using System.Reactive.Linq;
using System.Reactive.Concurrency;

namespace StarsAssistant.Helpers;

public class FsWatcher
{
    /// <summary>
    /// Throttle a FileSystemObserver and eliminate all duplicates.
    /// </summary>
    /// <remarks>
    /// Curtesy of https://endjin.com/blog/2024/05/observe-file-system-changes-with-rx-dotnet
    /// </remarks>
    /// <param name="watcher">An observer created with <c>ObserveFileSystem</c></param>
    /// <param name="inactivitySeconds">Throttling window, defaults to 1 second.</param>
    /// <param name="scheduler">Scheduler to user for throttling, default is set by <c>Quiescent</c></param>
    /// <returns>A throttled observer with duplicate elimination.</returns>
    public static IObservable<IEnumerable<FileSystemEventArgs>> ThrottleAndDistinctObserver (
        IObservable<FileSystemEventArgs> watcher, 
        int inactivitySeconds = 1,
        IScheduler? scheduler = null)
    {
        return watcher
                .Quiescent(TimeSpan.FromSeconds(inactivitySeconds), scheduler)
                .Select(changes => changes.DistinctBy(x => (x.ChangeType, x.FullPath)));
    }

    /// <summary>
    /// Helper to convert a FileSystemWatcher into an obervable stream. See FileSystemWatcher
    /// documentation for further details on the parameters.
    /// </summary>
    /// <remarks>
    /// Curtesy of https://endjin.com/blog/2024/05/observe-file-system-changes-with-rx-dotnet
    /// </remarks>
    /// <param name="directoryPath">The base directory to watch over.</param>
    /// <param name="filters">An optional list of filter expressions to look for, default is no filter.</param>
    /// <param name="includeSubdirectories">Scan subdirectories, default is exclude subdirs</param>
    /// <param name="notifyFilter">Set this to override the default event selection to filter.</param>
    /// <returns>An observable for further use, usually throttled by Quiescent.</returns>
    public static IObservable<FileSystemEventArgs> ObserveFileSystem(
        string directoryPath, 
        IEnumerable<string>? filters = null, 
        bool includeSubdirectories = false, 
        NotifyFilters? notifyFilter = null)
    {
        return
            // Observable.Defer enables us to avoid doing any work
            // until we have a subscriber.
            Observable.Defer(() =>
                {
                    FileSystemWatcher fsw = new(directoryPath);
                    if (filters != null)
                        foreach (string filter in filters)
                            fsw.Filters.Add(filter);
                    if (notifyFilter != null)
                        fsw.NotifyFilter = (NotifyFilters) notifyFilter;
                    fsw.EnableRaisingEvents = true;
                    fsw.IncludeSubdirectories = includeSubdirectories;

                    return Observable.Return(fsw);
                })
                // Once the preceding part emits the FileSystemWatcher
                // (which will happen when someone first subscribes), we
                // want to wrap all the events as IObservable<T>s, for which
                // we'll use a projection. To avoid ending up with an
                // IObservable<IObservable<FileSystemEventArgs>>, we use
                // SelectMany, which effectively flattens it by one level.
                .SelectMany(fsw =>
                    Observable.Merge(new[]
                        {
                            Observable.FromEventPattern<FileSystemEventHandler, FileSystemEventArgs>(
                                h => fsw.Created += h, h => fsw.Created -= h),
                            Observable.FromEventPattern<FileSystemEventHandler, FileSystemEventArgs>(
                                h => fsw.Changed += h, h => fsw.Changed -= h),
                            Observable.FromEventPattern<RenamedEventHandler, FileSystemEventArgs>(
                                h => fsw.Renamed += h, h => fsw.Renamed -= h),
                            Observable.FromEventPattern<FileSystemEventHandler, FileSystemEventArgs>(
                                h => fsw.Deleted += h, h => fsw.Deleted -= h)
                        })
                        // FromEventPattern supplies both the sender and the event
                        // args. Extract just the latter.
                        .Select(ep => ep.EventArgs)
                        // The Finally here ensures the watcher gets shut down once
                        // we have no subscribers.
                        .Finally(fsw.Dispose))
                // This combination of Publish and RefCount means that multiple
                // subscribers will get to share a single FileSystemWatcher,
                // but that it gets shut down if all subscribers unsubscribe.
                .Publish()
                .RefCount();
    }

}