/*
Copyright 2013 Google Inc

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.
*/

using System;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;

using Google.Apis.Logging;
using Google.Apis.Util;

namespace Google.Apis.Http
{
    /// <summary>
    /// A thread-safe back-off handler which handles an abnormal HTTP response or an exception with 
    /// <see cref="Google.Apis.Util.IBackOff"/>.
    /// </summary>
    public class BackOffHandler : IHttpUnsuccessfulResponseHandler, IHttpExceptionHandler
    {
        private static readonly ILogger Logger = ApplicationContext.Logger.ForType<BackOffHandler>();

        /// <summary>An initializer class to initialize a back-off handler.</summary>
        public class Initializer
        {
            /// <summary>Gets the back-off policy used by this back-off handler.</summary>
            public IBackOff BackOff { get; private set; }

            /// <summary>
            /// Gets or sets the maximum time span to wait. If the back-off instance returns a greater time span than
            /// this value, this handler returns <c>false</c> to both <c>HandleExceptionAsync</c> and 
            /// <c>HandleResponseAsync</c>. Default value is 16 seconds per a retry request.
            /// </summary>
            public TimeSpan MaxTimeSpan { get; set; }

            /// <summary>
            /// Gets or sets a delegate function which indicates whether this back-off handler should handle an 
            /// abnormal HTTP response. The default is <see cref="DefaultHandleUnsuccessfulResponseFunc"/>. 
            /// </summary>
            public Func<HttpResponseMessage, bool> HandleUnsuccessfulResponseFunc { get; set; }

            /// <summary>
            /// Gets or sets a delegate function which indicates whether this back-off handler should handle an 
            /// exception. The default is <see cref="DefaultHandleExceptionFunc"/>. 
            /// </summary>
            public Func<Exception, bool> HandleExceptionFunc { get; set; }

            /// <summary>Default function which handles server errors (503).</summary>
            public static readonly Func<HttpResponseMessage, bool> DefaultHandleUnsuccessfulResponseFunc =
                (r) => (int)r.StatusCode == 503;

            /// <summary>
            /// Default function which handles exception which aren't 
            /// <see cref="System.Threading.Tasks.TaskCanceledException"/> or 
            /// <see cref="System.OperationCanceledException"/>. Those exceptions represent a task or an operation
            /// which was canceled and shouldn't be retried.
            /// </summary>
            public static readonly Func<Exception, bool> DefaultHandleExceptionFunc =
                (ex) => !(ex is TaskCanceledException || ex is OperationCanceledException);

            /// <summary>Constructs a new initializer by the given back-off.</summary>
            public Initializer(IBackOff backOff)
            {
                BackOff = backOff;
                HandleExceptionFunc = DefaultHandleExceptionFunc;
                HandleUnsuccessfulResponseFunc = DefaultHandleUnsuccessfulResponseFunc;
                MaxTimeSpan = TimeSpan.FromSeconds(16);
            }
        }

        /// <summary>Gets the back-off policy used by this back-off handler.</summary>
        public IBackOff BackOff { get; private set; }

        /// <summary>
        /// Gets the maximum time span to wait. If the back-off instance returns a greater time span, the handle method
        /// returns <c>false</c>. Default value is 16 seconds per a retry request.
        /// </summary>
        public TimeSpan MaxTimeSpan { get; private set; }

        /// <summary>
        /// Gets a delegate function which indicates whether this back-off handler should handle an abnormal HTTP 
        /// response. The default is <see cref="Initializer.DefaultHandleUnsuccessfulResponseFunc"/>. 
        /// </summary>
        public Func<HttpResponseMessage, bool> HandleUnsuccessfulResponseFunc { get; private set; }

        /// <summary>
        /// Gets a delegate function which indicates whether this back-off handler should handle an exception. The 
        /// default is <see cref="Initializer.DefaultHandleExceptionFunc"/>. 
        /// </summary>
        public Func<Exception, bool> HandleExceptionFunc { get; private set; }

        /// <summary>Constructs a new back-off handler with the given back-off.</summary>
        /// <param name="backOff">The back-off policy.</param>
        public BackOffHandler(IBackOff backOff)
            : this(new Initializer(backOff))
        {
        }

        /// <summary>Constructs a new back-off handler with the given initializer.</summary>
        public BackOffHandler(Initializer initializer)
        {
            BackOff = initializer.BackOff;
            MaxTimeSpan = initializer.MaxTimeSpan;
            HandleExceptionFunc = initializer.HandleExceptionFunc;
            HandleUnsuccessfulResponseFunc = initializer.HandleUnsuccessfulResponseFunc;
        }

        #region IHttpUnsuccessfulResponseHandler

        /// <inheritdoc/>
        public virtual async Task<bool> HandleResponseAsync(HandleUnsuccessfulResponseArgs args)
        {
            // if the func returns true try to handle this current failed try
            if (HandleUnsuccessfulResponseFunc != null && HandleUnsuccessfulResponseFunc(args.Response))
            {
                return await HandleAsync(args.SupportsRetry, args.CurrentFailedTry, args.CancellationToken)
                    .ConfigureAwait(false);
            }
            return false;
        }

        #endregion

        #region IHttpExceptionHandler

        /// <inheritdoc/>
        public virtual async Task<bool> HandleExceptionAsync(HandleExceptionArgs args)
        {
            // if the func returns true try to handle this current failed try
            if (HandleExceptionFunc != null && HandleExceptionFunc(args.Exception))
            {
                return await HandleAsync(args.SupportsRetry, args.CurrentFailedTry, args.CancellationToken)
                    .ConfigureAwait(false);
            }
            return false;
        }

        #endregion

        /// <summary>
        /// Handles back-off. In case the request doesn't support retry or the back-off time span is greater than the
        /// maximum time span allowed for a request, the handler returns <c>false</c>. Otherwise the current thread 
        /// will block for x milliseconds (x is defined by the <see cref="BackOff"/> instance), and this handler 
        /// returns <c>true</c>.
        /// </summary>
        private async Task<bool> HandleAsync(bool supportsRetry, int currentFailedTry,
            CancellationToken cancellationToken)
        {
            if (!supportsRetry || BackOff.MaxNumOfRetries < currentFailedTry)
            {
                return false;
            }

            TimeSpan ts = BackOff.GetNextBackOff(currentFailedTry);
            if (ts > MaxTimeSpan || ts < TimeSpan.Zero)
            {
                return false;
            }

            await Wait(ts, cancellationToken).ConfigureAwait(false);
            Logger.Debug("Back-Off handled the error. Waited {0}ms before next retry...", ts.TotalMilliseconds);
            return true;
        }

        /// <summary>Waits the given time span. Overriding this method is recommended for mocking purposes.</summary>
        /// <param name="ts">TimeSpan to wait (and block the current thread).</param>
        /// <param name="cancellationToken">The cancellation token in case the user wants to cancel the operation in 
        /// the middle.</param>
        protected virtual async Task Wait(TimeSpan ts, CancellationToken cancellationToken)
        {
            await Task.Delay(ts, cancellationToken).ConfigureAwait(false);
        }
    }
}