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+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.
+// http://code.google.com/p/protobuf/
+//
+// 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.protobuf;
+
+import com.google.protobuf.Descriptors.Descriptor;
+import com.google.protobuf.Descriptors.FieldDescriptor;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * A table of known extensions, searchable by name or field number.  When
+ * parsing a protocol message that might have extensions, you must provide
+ * an {@code ExtensionRegistry} in which you have registered any extensions
+ * that you want to be able to parse.  Otherwise, those extensions will just
+ * be treated like unknown fields.
+ *
+ * <p>For example, if you had the {@code .proto} file:
+ *
+ * <pre>
+ * option java_class = "MyProto";
+ *
+ * message Foo {
+ *   extensions 1000 to max;
+ * }
+ *
+ * extend Foo {
+ *   optional int32 bar;
+ * }
+ * </pre>
+ *
+ * Then you might write code like:
+ *
+ * <pre>
+ * ExtensionRegistry registry = ExtensionRegistry.newInstance();
+ * registry.add(MyProto.bar);
+ * MyProto.Foo message = MyProto.Foo.parseFrom(input, registry);
+ * </pre>
+ *
+ * <p>Background:
+ *
+ * <p>You might wonder why this is necessary.  Two alternatives might come to
+ * mind.  First, you might imagine a system where generated extensions are
+ * automatically registered when their containing classes are loaded.  This
+ * is a popular technique, but is bad design; among other things, it creates a
+ * situation where behavior can change depending on what classes happen to be
+ * loaded.  It also introduces a security vulnerability, because an
+ * unprivileged class could cause its code to be called unexpectedly from a
+ * privileged class by registering itself as an extension of the right type.
+ *
+ * <p>Another option you might consider is lazy parsing: do not parse an
+ * extension until it is first requested, at which point the caller must
+ * provide a type to use.  This introduces a different set of problems.  First,
+ * it would require a mutex lock any time an extension was accessed, which
+ * would be slow.  Second, corrupt data would not be detected until first
+ * access, at which point it would be much harder to deal with it.  Third, it
+ * could violate the expectation that message objects are immutable, since the
+ * type provided could be any arbitrary message class.  An unpriviledged user
+ * could take advantage of this to inject a mutable object into a message
+ * belonging to priviledged code and create mischief.
+ *
+ * @author kenton@google.com Kenton Varda
+ */
+public final class ExtensionRegistry {
+  /** Construct a new, empty instance. */
+  public static ExtensionRegistry newInstance() {
+    return new ExtensionRegistry(
+      new HashMap<String, ExtensionInfo>(),
+      new HashMap<DescriptorIntPair, ExtensionInfo>());
+  }
+
+  /** Get the unmodifiable singleton empty instance. */
+  public static ExtensionRegistry getEmptyRegistry() {
+    return EMPTY;
+  }
+
+  /** Returns an unmodifiable view of the registry. */
+  public ExtensionRegistry getUnmodifiable() {
+    return new ExtensionRegistry(
+      Collections.unmodifiableMap(extensionsByName),
+      Collections.unmodifiableMap(extensionsByNumber));
+  }
+
+  /** A (Descriptor, Message) pair, returned by lookup methods. */
+  public static final class ExtensionInfo {
+    /** The extension's descriptor. */
+    public final FieldDescriptor descriptor;
+
+    /**
+     * A default instance of the extension's type, if it has a message type.
+     * Otherwise, {@code null}.
+     */
+    public final Message defaultInstance;
+
+    private ExtensionInfo(FieldDescriptor descriptor) {
+      this.descriptor = descriptor;
+      this.defaultInstance = null;
+    }
+    private ExtensionInfo(FieldDescriptor descriptor, Message defaultInstance) {
+      this.descriptor = descriptor;
+      this.defaultInstance = defaultInstance;
+    }
+  }
+
+  /**
+   * Find an extension by fully-qualified field name, in the proto namespace.
+   * I.e. {@code result.descriptor.fullName()} will match {@code fullName} if
+   * a match is found.
+   *
+   * @return Information about the extension if found, or {@code null}
+   *         otherwise.
+   */
+  public ExtensionInfo findExtensionByName(String fullName) {
+    return extensionsByName.get(fullName);
+  }
+
+  /**
+   * Find an extension by containing type and field number.
