/*
 * Copyright (C) 2012 The Guava Authors
 * Copyright 2020-2022 Equinix, Inc
 * Copyright 2014-2022 The Billing Project, LLC
 *
 * The Billing Project licenses this file to you 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 org.killbill.commons.utils.reflect;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.Arrays;

import javax.annotation.CheckForNull;

/**
 * Verbatim copy of Guava's {@code com.google.common.reflect.AbstractInvocationHandler}.
 *
 * Abstract implementation of {@link InvocationHandler} that handles {@link Object#equals}, {@link
 * Object#hashCode} and {@link Object#toString}. For example:
 *
 * <pre>
 * class Unsupported extends AbstractInvocationHandler {
 *   protected Object handleInvocation(Object proxy, Method method, Object[] args) {
 *     throw new UnsupportedOperationException();
 *   }
 * }
 *
 * CharSequence unsupported = Reflection.newProxy(CharSequence.class, new Unsupported());
 * </pre>
 *
 * @author Ben Yu
 * @since 12.0
 */
public abstract class AbstractInvocationHandler implements InvocationHandler {

    private static final Object[] NO_ARGS = {};

    /**
     * {@inheritDoc}
     *
     * <ul>
     *   <li>{@code proxy.hashCode()} delegates to {@code com.google.common.reflect.AbstractInvocationHandler#hashCode}
     *   <li>{@code proxy.toString()} delegates to {@code com.google.common.reflect.AbstractInvocationHandler#toString}
     *   <li>{@code proxy.equals(argument)} returns true if:
     *       <ul>
     *         <li>{@code proxy} and {@code argument} are of the same type
     *         <li>and {@code com.google.common.reflect.AbstractInvocationHandler#equals} returns true for the {@link
     *             InvocationHandler} of {@code argument}
     *       </ul>
     *   <li>other method calls are dispatched to {@link #handleInvocation}.
     * </ul>
     */
    @Override
    @CheckForNull
    public final Object invoke(final Object proxy, final Method method, @CheckForNull Object[] args) throws Throwable {
        if (args == null) {
            args = NO_ARGS;
        }
        if (args.length == 0 && "hashCode".equals(method.getName())) {
            return hashCode();
        }
        if (args.length == 1
            && "equals".equals(method.getName())
            && method.getParameterTypes()[0] == Object.class) {
            final Object arg = args[0];
            if (arg == null) {
                return false;
            }
            if (proxy == arg) {
                return true;
            }
            return isProxyOfSameInterfaces(arg, proxy.getClass())
                   && equals(Proxy.getInvocationHandler(arg));
        }
        if (args.length == 0 && "toString".equals(method.getName())) {
            return toString();
        }
        return handleInvocation(proxy, method, args);
    }

    /**
     * {@link #invoke} delegates to this method upon any method invocation on the proxy instance,
     * except {@link Object#equals}, {@link Object#hashCode} and {@link Object#toString}. The result
     * will be returned as the proxied method's return value.
     *
     * <p>Unlike {@link #invoke}, {@code args} will never be null. When the method has no parameter,
     * an empty array is passed in.
     */
    @CheckForNull
    protected abstract Object handleInvocation(Object proxy, Method method, Object[] args) throws Throwable;

    /**
     * By default delegates to {@link Object#equals} so instances are only equal if they are
     * identical. {@code proxy.equals(argument)} returns true if:
     *
     * <ul>
     *   <li>{@code proxy} and {@code argument} are of the same type
     *   <li>and this method returns true for the {@link InvocationHandler} of {@code argument}
     * </ul>
     *
     * <p>Subclasses can override this method to provide custom equality.
     */
    @Override
    public boolean equals(@CheckForNull final Object obj) {
        return super.equals(obj);
    }

    /**
     * By default delegates to {@link Object#hashCode}. The dynamic proxies' {@code hashCode()} will
     * delegate to this method. Subclasses can override this method to provide custom equality.
     */
    @Override
    public int hashCode() {
        return super.hashCode();
    }

    /**
     * By default delegates to {@link Object#toString}. The dynamic proxies' {@code toString()} will
     * delegate to this method. Subclasses can override this method to provide custom string
     * representation for the proxies.
     */
    @Override
    public String toString() {
        return super.toString();
    }

    private static boolean isProxyOfSameInterfaces(final Object arg, final Class<?> proxyClass) {
        return proxyClass.isInstance(arg)
               // Equal proxy instances should mostly be instance of proxyClass
               // Under some edge cases (such as the proxy of JDK types serialized and then deserialized)
               // the proxy type may not be the same.
               // We first check isProxyClass() so that the common case of comparing with non-proxy objects
               // is efficient.
               || (Proxy.isProxyClass(arg.getClass())
                   && Arrays.equals(arg.getClass().getInterfaces(), proxyClass.getInterfaces()));
    }
}
