LightlessClient/LightlessSync/WebAPI/Files/FileDownloadDeduplicator.cs

using System.Collections.Concurrent;

namespace LightlessSync.WebAPI.Files;

public readonly record struct DownloadClaim(bool IsOwner, Task<bool> Completion);

public sealed class FileDownloadDeduplicator
{
    private readonly ConcurrentDictionary<string, TaskCompletionSource<bool>> _inFlight =
        new(StringComparer.OrdinalIgnoreCase);

    public DownloadClaim Claim(string hash)
    {
        if (string.IsNullOrWhiteSpace(hash))
        {
            return new DownloadClaim(false, Task.FromResult(true));
        }

        var tcs = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
        var existing = _inFlight.GetOrAdd(hash, tcs);
        var isOwner = ReferenceEquals(existing, tcs);
        return new DownloadClaim(isOwner, existing.Task);
    }

    public void Complete(string hash, bool success)
    {
        if (string.IsNullOrWhiteSpace(hash))
        {
            return;
        }

        if (_inFlight.TryRemove(hash, out var tcs))
        {
            tcs.TrySetResult(success);
        }
    }

    public void CompleteAll(bool success)
    {
        foreach (var entry in _inFlight.ToArray())
        {
            if (_inFlight.TryRemove(entry.Key, out var tcs))
            {
                tcs.TrySetResult(success);
            }
        }
    }
}