DownloadClient.cs

using System;
using System.IO;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;
#if !NET6_0_OR_GREATER
using System.Threading.Tasks.Dataflow;
#endif
// ReSharper disable MemberCanBePrivate.Global

// ReSharper disable AutoPropertyCanBeMadeGetOnly.Local
// ReSharper disable UnusedMember.Global
// ReSharper disable ConvertToUsingDeclaration
// ReSharper disable IdentifierTypo
// ReSharper disable ArrangeObjectCreationWhenTypeEvident

namespace Hi3Helper.Http
{
    /// <summary>
    ///     An instance of the download client. This instance can be re-used. But besides it can be re-used, make sure to set
    ///     the sufficient <c>MaxConnection</c> of the <seealso cref="HttpClient" />
    ///     assigned into the current <seealso cref="DownloadClient" /> instance.
    /// </summary>
    public sealed class DownloadClient
    {
        private const int DefaultConnectionSessions = 4;
        private const int DefaultRetryCountMax = 5;
        internal const int MinimumDownloadSpeedLimit = 256 << 10; // 262144 bytes/s (256 KiB/s)

        private TimeSpan RetryAttemptInterval { get; init; }
        private int RetryCountMax { get; init; }
        private TimeSpan TimeoutAfterInterval { get; init; }
        private HttpClient CurrentHttpClientInstance { get; init; }

        private const int DefaultSessionChunkSize = 4 << 20; // 4 MiB for each chunk size

        private DownloadClient(HttpClient httpClient, int retryCountMax = DefaultRetryCountMax,
            TimeSpan? retryAttemptInterval = null, TimeSpan? timeoutAfterInterval = null)
        {
            retryAttemptInterval ??= TimeSpan.FromSeconds(1);
            RetryAttemptInterval = retryAttemptInterval.Value;

            timeoutAfterInterval ??= TimeSpan.FromSeconds(10);
            TimeoutAfterInterval = timeoutAfterInterval.Value;

            RetryCountMax = retryCountMax;
            CurrentHttpClientInstance = httpClient;
        }

        /// <summary>
        ///     Create an instance of a Http Download Client instance from the given <seealso cref="HttpClient" /> instance.
        /// </summary>
        /// <param name="httpClient">
        ///     Use the HttpClient from the parent caller from the given <seealso cref="HttpClient" /> instance.
        /// </param>
        /// <param name="retryCountMax">
        ///     Count of how many times the retry attempt should be accepted.
        /// </param>
        /// <param name="retryAttemptInterval">
        ///     Determine how long the pause will run before the next retry attempt is executed. The default value is 1 second.
        /// </param>
        /// <param name="timeoutAfterInterval">
        ///     Determine how long the method will time out while it's getting called.
        /// </param>
        /// <returns>An instance of a Http Download Client</returns>
        /// <exception cref="NullReferenceException">Throw if the <paramref name="httpClient" /> argument is <c>null</c>.</exception>
        public static DownloadClient CreateInstance(HttpClient httpClient, int retryCountMax = DefaultRetryCountMax,
            TimeSpan? retryAttemptInterval = null, TimeSpan? timeoutAfterInterval = null)
        {
            // Throw if HttpClient argument is null
            ArgumentNullException.ThrowIfNull(httpClient, nameof(httpClient));

            // Return the instance
            return new DownloadClient(httpClient, retryCountMax, retryAttemptInterval, timeoutAfterInterval);
        }

