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diff --git a/src/main/java/com/google/devtools/build/lib/packages/AttributeContainer.java b/src/main/java/com/google/devtools/build/lib/packages/AttributeContainer.java
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+// Copyright 2014 Google Inc. 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.packages;
+
+import com.google.devtools.build.lib.events.Location;
+
+import java.util.BitSet;
+
+/**
+ * Provides attribute setting and retrieval for a Rule. Encapsulating attribute access
+ * here means it can be passed around independently of the Rule itself. In particular,
+ * it can be consumed by independent {@link AttributeMap} instances that can apply
+ * varying kinds of logic for determining the "value" of an attribute. For example,
+ * a configurable attribute's "value" may be a { config --> value } dictionary
+ * or a configuration-bound lookup on that dictionary, depending on the context in
+ * which it's requested.
+ *
+ * <p>This class provides the lowest-level access to attribute information. It is *not*
+ * intended to be a robust public interface, but rather just an input to {@link AttributeMap}
+ * instances. Use those instances for all domain-level attribute access.
+ */
+public class AttributeContainer {
+
+  private final RuleClass ruleClass;
+
+  // Attribute values, keyed by attribute index:
+  private final Object[] attributeValues;
+
+  // Whether an attribute value has been set explicitly in the BUILD file, keyed by attribute index.
+  private final BitSet attributeValueExplicitlySpecified;
+
+  // Attribute locations, keyed by attribute index:
+  private final Location[] attributeLocations;
+
+  /**
+   * Create a container for a rule of the given rule class.
+   */
+  AttributeContainer(RuleClass ruleClass) {
+    this.ruleClass = ruleClass;
+    this.attributeValues = new Object[ruleClass.getAttributeCount()];
+    this.attributeValueExplicitlySpecified = new BitSet(ruleClass.getAttributeCount());
+    this.attributeLocations = new Location[ruleClass.getAttributeCount()];
+  }
+
+  /**
+   * Returns an attribute value by instance, or null on no match.
+   */
+  public Object getAttr(Attribute attribute) {
+    return getAttr(attribute.getName());
+  }
+
+  /**
+   * Returns an attribute value by name, or null on no match.
+   */
+  public Object getAttr(String attrName) {
+    Integer idx = ruleClass.getAttributeIndex(attrName);
+    return idx != null ? attributeValues[idx] : null;
+  }
+
+  /**
+   * Returns true iff the given attribute exists for this rule and its value
+   * is explicitly set in the BUILD file (as opposed to its default value).
+   */
+  public boolean isAttributeValueExplicitlySpecified(Attribute attribute) {
+    return isAttributeValueExplicitlySpecified(attribute.getName());
+  }
+
+  public boolean isAttributeValueExplicitlySpecified(String attributeName) {
+    Integer idx = ruleClass.getAttributeIndex(attributeName);
+    return idx != null ? attributeValueExplicitlySpecified.get(idx) : false;
+  }
+
+  /**
+   * Returns the location of the attribute definition for this rule, or null if not found.
+   */
+  public Location getAttributeLocation(String attrName) {
+    Integer idx = ruleClass.getAttributeIndex(attrName);
+    return idx != null ? attributeLocations[idx] : null;
+  }
+
+  Object getAttributeValue(int index) {
+    return attributeValues[index];
+  }
+
+  void setAttributeValue(Attribute attribute, Object value, boolean explicit) {
+    Integer index = ruleClass.getAttributeIndex(attribute.getName());
+    attributeValues[index] = value;
+    attributeValueExplicitlySpecified.set(index, explicit);
+  }
+
+  void setAttributeValueByName(String attrName, Object value) {
+    Integer index = ruleClass.getAttributeIndex(attrName);
+    attributeValues[index] = value;
+    attributeValueExplicitlySpecified.set(index);
+  }
+
+  void setAttributeLocation(int attrIndex, Location location) {
+    attributeLocations[attrIndex] = location;
+  }
+
+  void setAttributeLocation(Attribute attribute, Location location) {
+    Integer index = ruleClass.getAttributeIndex(attribute.getName());
+    attributeLocations[index] = location;
+  }
+}