mmd-pdf/Services/WheelPageFlipGate.cs

namespace MmdPdf.Services
{
    /// <summary>
    /// Separates fast in-page wheel scrolling from page navigation at the edge. Momentum events
    /// immediately following a content scroll are ignored; after that, two standard wheel notches
    /// in the same direction confirm that the user intends to change pages (#205).
    /// </summary>
    internal sealed class WheelPageFlipGate
    {
        private static readonly TimeSpan MomentumQuietPeriod = TimeSpan.FromMilliseconds(250);
        private static readonly TimeSpan ConfirmationWindow = TimeSpan.FromMilliseconds(650);
        private const int ConfirmationDelta = 240;

        private DateTime _blockUntilUtc;
        private DateTime _lastEdgeWheelUtc;
        private int _direction;
        private int _accumulatedDelta;

        internal void NoteContentScroll(DateTime nowUtc)
        {
            _blockUntilUtc = nowUtc + MomentumQuietPeriod;
            ResetConfirmation();
        }

        internal bool TryConfirm(int delta, DateTime nowUtc)
        {
            if (delta == 0 || nowUtc < _blockUntilUtc)
            {
                ResetConfirmation();
                return false;
            }

            int direction = Math.Sign(delta);
            if (_direction != direction || nowUtc - _lastEdgeWheelUtc > ConfirmationWindow)
            {
                _direction = direction;
                _accumulatedDelta = 0;
            }

            _lastEdgeWheelUtc = nowUtc;
            _accumulatedDelta += Math.Abs(delta);
            if (_accumulatedDelta < ConfirmationDelta) return false;

            ResetConfirmation();
            return true;
        }

        private void ResetConfirmation()
        {
            _lastEdgeWheelUtc = default;
            _direction = 0;
            _accumulatedDelta = 0;
        }
    }
}