        /// <summary>
        ///     Start the download process of the file asynchronously. This method can be used for single-session or multi-session
        ///     download.
        /// </summary>
        /// <param name="url">The URL of the file to be downloaded.</param>
        /// <param name="fileOutputPath">
        ///     Output path of the file to download.
        /// </param>
        /// <param name="useOverwrite">
        ///     Overwrite the download even the previous session can be continued.
        ///     If set to true, the previous download will start from beginning.
        /// </param>
        /// <param name="offsetStart">
        ///     The start position of the data to be downloaded. If this argument is set with <paramref name="offsetEnd" /> to
        ///     <c>null</c>,<br />
        ///     the download will start from the beginning of the data. The <paramref name="offsetStart" /> argument cannot be set
        ///     more than or equal as <paramref name="offsetEnd" />.<br /><br />
        ///     Default: <c>null</c>
        /// </param>
        /// <param name="offsetEnd">
        ///     The end position of the data to be downloaded. If this argument is set to <c>null</c>,<br />
        ///     the download will write the data until the end of the data. The <paramref name="offsetEnd" /> argument cannot be
        ///     set less than or equal as <paramref name="offsetStart" />.<br /><br />
        ///     Default: <c>null</c>
        /// </param>
        /// <param name="progressDelegateAsync">
        ///     The delegate callback to process the download progress information.<br /><br />
        ///     Default: <c>null</c>
        /// </param>
        /// <param name="downloadSpeedLimiter">
        ///     If the download speed limiter is null, the download speed will be set to unlimited.
        /// </param>
        /// <param name="maxConnectionSessions">
        ///     How much connection session to be started for the download process. If it's being set to less than or equal as 0,
        ///     then it will fall back to the default value: 4.<br /><br />
        ///     Default: <c>4</c>
        /// </param>
        /// <param name="sessionChunkSize">
        ///     How big the size of each session chunk.<br /><br />
        ///     Default: <c>4,194,304 bytes</c> or <c>4 MiB</c>
        /// </param>
        /// <param name="cancelToken">
        ///     Cancellation token. If not assigned, a cancellation token will not be assigned and the download becomes
        ///     non-cancellable.
        /// </param>
        /// <exception cref="ArgumentNullException"><paramref name="url" /> or <paramref name="fileOutputPath" /> is null.</exception>
        /// <exception cref="ArgumentException">
        ///     <paramref name="url" /> or <paramref name="fileOutputPath" /> is empty or only have
        ///     whitespaces.
        /// </exception>
        public async Task DownloadAsync(
            string url,
            string fileOutputPath,
            bool useOverwrite = false,
            long? offsetStart = null,
            long? offsetEnd = null,
            DownloadProgressDelegate? progressDelegateAsync = null,
            int maxConnectionSessions = DefaultConnectionSessions,
            int sessionChunkSize = DefaultSessionChunkSize,
            DownloadSpeedLimiter? downloadSpeedLimiter = null,
            CancellationToken cancelToken = default)
        {
            ArgumentException.ThrowIfNullOrEmpty(url, nameof(url));
            ArgumentException.ThrowIfNullOrWhiteSpace(url, nameof(url));
            ArgumentException.ThrowIfNullOrEmpty(fileOutputPath, nameof(fileOutputPath));
            ArgumentException.ThrowIfNullOrWhiteSpace(fileOutputPath, nameof(fileOutputPath));

            Uri uri = url.ToUri();

            FileStreamOptions fileStreamOptions = new FileStreamOptions
            {
                Mode = FileMode.OpenOrCreate,
                Access = FileAccess.Write,
                Share = FileShare.ReadWrite,
                Options = FileOptions.WriteThrough
            };

            DownloadProgress downloadProgressStruct = new DownloadProgress();

#if !NET6_0_OR_GREATER
            ActionBlock<ChunkSession> actionBlock = new ActionBlock<ChunkSession>(async chunk =>
            {
                await PerformDownloadWriteDelegate(
                    progressDelegateAsync,
                    maxConnectionSessions,
                    downloadSpeedLimiter,
                    chunk,
                    fileStreamOptions,
                    downloadProgressStruct,
                    cancelToken);
            },
                new ExecutionDataflowBlockOptions
                {
                    CancellationToken = cancelToken,
                    MaxDegreeOfParallelism = maxConnectionSessions,
                    MaxMessagesPerTask = maxConnectionSessions,
                    BoundedCapacity = maxConnectionSessions
                });

