1 files changed, 59 insertions, 0 deletions
diff --git a/src/main/java/com/google/devtools/build/lib/collect/CompactImmutableMap.java b/src/main/java/com/google/devtools/build/lib/collect/CompactImmutableMap.java
new file mode 100644
index 0000000000..d410d0cdba
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/collect/CompactImmutableMap.java
@@ -0,0 +1,59 @@
+// Copyright 2017 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.devtools.build.lib.collect;
+
+import java.util.Iterator;
+
+/**
+ * A minimal map interface that avoids methods whose implementation tends to force GC churn, or
+ * otherwise overly constrain implementation freedom.
+ *
+ * <p>TODO: Convert to interface once we move to Java 8.
+ */
+public abstract class CompactImmutableMap<K, V> implements Iterable<K> {
+
+  public boolean containsKey(K key) {
+    return get(key) != null;
+  }
+
+  public abstract V get(K key);
+
+  public abstract int size();
+
+  public abstract K keyAt(int index);
+
+  public abstract V valueAt(int index);
+
+  @Override
+  public Iterator<K> iterator() {
+    return new ImmutableMapSharedKeysIterator();
+  }
+
+  private class ImmutableMapSharedKeysIterator implements Iterator<K> {
+    int index = 0;
+
+    @Override
+    public boolean hasNext() {
+      return index < size();
+    }
+
+    @Override
+    public K next() {
+      K key = keyAt(index);
+      ++index;
+      return key;
+    }
+  }
+}