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// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
// http://code.google.com/p/protobuf/
//
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package com.google.protobuf;

/**
 * Grab-bag of utility functions useful when dealing with RPCs.
 *
 * @author [email protected] Kenton Varda
 */
public final class RpcUtil {
    private RpcUtil() {
    }

    /**
     * Take an {@code RpcCallback} and convert it to an
     * {@code RpcCallback} accepting a specific message type.  This is always
     * type-safe (parameter type contravariance).
     */
    @SuppressWarnings("unchecked")
    public static  RpcCallback
    specializeCallback(final RpcCallback originalCallback) {
        return (RpcCallback) originalCallback;
        // The above cast works, but only due to technical details of the Java
        // implementation.  A more theoretically correct -- but less efficient --
        // implementation would be as follows:
        //   return new RpcCallback() {
        //     public void run(Type parameter) {
        //       originalCallback.run(parameter);
        //     }
        //   };
    }

    /**
     * Take an {@code RpcCallback} accepting a specific message type and convert
     * it to an {@code RpcCallback}.  The generalized callback will
     * accept any message object which has the same descriptor, and will convert
     * it to the correct class before calling the original callback.  However,
     * if the generalized callback is given a message with a different descriptor,
     * an exception will be thrown.
     */
    public static 
    RpcCallback generalizeCallback(
            final RpcCallback originalCallback,
            final Class originalClass,
            final Type defaultInstance) {
        return new RpcCallback() {
            public void run(final Message parameter) {
                Type typedParameter;
                try {
                    typedParameter = originalClass.cast(parameter);
                } catch (ClassCastException ignored) {
                    typedParameter = copyAsType(defaultInstance, parameter);
                }
                originalCallback.run(typedParameter);
            }
        };
    }

    /**
     * Creates a new message of type "Type" which is a copy of "source".  "source"
     * must have the same descriptor but may be a different class (e.g.
     * DynamicMessage).
     */
    @SuppressWarnings("unchecked")
    private static  Type copyAsType(
            final Type typeDefaultInstance, final Message source) {
        return (Type) typeDefaultInstance.newBuilderForType()
                .mergeFrom(source)
                .build();
    }

    /**
     * Creates a callback which can only be called once.  This may be useful for
     * security, when passing a callback to untrusted code:  most callbacks do
     * not expect to be called more than once, so doing so may expose bugs if it
     * is not prevented.
     */
    public static 
    RpcCallback newOneTimeCallback(
            final RpcCallback originalCallback) {
        return new RpcCallback() {
            private boolean alreadyCalled = false;

            public void run(final ParameterType parameter) {
                synchronized (this) {
                    if (alreadyCalled) {
                        throw new AlreadyCalledException();
                    }
                    alreadyCalled = true;
                }

                originalCallback.run(parameter);
            }
        };
    }

    /**
     * Exception thrown when a one-time callback is called more than once.
     */
    public static final class AlreadyCalledException extends RuntimeException {
        private static final long serialVersionUID = 5469741279507848266L;

        public AlreadyCalledException() {
            super("This RpcCallback was already called and cannot be called " +
                    "multiple times.");
        }
    }
}