            await foreach (ChunkSession session in ChunkSession.EnumerateMultipleChunks(
                               CurrentHttpClientInstance,
                               uri,
                               fileOutputPath,
                               useOverwrite,
                               sessionChunkSize,
                               downloadProgressStruct,
                               progressDelegateAsync,
                               RetryCountMax,
                               RetryAttemptInterval,
                               TimeoutAfterInterval
                           ).WithCancellation(cancelToken))
            {
                await actionBlock.SendAsync(session, cancelToken);
            }

            actionBlock.Complete();
            await actionBlock.Completion;
            if (actionBlock.Completion.Exception != null)
            {
                throw actionBlock.Completion.Exception;
            }
#else
            try
            {
                await Parallel.ForEachAsync(
                    ChunkSession.EnumerateMultipleChunks(
                                   CurrentHttpClientInstance,
                                   uri,
                                   fileOutputPath,
                                   useOverwrite,
                                   sessionChunkSize,
                                   downloadProgressStruct,
                                   progressDelegateAsync,
                                   RetryCountMax,
                                   RetryAttemptInterval,
                                   TimeoutAfterInterval,
                                   cancelToken
                               ),
                               new ParallelOptions
                               {
                                   CancellationToken = cancelToken,
                                   MaxDegreeOfParallelism = maxConnectionSessions
                               },
                               async (chunk, coopCancelToken) =>
                               await PerformDownloadWriteDelegate(
                                   progressDelegateAsync,
                                   maxConnectionSessions,
                                   downloadSpeedLimiter,
                                   chunk,
                                   fileStreamOptions,
                                   downloadProgressStruct,
                                   coopCancelToken));
            }
            catch (AggregateException ex)
            {
                throw ex.Flatten().InnerExceptions.First();
            }
#endif

            Metadata.DeleteMetadataFile(fileOutputPath);
        }

        private static async Task PerformDownloadWriteDelegate(
            DownloadProgressDelegate? progressDelegateAsync,
            int maxConnectionSessions,
            DownloadSpeedLimiter? downloadSpeedLimiter,
            ChunkSession chunk,
            FileStreamOptions fileStreamOptions,
            DownloadProgress downloadProgressStruct,
            CancellationToken cancelToken)
        {
            await using (FileStream stream = new FileStream(chunk.CurrentMetadata?.OutputFilePath!, fileStreamOptions))
            {
                if (chunk.CurrentMetadata == null)
                {
                    throw new NullReferenceException("chunk.CurrentMetadata reference is null");
                }

                await chunk.CurrentMetadata.SaveLastMetadataStateAsync(cancelToken);
                await IO.WriteStreamToFileChunkSessionAsync(
                    chunk,
                    downloadSpeedLimiter,
                    maxConnectionSessions,
                    null,
                    false,
                    stream,
                    downloadProgressStruct,
                    progressDelegateAsync,
                    cancelToken);

                if (chunk.CurrentPositions.End - 1 > chunk.CurrentPositions.Start)
                    throw new Exception();

                chunk.CurrentMetadata.PopRange(chunk.CurrentPositions);
                await chunk.CurrentMetadata.SaveLastMetadataStateAsync(cancelToken);
            }
        }

