watchparty/src/WatchParty/Parties/PartyMember.cs

using System;

namespace WatchParty.Parties
{
    public class PartyMember
    {
        public Guid UserId { get; set; }

        public string UserName { get; set; }

        /// <summary>
        /// The Emby session this member is watching from. Resolved at join time
        /// from the caller's active sessions - see
        /// docs/architecture.md#which-session-is-mine for the current, fairly
        /// blunt, v1 heuristic.
        /// </summary>
        public string SessionId { get; set; }

        public DateTime JoinedUtc { get; set; }

        /// <summary>Last position reported for this member, in ticks (100ns units, Emby convention).</summary>
        public long LastKnownPositionTicks { get; set; }

        public bool LastKnownIsPaused { get; set; }

        /// <summary>When LastKnownPositionTicks/LastKnownIsPaused were last updated.</summary>
        public DateTime LastUpdatedUtc { get; set; }

        /// <summary>When we last sent this member a corrective command, for cooldown purposes.</summary>
        public DateTime? LastResyncUtc { get; set; }

        /// <summary>
        /// Extrapolates where this member's playback should be *right now*,
        /// assuming unchanged play/pause state since LastUpdatedUtc.
        /// </summary>
        public long EstimatedPositionTicks(DateTime utcNow)
        {
            if (LastKnownIsPaused)
            {
                return LastKnownPositionTicks;
            }

            var elapsed = utcNow - LastUpdatedUtc;
            if (elapsed <= TimeSpan.Zero)
            {
                return LastKnownPositionTicks;
            }

            return LastKnownPositionTicks + elapsed.Ticks;
        }
    }
}