// Copyright Epic Games, Inc. All Rights Reserved.

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using EpicGames.Horde.Storage;
using EpicGames.Serialization;
using Jupiter.Common.Implementation;

namespace Jupiter.Implementation.TransactionLog
{
	public class ReplicationLogSnapshotBuilder
	{
		private readonly IReplicationLog _replicationLog;
		private readonly IBlobService _blobService;
		private readonly IRefService _refService;
		private readonly BufferedPayloadFactory _bufferedPayloadFactory;
		private readonly ReplicationLogFactory _replicationLogFactory;

		public ReplicationLogSnapshotBuilder(IReplicationLog replicationLog, IBlobService blobService, IRefService refService, BufferedPayloadFactory bufferedPayloadFactory, ReplicationLogFactory replicationLogFactory)
		{
			_replicationLog = replicationLog;
			_blobService = blobService;
			_refService = refService;
			_bufferedPayloadFactory = bufferedPayloadFactory;
			_replicationLogFactory = replicationLogFactory;
		}

		public async Task<BlobId> BuildSnapshotAsync(NamespaceId ns, NamespaceId storeInNamespace, CancellationToken cancellationToken = default(CancellationToken))
		{
			// builds a snapshot and commits it to the blob store with the identifier specified
			SnapshotInfo? snapshotInfo = await _replicationLog.GetLatestSnapshotAsync(ns);

			if (cancellationToken.IsCancellationRequested)
			{
				throw new TaskCanceledException();
			}

			FileInfo? tempFile = null;
			ReplicationLogSnapshot? snapshot = null;

			try
			{
				BucketId bucketName = new BucketId("snapshot");
				string? lastBucket;
				Guid? lastEvent;
				if (snapshotInfo != null)
				{
					// append to the previous snapshot if one is available
					await using BlobContents blobContents = await _blobService.GetObjectAsync(snapshotInfo.BlobNamespace, snapshotInfo.SnapshotBlob,
						bucketHint: bucketName, cancellationToken: cancellationToken);
					if (cancellationToken.IsCancellationRequested)
					{
						throw new TaskCanceledException();
					}

					using IBufferedPayload snapshotPayload =
						await _bufferedPayloadFactory.CreateFilesystemBufferedPayloadAsync(blobContents.Stream, "snapshot-build", cancellationToken);
					await using Stream s = snapshotPayload.GetStream();
					snapshot = _replicationLogFactory.DeserializeSnapshotFromStream(s);
					lastBucket = snapshot.LastBucket;
					lastEvent = snapshot.LastEvent;
				}
				else
				{
					snapshot = ReplicationLogFactory.CreateEmptySnapshot(ns);

					lastBucket = null;
					lastEvent = null;
				}

				if (cancellationToken.IsCancellationRequested)
				{
					throw new TaskCanceledException();
				}

				await foreach (ReplicationLogEvent entry in _replicationLog.GetAsync(ns, lastBucket, lastEvent).WithCancellation(cancellationToken))
				{
					if (cancellationToken.IsCancellationRequested)
					{
						throw new TaskCanceledException();
					}

					snapshot.ProcessEvent(entry);
				}

				tempFile = new FileInfo(Path.GetTempFileName());

				{
					await using FileStream fs = tempFile.OpenWrite();
					snapshot.Serialize(fs);
					await fs.FlushAsync(cancellationToken);
				}

				Stream tempFileStream = tempFile.OpenRead();
				using IBufferedPayload payload = await _bufferedPayloadFactory.CreateFromStreamAsync(tempFileStream, tempFile.Length, "snapshot-serialize", cancellationToken);
				tempFileStream.Close();

				{
					BlobId blobIdentifier;
					{
						await using Stream stream = payload.GetStream();
						blobIdentifier = await BlobId.FromStreamAsync(stream, cancellationToken);
					}

					CbWriter writer = new CbWriter();
					writer.BeginObject();
					writer.WriteBinaryAttachment("snapshotBlob", blobIdentifier.AsIoHash());
					writer.WriteDateTime("timestamp", DateTime.Now);
					writer.EndObject();

					byte[] cbObjectBytes = writer.ToByteArray();
					BlobId cbBlobId = BlobId.FromBlob(cbObjectBytes);

					if (cancellationToken.IsCancellationRequested)
					{
						throw new TaskCanceledException();
					}

					// upload the attachment first so we are not missing any references when we go to create the ref
					await _blobService.PutObjectAsync(storeInNamespace, payload, blobIdentifier, bucketHint: bucketName, cancellationToken: cancellationToken);

					(ContentId[] missingContentIds, BlobId[] missingBlobs) = await _refService.PutAsync(storeInNamespace, bucketName,
						new RefId(blobIdentifier.ToString()), cbBlobId, new CbObject(cbObjectBytes), cancellationToken: cancellationToken);
					List<ContentHash> missingHashes = new List<ContentHash>(missingContentIds);
					missingHashes.AddRange(missingBlobs);
					if (missingHashes.Count != 0)
					{
						throw new Exception(
							$"Failed to upload snapshot to object service, missing references {string.Join(',', missingHashes.Select(b => b.ToString()))}");
					}

					if (cancellationToken.IsCancellationRequested)
					{
						throw new TaskCanceledException();
					}

					// update the replication log with the new snapshot
					await _replicationLog.AddSnapshotAsync(new SnapshotInfo(ns, storeInNamespace, blobIdentifier, DateTime.Now));

					return blobIdentifier;
				}
			}
			finally
			{
				snapshot?.Dispose();
				tempFile?.Delete();
			}
		}
	}
}