        /// <summary>
        ///     Start the download process to a <seealso cref="Stream" />.
        /// </summary>
        /// <param name="url">The URL of the file to be downloaded into stream.</param>
        /// <param name="outputStream">
        ///     The output stream where the data will be downloaded. The <paramref name="outputStream" /> must be writable.
        /// </param>
        /// <param name="allowContinue">
        ///     Allow to resume the last position of the download.
        ///     However, this argument will be ignored if the <paramref name="outputStream" /> is not seekable and
        ///     <paramref name="offsetStart" /> is set to > 0.
        /// </param>
        /// <param name="offsetStart">
        ///     The start position of the data to be downloaded. If this argument is set with <paramref name="offsetEnd" /> to
        ///     <c>null</c>,<br />
        ///     the download will start from the beginning of the data. The <paramref name="offsetStart" /> argument cannot be set
        ///     more than or equal as <paramref name="offsetEnd" />.<br /><br />
        ///     Default: <c>null</c>
        /// </param>
        /// <param name="offsetEnd">
        ///     The end position of the data to be downloaded. If this argument is set to <c>null</c>,<br />
        ///     the download will write the data until the end of the data. The <paramref name="offsetEnd" /> argument cannot be
        ///     set less than or equal as <paramref name="offsetStart" />.<br /><br />
        ///     Default: <c>null</c>
        /// </param>
        /// <param name="progressDelegateAsync">
        ///     The delegate callback to process the download progress information.<br /><br />
        ///     Default: <c>null</c>
        /// </param>
        /// <param name="downloadSpeedLimiter">
        ///     If the download speed limiter is null, the download speed will be set to unlimited.
        /// </param>
        /// <param name="cancelToken">
        ///     Cancellation token. If not assigned, a cancellation token will not be assigned and the download becomes
        ///     non-cancellable.
        /// </param>
        /// <exception cref="ArgumentNullException"><paramref name="url" /> or <paramref name="outputStream" /> is null.</exception>
        /// <exception cref="ArgumentException"><paramref name="url" /> is empty or only have whitespaces.</exception>
        public async Task DownloadAsync(
            string url,
            Stream outputStream,
            bool allowContinue,
            DownloadProgressDelegate? progressDelegateAsync = null,
            long? offsetStart = null,
            long? offsetEnd = null,
            DownloadSpeedLimiter? downloadSpeedLimiter = null,
            CancellationToken cancelToken = default)
        {
            ArgumentException.ThrowIfNullOrEmpty(url, nameof(url));
            ArgumentException.ThrowIfNullOrWhiteSpace(url, nameof(url));
            ArgumentNullException.ThrowIfNull(outputStream, nameof(outputStream));

            Uri uri = url.ToUri();

            offsetStart ??= 0;

            bool isSeeked = false;

            // If the download allows the download to continue and the outputStream is seekable,
            // then seek the outputStream to the very end of its position.
            if (allowContinue
                && outputStream.CanSeek
                && outputStream.IsStreamCanSeeLength()
                && offsetStart == 0)
            {
                outputStream.Seek(0, SeekOrigin.End);
                isSeeked = true;
            }

            // Create the session and stream tuple
            (ChunkSession, HttpResponseInputStream)? networkStream = await ChunkSession
                .CreateSingleSessionAsync(
                    CurrentHttpClientInstance,
                    uri,
                    offsetStart,
                    offsetEnd,
                    RetryCountMax,
                    RetryAttemptInterval,
                    TimeoutAfterInterval,
                    cancelToken
                );

            // If the network stream tuple is null, then ignore
            if (networkStream == null)
            {
                return;
            }

            // Set the download progress struct and set the bytes total to download.
            DownloadProgress downloadProgressStruct = new DownloadProgress();
            downloadProgressStruct.SetBytesTotal(networkStream.Value.Item2.Length);

            // If the outputStream is seekable, then advance the downloaded progress.
            if (isSeeked)
            {
                downloadProgressStruct.AdvanceBytesDownloaded(outputStream.Length);
            }

            // Start the download
            await IO.WriteStreamToFileChunkSessionAsync(
                networkStream.Value.Item1,
                downloadSpeedLimiter,
                1,
                networkStream.Value.Item2,
                true,
                outputStream,
                downloadProgressStruct,
                progressDelegateAsync,
                cancelToken);
        }

        /// <summary>
        /// Get the current <seealso cref="HttpClient"/> instance used by the <seealso cref="DownloadClient"/>
        /// </summary>
        /// <returns>The current <seealso cref="HttpClient"/> instance used by the <seealso cref="DownloadClient"/></returns>
        public HttpClient GetHttpClient() => CurrentHttpClientInstance;

