using System.Collections.Generic;

namespace AcDream.App.Streaming;

/// <summary>
/// Decides which retained server-object spawns to re-hydrate (rebuild render
/// entities for) when a landblock (re)loads. The pure selection half of the
/// #138 fix; <c>GameWindow</c> owns the replay half (it alone can rebuild a
/// mesh from a <c>CreateObject</c>).
///
/// <para>
/// <b>Why this exists (retail/ACE model).</b> A real AC client keeps its
/// <c>weenie_object_table</c> across a teleport and re-projects its rendered
/// world from that table; the server does NOT re-broadcast objects it believes
/// the client already knows (ACE's per-player <c>KnownObjects</c> set is never
/// cleared on a normal teleport — ACE relies on the client retaining the
/// table, cross-checked against <c>references/holtburger</c> +
/// <c>references/ACE/Source/ACE.Server/Physics/Common/ObjectMaint.cs</c>).
/// </para>
///
/// <para>
/// acdream's dungeon-collapse (and Near→Far demote) drops a landblock's
/// RENDER entities for FPS but retains the parsed spawns
/// (<c>GameWindow._lastSpawnByGuid</c> — the table is only pruned by an
/// explicit server <c>DeleteObject</c> or a spawn de-dup). On reload, ACE will
/// not re-send the objects it still thinks we have, so the render side stays
/// empty — the #138 symptom. This selects the retained spawns whose render
/// entity is currently absent so <c>GameWindow</c> can replay them, making
/// re-delivery independent of the server.
/// </para>
/// </summary>
public static class LandblockEntityRehydrator
{
    /// <summary>
    /// A retained spawn reduced to just the fields the selection needs.
    /// <paramref name="HasWorldMesh"/> is true only when the spawn carries both
    /// a world position AND a Setup id — i.e. it would actually build a visible
    /// render entity (inventory items and setup-less spawns produce none and
    /// are skipped, matching <c>OnLiveEntitySpawnedLocked</c>'s own guard).
    /// </summary>
    public readonly record struct RetainedSpawn(uint Guid, uint SpawnLandblockId, bool HasWorldMesh);

    /// <summary>
    /// Canonical landblock id (low 16 bits forced to <c>0xFFFF</c>) — the same
    /// keying <see cref="GpuWorldState.AppendLiveEntity"/> uses, so a
    /// cell-resolved spawn id (<c>0xAAAA00CC</c>) and a streamed landblock id
    /// (<c>0xAAAAFFFF</c>) compare equal when they name the same landblock.
    /// </summary>
    public static uint Canonicalize(uint landblockId) => (landblockId & 0xFFFF0000u) | 0xFFFFu;

    /// <summary>
    /// Select the guids whose retained spawn should be replayed for the
    /// just-loaded <paramref name="loadedLandblockId"/>.
    /// </summary>
    /// <param name="loadedLandblockId">The landblock that just (re)loaded.</param>
    /// <param name="retainedSpawns">Snapshot of the retained world-object spawns.</param>
    /// <param name="presentServerGuids">
    /// Server guids that already have a live render entity in
    /// <see cref="GpuWorldState"/>. A guid here was either never dropped or was
    /// already re-delivered by the server, so replaying it would be redundant.
    /// </param>
    /// <param name="playerServerGuid">
    /// The local player's guid — never re-hydrated here; it is owned by the
    /// persistent-entity rescue path (<see cref="GpuWorldState.MarkPersistent"/>
    /// + <c>DrainRescued</c>), which preserves the player's live pose/position
    /// rather than resetting it to the spawn snapshot.
    /// </param>
    public static List<uint> SelectGuidsToRehydrate(
        uint loadedLandblockId,
        IReadOnlyCollection<RetainedSpawn> retainedSpawns,
        IReadOnlySet<uint> presentServerGuids,
        uint playerServerGuid)
    {
        uint loadedCanonical = Canonicalize(loadedLandblockId);
        var result = new List<uint>();
        foreach (var s in retainedSpawns)
        {
            if (!s.HasWorldMesh) continue;                              // no visible entity to rebuild
            if (s.Guid == playerServerGuid) continue;                  // player: persistent-rescue path owns it
            if (Canonicalize(s.SpawnLandblockId) != loadedCanonical) continue; // different landblock
            if (presentServerGuids.Contains(s.Guid)) continue;         // already rendered
            result.Add(s.Guid);
        }
        return result;
    }
}