protobuf/src/ProtocolBuffers/ExtensionRegistryLite.cs

#region Copyright notice and license

// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
// http://github.com/jskeet/dotnet-protobufs/
// Original C++/Java/Python code:
// http://code.google.com/p/protobuf/
//
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// met:
//
//     * Redistributions of source code must retain the above copyright
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// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
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#endregion

using System.Collections.Generic;
using System;
using Google.ProtocolBuffers.Descriptors;

namespace Google.ProtocolBuffers
{
    /// <summary>
    /// A table of known extensions, searchable by name or field number.  When
    /// parsing a protocol message that might have extensions, you must provide
    /// an <see cref="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.
    /// </summary>
    /// <example>
    /// For example, if you had the <c>.proto</c> file:
    /// <code>
    /// option java_class = "MyProto";
    ///
    /// message Foo {
    ///   extensions 1000 to max;
    /// }
    ///
    /// extend Foo {
    ///   optional int32 bar;
    /// }
    /// </code>
    ///
    /// Then you might write code like:
    ///
    /// <code>
    /// extensionRegistry registry = extensionRegistry.CreateInstance();
    /// registry.Add(MyProto.Bar);
    /// MyProto.Foo message = MyProto.Foo.ParseFrom(input, registry);
    /// </code>
    /// </example>
    ///
    /// <remarks>
    /// <para>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.
    /// </para>
    /// <para>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 unprivileged user
    /// could take advantage of this to inject a mutable object into a message
    /// belonging to privileged code and create mischief.</para>
    /// </remarks>
    public sealed partial class ExtensionRegistry
    {
        class ExtensionByNameMap : Dictionary<object, Dictionary<string, IGeneratedExtensionLite>> { }
        class ExtensionByIdMap : Dictionary<ExtensionIntPair, IGeneratedExtensionLite> { }

        private static readonly ExtensionRegistry empty = new ExtensionRegistry(
            new ExtensionByNameMap(),
            new ExtensionByIdMap(),
            true);

        private readonly ExtensionByNameMap extensionsByName;
        private readonly ExtensionByIdMap extensionsByNumber;

        private readonly bool readOnly;

        private ExtensionRegistry(ExtensionByNameMap byName, ExtensionByIdMap byNumber, bool readOnly)
        {
            this.extensionsByName = byName;
            this.extensionsByNumber = byNumber;
            this.readOnly = readOnly;
        }

        /// <summary>
        /// Construct a new, empty instance.
        /// </summary>
        public static ExtensionRegistry CreateInstance()
        {
            return new ExtensionRegistry(new ExtensionByNameMap(), new ExtensionByIdMap(), false);
        }

        public ExtensionRegistry AsReadOnly()
        {
            return new ExtensionRegistry(extensionsByName, extensionsByNumber, true);
        }

        /// <summary>
        /// Get the unmodifiable singleton empty instance.
        /// </summary>
        public static ExtensionRegistry Empty
        {
            get { return empty; }
        }

        /// <summary>
        /// Finds an extension by containing type and field number.
        /// A null reference is returned if the extension can't be found.
        /// </summary>
        public IGeneratedExtensionLite this[IMessageLite containingType, int fieldNumber]
        {
            get
            {
                IGeneratedExtensionLite ret;
                extensionsByNumber.TryGetValue(new ExtensionIntPair(containingType, fieldNumber), out ret);
                return ret;
            }
        }

        public IGeneratedExtensionLite FindByName(IMessageLite defaultInstanceOfType, string fieldName)
        { return FindExtensionByName(defaultInstanceOfType, fieldName); }

        IGeneratedExtensionLite FindExtensionByName(object forwhat, string fieldName)
        {
            IGeneratedExtensionLite extension = null;
            Dictionary<string, IGeneratedExtensionLite> map;
            if (extensionsByName.TryGetValue(forwhat, out map) && map.TryGetValue(fieldName, out extension))
                return extension;
            return null;
        }

        /// <summary>
        /// Add an extension from a generated file to the registry.
        /// </summary>
        public void Add(IGeneratedExtensionLite extension)
        {
            if (readOnly)
            {
                throw new InvalidOperationException("Cannot add entries to a read-only extension registry");
            }
            extensionsByNumber.Add(new ExtensionIntPair(extension.ContainingType, extension.Number), extension);

            Dictionary<string, IGeneratedExtensionLite> map;
            if (!extensionsByName.TryGetValue(extension.ContainingType, out map))
                extensionsByName.Add(extension.ContainingType, map = new Dictionary<string, IGeneratedExtensionLite>());
            map[extension.Descriptor.Name] = extension;
            map[extension.Descriptor.FullName] = extension;
        }

        /// <summary>
        /// Nested type just used to represent a pair of MessageDescriptor and int, as
        /// the key into the "by number" map.
        /// </summary>
        private struct ExtensionIntPair : IEquatable<ExtensionIntPair>
        {
            private readonly object msgType;
            private readonly int number;

            internal ExtensionIntPair(object msgType, int number)
            {
                this.msgType = msgType;
                this.number = number;
            }

            public override int GetHashCode()
            {
                return msgType.GetHashCode() * ((1 << 16) - 1) + number;
            }

            public override bool Equals(object obj)
            {
                if (!(obj is ExtensionIntPair))
                {
                    return false;
                }
                return Equals((ExtensionIntPair)obj);
            }

            public bool Equals(ExtensionIntPair other)
            {
                return msgType.Equals(other.msgType) && number == other.number;
            }
        }
    }
}