        /// <summary>
        /// Get the Http's <seealso cref="HttpResponseMessage.StatusCode"/> of the URL.
        /// </summary>
        /// <param name="url">The URL to check</param>
        /// <param name="cancelToken">The cancellation token</param>
        /// <returns>A tuple contains a <seealso cref="HttpResponseMessage.StatusCode"/> and an <seealso cref="bool"/> of the status code (true = success, false = failed)</returns>
        public async ValueTask<(HttpStatusCode, bool)> GetURLStatus(string url, CancellationToken cancelToken)
            => await GetURLStatus(CurrentHttpClientInstance, url, cancelToken);

        /// <summary>
        /// Get the Http's <seealso cref="HttpResponseMessage.StatusCode"/> of the URL.
        /// </summary>
        /// <param name="url">The URL to check</param>
        /// <param name="cancelToken">The cancellation token</param>
        /// <returns>A tuple contains a <seealso cref="HttpResponseMessage.StatusCode"/> and an <seealso cref="bool"/> of the status code (true = success, false = failed)</returns>
        public async ValueTask<(HttpStatusCode, bool)> GetURLStatus(Uri url, CancellationToken cancelToken)
            => await GetURLStatus(CurrentHttpClientInstance, url, cancelToken);

        /// <summary>
        /// Get the Http's <seealso cref="HttpResponseMessage.StatusCode"/> of the URL from an <seealso cref="HttpClient"/> instance.
        /// </summary>
        /// <param name="httpClient">The <seealso cref="HttpClient"/> instance to be used for URL checking</param>
        /// <param name="url">The URL to check</param>
        /// <param name="cancelToken">The cancellation token</param>
        /// <returns>A tuple contains a <seealso cref="HttpResponseMessage.StatusCode"/> and an <seealso cref="bool"/> of the status code (true = success, false = failed)</returns>
        public static async ValueTask<(HttpStatusCode, bool)> GetURLStatus(HttpClient httpClient, string url, CancellationToken cancelToken)
            => await GetURLStatus(httpClient, url.ToUri(), cancelToken);

        /// <summary>
        /// Get the Http's <seealso cref="HttpResponseMessage.StatusCode"/> of the URL from an <seealso cref="HttpClient"/> instance.
        /// </summary>
        /// <param name="httpClient">The <seealso cref="HttpClient"/> instance to be used for URL checking</param>
        /// <param name="url">The URL to check</param>
        /// <param name="cancelToken">The cancellation token</param>
        /// <returns>A tuple contains a <seealso cref="HttpResponseMessage.StatusCode"/> and an <seealso cref="bool"/> of the status code (true = success, false = failed)</returns>
        public static async ValueTask<(HttpStatusCode, bool)> GetURLStatus(HttpClient httpClient, Uri url, CancellationToken cancelToken)
        {
            using (HttpResponseMessage response = await httpClient.SendAsync(
                new HttpRequestMessage()
                {
                    RequestUri = url
                },
                HttpCompletionOption.ResponseHeadersRead,
                cancelToken))
            {
                return (response.StatusCode, response.IsSuccessStatusCode);
            }
        }

        /// <summary>
        /// Get the total size of an existing downloaded file using the current instance of a <seealso cref="DownloadClient"/>.
        /// </summary>
        /// <param name="fileUrl">The URL of the file.</param>
        /// <param name="filePath">The path of the existing download file.</param>
        /// <param name="expectedLength">The expected value of the total size. If it's set to 0, then a GET HTTP routine will not be requested.</param>
        /// <param name="cancelToken">The cancellation token for the routine.</param>
        /// <returns>A size of a downloaded file.</returns>
        /// <exception cref="InvalidOperationException">If <paramref name="expectedLength"/> is set to 0 and the current <seealso cref="HttpClient"/> of this <seealso cref="DownloadClient"/> is null.</exception>
        public async ValueTask<long> GetDownloadedFileSize(
            string fileUrl,
            string filePath,
            long expectedLength = 0,
            CancellationToken cancelToken = default)
            => await GetDownloadedFileSize(
                fileUrl,
                CurrentHttpClientInstance,
                filePath,
                expectedLength,
                RetryCountMax,
                RetryAttemptInterval,
                TimeoutAfterInterval,
                cancelToken);

