nadekobot/src/NadekoBot/Common/DownloadTracker.cs

namespace NadekoBot.Common;

public class DownloadTracker : INService
{
    private ConcurrentDictionary<ulong, DateTime> LastDownloads { get; } = new ConcurrentDictionary<ulong, DateTime>();
    private SemaphoreSlim downloadUsersSemaphore = new SemaphoreSlim(1, 1);

    /// <summary>
    /// Ensures all users on the specified guild were downloaded within the last hour. 
    /// </summary>
    /// <param name="guild">Guild to check and potentially download users from</param>
    /// <returns>Task representing download state</returns>
    public async Task EnsureUsersDownloadedAsync(IGuild guild)
    {
        await downloadUsersSemaphore.WaitAsync();
        try
        {
            var now = DateTime.UtcNow;

            // download once per hour at most
            var added = LastDownloads.AddOrUpdate(
                guild.Id,
                now,
                (key, old) => now - old > TimeSpan.FromHours(1) ? now : old);

            // means that this entry was just added - download the users
            if (added == now)
                await guild.DownloadUsersAsync();
        }
        finally
        {
            downloadUsersSemaphore.Release();
        }
    }
}