Beam/Beam.Downloaders/UnitDownloader.cs

using Beam.Abstractions;
using Beam.Models;
using HtmlAgilityPack;
using File = System.IO.File;

namespace Beam.Downloaders {
    /// <summary>
    /// A download managing class that manages a singular download with failure-detection and exponential-backoff retries. This class is safe to instantiate per request.
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="web"></param>
    /// <param name="transformer"></param>
    /// <param name="failurePredicate"></param>
    public class UnitDownloader<RawType, OutType>(UnitDownloaderOptions<RawType, OutType> options) : IUnitDownloader<OutType> where RawType : IDocument {
        public UnitDownloaderOptions<RawType, OutType> Options { get; } = options;
        public HttpClient Client => Options.Client;
        public virtual AsyncTransformer<RawType, OutType> Transformer => Options.AsyncTransformer;

        public virtual AsyncDownloadFailurePredicate<RawType>?[]? FailurePredicates =>
            Options?.FailurePredicateOptions?.AsyncDownloadFailurePredicates;

        public int LinksPerDownload { get; } = 1;

        protected virtual async Task DownloadToStream(string url, int bufferSize, Stream destinationStream, IProgress<IDownloadReport> progress,
            CancellationToken ct) {

            var stream = await Client.GetStreamAsync(url, ct);
            byte[] buffer = new byte[bufferSize];
            int inBuffer = 0;
            long downloaded = 0;

            long? remaining() {
                try {
                    return stream.Length - downloaded;
                }
                catch {
                    return null;
                }
            }   
            
            while ((inBuffer = stream.Read(buffer)) > 0) {
                downloaded += inBuffer;
                await destinationStream.WriteAsync(buffer.AsMemory(0, inBuffer), ct);
                progress?.Report(new DownloadReport() {
                    BytesDownloaded = inBuffer,
                    BytesRemaining = remaining()
                });
                
                ct.ThrowIfCancellationRequested();
            }
        }

        protected virtual async Task DownloadToFile(string url, int bufferSize, string path,
            IProgress<IDownloadReport> progress, CancellationToken ct) {

            if (!Directory.Exists(Path.GetDirectoryName(path)))
                throw new InvalidOperationException(
                    string.Format(Exceptions.Exceptions.unit_download_directory_nonexistant, path));
            await using var file = File.OpenWrite(path);
            await DownloadToStream(url, bufferSize, file, progress, ct);
        }

        protected virtual async Task<ByteDocument> DownloadToMemory(string url, int bufferSize,
            IProgress<IDownloadReport> progress, CancellationToken ct) {

            await using var ms = new MemoryStream();
            await DownloadToStream(url, bufferSize, ms, progress, ct);
            if (!ms.TryGetBuffer(out var bytes))
                throw new Exception(Exceptions.Exceptions.unit_download_invalid_memory_stream);
            return new ByteDocument(url, bytes);
        }
        
        protected virtual async Task<bool> IsFailure(RawType doc, CancellationToken ct) {
            if (FailurePredicates is null)
                return false;
            if (!(Options?.FailurePredicateOptions?.ProcessInParallel ?? false))
                foreach (var pred in FailurePredicates) {
                    if (pred is null)
                        continue;
                    if (await pred(doc))
                        return true;
                }
            else {
                var failed = false;
                await Parallel.ForEachAsync(FailurePredicates, new ParallelOptions() {
                        MaxDegreeOfParallelism = Options?.FailurePredicateOptions?.ParallelThreads ?? 4,
                        CancellationToken = ct
                    },
                    async (predicate, token) => {
                        if (token.IsCancellationRequested)
                            return;
                        if (failed)
                            return;
                        if (predicate == null)
                            return;
                        if (await predicate(doc))
                            Interlocked.CompareExchange(ref failed, true, false);
                    }
                );
                return failed;
            }

            return false;
        }

        protected virtual async Task<RawType> _Download(string link, IProgress<IDownloadReport> progress, CancellationToken ct) {
            if (Options.DownloadFolder is not null && this is UnitDownloader<StringDocument, OutType>) {
                var path = Path.Combine(Options.DownloadFolder, Path.GetRandomFileName());
                await DownloadToFile(link, Options.BufferSize, path, progress, ct);
                return (RawType)(object)new StringDocument(link, path);
            }
            if (this is UnitDownloader<ByteDocument, OutType>) {
                return (RawType)(object)(await DownloadToMemory(link, Options.BufferSize, progress, ct));
            }
            throw new NotSupportedException(Exceptions.Exceptions.unit_downloader_limited_support);
        }
        
        protected virtual async Task<(bool, OutType?)> Transform(RawType download, CancellationToken ct) {
            try {
                if (FailurePredicates is null || !(await IsFailure(download, ct)))
                    return (true, await Transformer(download));
                else
                    return (false, default);
            } catch(Exception) {
                return (false, default);
            }
        }

        public async Task<(bool, OutType?)> TryDownload(IOrdered<string>[] link, CancellationToken ct, int maximumRetryCount = 7, IProgress<IDownloadReport>? downProgress = null, IProgress<IRetryReport>? tryProgress = null) {
            if (link.Length == 0)
                return (false, default);

            downProgress ??= new Progress<IDownloadReport>();
            
            OutType? ot = default;
            int tryCount = 0;
            while (tryCount < maximumRetryCount) {
                ct.ThrowIfCancellationRequested();
                var rt = await _Download(link[0].Data, downProgress, ct);
                (var success, ot) = await Transform(rt, ct);
                if (success && ot != null)
                    return (true, ot);
                ++tryCount;
                tryProgress?.Report(new RetryReport(tryCount, link[0].Data));
                await Task.Delay((int)Math.Pow(2, tryCount) * 1000);
            }

            return (false, ot);
        }
    }
}