+   *
+   * @return Information about the extension if found, or {@code null}
+   *         otherwise.
+   */
+  public ExtensionInfo findExtensionByNumber(Descriptor containingType,
+                                             int fieldNumber) {
+    return extensionsByNumber.get(
+      new DescriptorIntPair(containingType, fieldNumber));
+  }
+
+  /** Add an extension from a generated file to the registry. */
+  public void add(GeneratedMessage.GeneratedExtension<?, ?> extension) {
+    if (extension.getDescriptor().getJavaType() ==
+        FieldDescriptor.JavaType.MESSAGE) {
+      add(new ExtensionInfo(extension.getDescriptor(),
+                            extension.getMessageDefaultInstance()));
+    } else {
+      add(new ExtensionInfo(extension.getDescriptor(), null));
+    }
+  }
+
+  /** Add a non-message-type extension to the registry by descriptor. */
+  public void add(FieldDescriptor type) {
+    if (type.getJavaType() == FieldDescriptor.JavaType.MESSAGE) {
+      throw new IllegalArgumentException(
+        "ExtensionRegistry.add() must be provided a default instance when " +
+        "adding an embedded message extension.");
+    }
+    add(new ExtensionInfo(type, null));
+  }
+
+  /** Add a message-type extension to the registry by descriptor. */
+  public void add(FieldDescriptor type, Message defaultInstance) {
+    if (type.getJavaType() != FieldDescriptor.JavaType.MESSAGE) {
+      throw new IllegalArgumentException(
+        "ExtensionRegistry.add() provided a default instance for a " +
+        "non-message extension.");
+    }
+    add(new ExtensionInfo(type, defaultInstance));
+  }
+
+  // =================================================================
+  // Private stuff.
+
+  private ExtensionRegistry(
+      Map<String, ExtensionInfo> extensionsByName,
+      Map<DescriptorIntPair, ExtensionInfo> extensionsByNumber) {
+    this.extensionsByName = extensionsByName;
+    this.extensionsByNumber = extensionsByNumber;
+  }
+
+  private final Map<String, ExtensionInfo> extensionsByName;
+  private final Map<DescriptorIntPair, ExtensionInfo> extensionsByNumber;
+
+  private static final ExtensionRegistry EMPTY =
+    new ExtensionRegistry(
+      Collections.<String, ExtensionInfo>emptyMap(),
+      Collections.<DescriptorIntPair, ExtensionInfo>emptyMap());
+
+  private void add(ExtensionInfo extension) {
+    if (!extension.descriptor.isExtension()) {
+      throw new IllegalArgumentException(
+        "ExtensionRegistry.add() was given a FieldDescriptor for a regular " +
+        "(non-extension) field.");
+    }
+
+    extensionsByName.put(extension.descriptor.getFullName(), extension);
+    extensionsByNumber.put(
+      new DescriptorIntPair(extension.descriptor.getContainingType(),
+                            extension.descriptor.getNumber()),
+      extension);
+
+    FieldDescriptor field = extension.descriptor;
+    if (field.getContainingType().getOptions().getMessageSetWireFormat() &&
+        field.getType() == FieldDescriptor.Type.MESSAGE &&
+        field.isOptional() &&
+        field.getExtensionScope() == field.getMessageType()) {
+      // This is an extension of a MessageSet type defined within the extension
+      // type's own scope.  For backwards-compatibility, allow it to be looked
+      // up by type name.
+      extensionsByName.put(field.getMessageType().getFullName(), extension);
+    }
+  }
+
+  /** A (GenericDescriptor, int) pair, used as a map key. */
+  private static final class DescriptorIntPair {
+    final Descriptor descriptor;
+    final int number;
+
+    DescriptorIntPair(Descriptor descriptor, int number) {
+      this.descriptor = descriptor;
+      this.number = number;
+    }
+
+    public int hashCode() {
+      return descriptor.hashCode() * ((1 << 16) - 1) + number;
+    }
+    public boolean equals(Object obj) {
+      if (!(obj instanceof DescriptorIntPair)) return false;
+      DescriptorIntPair other = (DescriptorIntPair)obj;
+      return descriptor == other.descriptor && number == other.number;
+    }
+  }
+}