging two maps where one map defines default values that are added to
+/// (and possibly overriden) by an overriding map.
+class Merger {
+  /// Merges a default [o1] with an overriding object [o2].
+  ///
+  ///   * lists are merged (without duplicates).
+  ///   * lists of scalar values can be promoted to simple maps when merged with
+  ///     maps of strings to booleans (e.g., ['opt1', 'opt2'] becomes
+  ///     {'opt1': true, 'opt2': true}.
+  ///   * maps are merged recursively.
+  ///   * if map values cannot be merged, the overriding value is taken.
+  ///
+  Object merge(Object o1, Object o2) {
+    // Handle promotion first.
+    if (o1 is List && isMapToBools(o2)) {
+      o1 = new Map.fromIterable(o1, key: (item) => item, value: (item) => true);
+    } else if (isMapToBools(o1) && o2 is List) {
+      o2 = new Map.fromIterable(o2, key: (item) => item, value: (item) => true);
+    }
+
+    if (o1 is Map && o2 is Map) {
+      return mergeMap(o1, o2);
+    }
+    if (o1 is List && o2 is List) {
+      return mergeList(o1, o2);
+    }
+    // Default to override.
+    return o2;
+  }
+
+  /// Merge lists, avoiding duplicates.
+  List mergeList(List l1, List l2) =>
+      new List()..addAll(l1)..addAll(l2.where((item) => !l1.contains(item)));
+
+  /// Merge maps (recursively).
+  Map mergeMap(Map m1, Map m2) {
+    Map merged = new HashMap()..addAll(m1);
+    m2.forEach((k, v) {
+      merged[k] = merge(merged[k], v);
+    });
+    return merged;
+  }
+
+  static bool isMapToBools(Object o) =>
+      o is Map && o.values.every((v) => v is bool);
+}