        /// <summary>
        /// Get the total size of an existing downloaded file using the current instance of a <seealso cref="DownloadClient"/>.
        /// </summary>
        /// <param name="fileUrl">The URL of the file.</param>
        /// <param name="filePath">The path of the existing download file.</param>
        /// <param name="expectedLength">The expected value of the total size. If it's set to 0, then a GET HTTP routine will not be requested.</param>
        /// <param name="cancelToken">The cancellation token for the routine.</param>
        /// <returns>A size of a downloaded file.</returns>
        /// <exception cref="InvalidOperationException">If <paramref name="expectedLength"/> is set to 0 and the current <seealso cref="HttpClient"/> of this <seealso cref="DownloadClient"/> is null.</exception>
        public async ValueTask<long> GetDownloadedFileSize(
            Uri fileUrl,
            string filePath,
            long expectedLength = 0,
            CancellationToken cancelToken = default)
            => await GetDownloadedFileSize(
                fileUrl,
                CurrentHttpClientInstance,
                filePath,
                expectedLength,
                RetryCountMax,
                RetryAttemptInterval,
                TimeoutAfterInterval,
                cancelToken);

        /// <summary>
        /// Get the total size of an existing downloaded file
        /// </summary>
        /// <param name="fileUrl">The URL of the file.</param>
        /// <param name="httpClient"><seealso cref="HttpClient"/> instance to be used for checking the total size of the file if <paramref name="expectedLength"/> is 0.</param>
        /// <param name="filePath">The path of the existing download file.</param>
        /// <param name="expectedLength">The expected value of the total size. If it's set to 0, then a GET HTTP routine will not be requested.</param>
        /// <param name="retryCountMax">An expected amount of time for retrying the check.</param>
        /// <param name="retryAttemptInterval">How much time for delay to run before the next retry attempt.</param>
        /// <param name="timeoutAfterInterval">How much time for a timeout to trigger for retrying a routine.</param>
        /// <param name="cancelToken">The cancellation token for the routine.</param>
        /// <returns>A size of a downloaded file.</returns>
        /// <exception cref="InvalidOperationException">If <paramref name="httpClient"/> is null and <paramref name="expectedLength"/> is set to 0</exception>
        public static async ValueTask<long> GetDownloadedFileSize(
            string fileUrl,
            HttpClient httpClient,
            string filePath,
            long expectedLength = 0,
            int? retryCountMax = null,
            TimeSpan? retryAttemptInterval = default,
            TimeSpan? timeoutAfterInterval = default,
            CancellationToken cancelToken = default)
            => await GetDownloadedFileSize(
                fileUrl.ToUri(),
                httpClient,
                filePath,
                expectedLength,
                retryCountMax,
                retryAttemptInterval,
                timeoutAfterInterval,
                cancelToken);

        /// <summary>
        /// Get the total size of an existing downloaded file
        /// </summary>
        /// <param name="fileUrl">The URL of the file.</param>
        /// <param name="httpClient"><seealso cref="HttpClient"/> instance to be used for checking the total size of the file if <paramref name="expectedLength"/> is 0.</param>
        /// <param name="filePath">The path of the existing download file.</param>
        /// <param name="expectedLength">The expected value of the total size. If it's set to 0, then a GET HTTP routine will not be requested.</param>
        /// <param name="retryCountMax">An expected amount of time for retrying the check.</param>
        /// <param name="retryAttemptInterval">How much time for delay to run before the next retry attempt.</param>
        /// <param name="timeoutAfterInterval">How much time for a timeout to trigger for retrying a routine.</param>
        /// <param name="cancelToken">The cancellation token for the routine.</param>
        /// <returns>A size of a downloaded file.</returns>
        /// <exception cref="InvalidOperationException">If <paramref name="httpClient"/> is null and <paramref name="expectedLength"/> is set to 0</exception>
        public static async ValueTask<long> GetDownloadedFileSize(
            Uri fileUrl,
            HttpClient httpClient,
            string filePath,
            long expectedLength = 0,
            int? retryCountMax = null,
            TimeSpan? retryAttemptInterval = default,
            TimeSpan? timeoutAfterInterval = default,
            CancellationToken cancelToken = default)
        {
            // Set default values
            retryCountMax ??= DefaultRetryCountMax;
            retryAttemptInterval ??= TimeSpan.FromSeconds(1);
            timeoutAfterInterval ??= TimeSpan.FromSeconds(10);

