using System.Diagnostics;

namespace Terminal.Gui.App;

/// <summary>
///     Manages scheduled timeouts (timed callbacks) for the application.
///     <para>
///         Allows scheduling of callbacks to be invoked after a specified delay, with optional repetition.
///         Timeouts are stored in a sorted list by their scheduled execution time (high-resolution ticks).
///         Thread-safe for concurrent access.
///     </para>
///     <para>
///         Typical usage:
///         <list type="number">
///             <item>
///                 <description>Call <see cref="Add(TimeSpan, Func{bool})"/> to schedule a callback.</description>
///             </item>
///             <item>
///                 <description>
///                     Call <see cref="RunTimers"/> periodically (e.g., from the main loop) to execute due
///                     callbacks.
///                 </description>
///             </item>
///             <item>
///                 <description>Call <see cref="Remove"/> to cancel a scheduled timeout.</description>
///             </item>
///         </list>
///     </para>
/// </summary>
/// <remarks>
///     Uses <see cref="Stopwatch.GetTimestamp"/> for high-resolution timing instead of <see cref="DateTime.UtcNow"/>
///     to provide microsecond-level precision and eliminate race conditions from timer resolution issues.
/// </remarks>
public class TimedEvents : ITimedEvents
{
    internal SortedList<long, Timeout> _timeouts = new ();
    private readonly object _timeoutsLockToken = new ();

    /// <summary>
    ///     Gets the list of all timeouts sorted by the <see cref="TimeSpan"/> time ticks. A shorter limit time can be
    ///     added at the end, but it will be called before an earlier addition that has a longer limit time.
    /// </summary>
    public SortedList<long, Timeout> Timeouts => _timeouts;

    /// <inheritdoc/>
    public event EventHandler<TimeoutEventArgs>? Added;

    /// <summary>
    ///     Gets the current high-resolution timestamp in TimeSpan ticks.
    ///     Uses <see cref="Stopwatch.GetTimestamp"/> for microsecond-level precision.
    /// </summary>
    /// <returns>Current timestamp in TimeSpan ticks (100-nanosecond units).</returns>
    private static long GetTimestampTicks ()
    {
        // Convert Stopwatch ticks to TimeSpan ticks (100-nanosecond units)
        // Stopwatch.Frequency gives ticks per second, so we need to scale appropriately
        // To avoid overflow, we perform the operation in double precision first and then cast to long.
        var ticks = (long)((double)Stopwatch.GetTimestamp () * TimeSpan.TicksPerSecond / Stopwatch.Frequency);

        // Ensure ticks is positive and not overflowed (very unlikely now)
        Debug.Assert (ticks > 0);

        return ticks;
    }

    /// <inheritdoc/>
    public void RunTimers ()
    {
        lock (_timeoutsLockToken)
        {
            if (_timeouts.Count > 0)
            {
                RunTimersImpl ();
            }
        }
    }

    /// <inheritdoc/>
    public bool Remove (object token)
    {
        lock (_timeoutsLockToken)
        {
            int idx = _timeouts.IndexOfValue ((token as Timeout)!);

            if (idx == -1)
            {
                return false;
            }

            _timeouts.RemoveAt (idx);
        }

        return true;
    }

    /// <inheritdoc/>
    public object Add (TimeSpan time, Func<bool> callback)
    {
        ArgumentNullException.ThrowIfNull (callback);

        var timeout = new Timeout { Span = time, Callback = callback };
        AddTimeout (time, timeout);

        return timeout;
    }

    /// <inheritdoc/>
    public object Add (Timeout timeout)
    {
        AddTimeout (timeout.Span, timeout);

        return timeout;
    }

    /// <inheritdoc/>
    public bool CheckTimers (out int waitTimeout)
    {
        long now = GetTimestampTicks ();

        waitTimeout = 0;

        lock (_timeoutsLockToken)
        {
            if (_timeouts.Count > 0)
            {
                waitTimeout = (int)((_timeouts.Keys [0] - now) / TimeSpan.TicksPerMillisecond);

                if (waitTimeout < 0)
                {
                    // This avoids 'poll' waiting infinitely if 'waitTimeout < 0' until some action is detected
                    // This can occur after IMainLoopDriver.Wakeup is executed where the pollTimeout is less than 0
                    // and no event occurred in elapsed time when the 'poll' is start running again.
                    waitTimeout = 0;
                }

                return true;
            }

            // ManualResetEventSlim.Wait, which is called by IMainLoopDriver.EventsPending, will wait indefinitely if
            // the timeout is -1.
            waitTimeout = -1;
        }

        return false;
    }

    /// <inheritdoc/>
    public TimeSpan? GetTimeout (object token)
    {
        lock (_timeoutsLockToken)
        {
            int idx = _timeouts.IndexOfValue ((token as Timeout)!);

            if (idx == -1)
            {
                return null;
            }

            return _timeouts.Values [idx].Span;
        }
    }

    private void AddTimeout (TimeSpan time, Timeout timeout)
    {
        lock (_timeoutsLockToken)
        {
            long k = GetTimestampTicks () + time.Ticks;

            // if user wants to run as soon as possible set timer such that it expires right away (no race conditions)
            if (time == TimeSpan.Zero)
            {
                // Use a more substantial buffer (1ms) to ensure it's truly in the past
                // even under debugger overhead and extreme timing variations
                k -= TimeSpan.TicksPerMillisecond;
            }

            _timeouts.Add (NudgeToUniqueKey (k), timeout);
            Added?.Invoke (this, new (timeout, k));
        }
    }

    /// <summary>
    ///     Finds the closest number to <paramref name="k"/> that is not present in <see cref="_timeouts"/>
    ///     (incrementally).
    /// </summary>
    /// <param name="k"></param>
    /// <returns></returns>
    private long NudgeToUniqueKey (long k)
    {
        lock (_timeoutsLockToken)
        {
            while (_timeouts.ContainsKey (k))
            {
                k++;
            }
        }

        return k;
    }

    private void RunTimersImpl ()
    {
        long now = GetTimestampTicks ();

        // Process due timeouts one at a time, without blocking the entire queue
        while (true)
        {
            Timeout? timeoutToExecute = null;
            long scheduledTime = 0;

            // Find the next due timeout
            lock (_timeoutsLockToken)
            {
                if (_timeouts.Count == 0)
                {
                    break; // No more timeouts
                }

                // Re-evaluate current time for each iteration
                now = GetTimestampTicks ();
                
                // Check if the earliest timeout is due
                scheduledTime = _timeouts.Keys [0];
                
                if (scheduledTime >= now)
                {
                    // Earliest timeout is not yet due, we're done
                    break;
                }

                // This timeout is due - remove it from the queue
                timeoutToExecute = _timeouts.Values [0];
                _timeouts.RemoveAt (0);
            }

            // Execute the callback outside the lock
            // This allows nested Run() calls to access the timeout queue
            if (timeoutToExecute != null)
            {
                bool repeat = timeoutToExecute.Callback! ();
                
                if (repeat)
                {
                    AddTimeout (timeoutToExecute.Span, timeoutToExecute);
                }
            }
        }
    }

    /// <inheritdoc/>
    public void StopAll ()
    {
        lock (_timeoutsLockToken)
        {
            _timeouts.Clear ();
        }
    }
}