            // Sanity check: Throw if httpClient is null while expectedLength param is 0
            if (httpClient == null && expectedLength == 0)
                throw new InvalidOperationException($"You cannot set {nameof(httpClient)} to null while {nameof(expectedLength)} is set to 0!");

            // Get the file size from the URL or expected value
            long contentLength = expectedLength > 0 ? expectedLength : await fileUrl.GetUrlContentLengthAsync(httpClient!, (int)retryCountMax,
                retryAttemptInterval.Value, timeoutAfterInterval.Value, cancelToken);

            // Get current file info
            FileInfo currentFileInfo = new FileInfo(filePath);

            // Get metadata file info
            string metadataFilePath = currentFileInfo.FullName + Metadata.MetadataExtension;
            FileInfo metadataFileInfo = new FileInfo(metadataFilePath);

            // Get the last session metadata info
            Metadata? currentSessionMetadata =
                await Metadata.ReadLastMetadataAsync(null, currentFileInfo, metadataFileInfo, 0, cancelToken);

            // Set the current length to 0;
            long currentLength = 0;

            // SANITY CHECK: Metadata and file state check
            if ((metadataFileInfo.Exists && metadataFileInfo.Length < 64 && currentFileInfo.Exists)
             || (currentFileInfo.Exists && currentFileInfo.Length > contentLength)
             || (metadataFileInfo.Exists && !currentFileInfo.Exists)
             || ((currentSessionMetadata?.Ranges?.Count ?? 0) == 0 && (currentSessionMetadata?.IsCompleted ?? false)))
            {
                // Return 0 as uncompleted
                return 0;
            }

            // If the completed flag is set, the ranges are empty, the output file exist with the length is equal,
            // then return from enumerating. Or if the ranges list is empty, return
            if ((currentSessionMetadata?.Ranges?.Count == 0
                 && currentFileInfo.Exists
                 && currentFileInfo.Length == contentLength)
                || currentSessionMetadata?.Ranges == null)
            {
                currentLength += contentLength;
                return currentLength;
            }

            // Enumerate last ranges
            // long lastEndOffset = currentSessionMetadata.Ranges.Count > 0
            //     ? currentSessionMetadata.Ranges.Max(x => x?.End ?? 0) + 1
            //     : 0;

            // If the metadata is not exist, but it has an uncompleted file with size > DefaultSessionChunkSize,
            // then try to resume the download and advance the lastEndOffset from the file last position.
            if (currentSessionMetadata.Ranges.Count == 0
                && currentFileInfo.Exists
                && currentSessionMetadata.LastEndOffset <= currentFileInfo.Length)
            {
                currentLength += currentFileInfo.Length;
            }
            // Else if the file exist with size downloaded less than LastEndOffset, then continue
            // the position based on metadata.
            else if (currentFileInfo.Exists)
            {
                ChunkRange lastRange = new ChunkRange();
                foreach (ChunkRange? range in currentSessionMetadata.Ranges)
                {
                    if (range == null)
                    {
                        continue;
                    }

                    long toAdd = range.Start - lastRange.End;
                    currentLength += toAdd;

                    lastRange = range;
                }

                return currentLength;
            }

            // Otherwise, return currentLength
            return currentLength;
        }
    }
}