using System.Collections.Generic;
using System.Linq;
using AcDream.Core.World;

namespace AcDream.App.Streaming;

/// <summary>
/// Render-thread-owned registry of currently-loaded landblocks and their
/// entities. All mutation happens in <see cref="StreamingController.Tick"/>
/// on the render thread; the renderer reads <see cref="Entities"/> once per
/// frame.
///
/// <para>
/// Replaces <c>GameWindow._entities</c>, which was a flat list updated in
/// multiple places. This class is the single point of truth for "what's in
/// the world right now" and the only thing that mutates it.
/// </para>
///
/// <remarks>
/// Threading: not thread-safe. All calls must happen on the render thread.
/// The streaming worker never touches this type.
/// </remarks>
/// </summary>
public sealed class GpuWorldState
{
    private readonly Dictionary<uint, LoadedLandblock> _loaded = new();

    // Cached flat view over all entities across all loaded landblocks,
    // rebuilt on each add/remove. The renderer holds a reference to this
    // list, so rebuilding it replaces the reference atomically.
    private IReadOnlyList<WorldEntity> _flatEntities = System.Array.Empty<WorldEntity>();

    public IReadOnlyList<WorldEntity> Entities => _flatEntities;
    public IReadOnlyCollection<uint> LoadedLandblockIds => _loaded.Keys;

    public bool IsLoaded(uint landblockId) => _loaded.ContainsKey(landblockId);

    public void AddLandblock(LoadedLandblock landblock)
    {
        _loaded[landblock.LandblockId] = landblock;
        RebuildFlatView();
    }

    public void RemoveLandblock(uint landblockId)
    {
        if (_loaded.Remove(landblockId))
            RebuildFlatView();
    }

    /// <summary>
    /// Append an entity to a specific landblock's slot. Used by the live
    /// CreateObject path where the server spawns entities into an already-
    /// loaded landblock after the initial hydration pass.
    /// </summary>
    public void AppendLiveEntity(uint landblockId, WorldEntity entity)
    {
        if (!_loaded.TryGetValue(landblockId, out var lb))
            return;

        // LoadedLandblock.Entities is an IReadOnlyList. Rebuild the
        // landblock record with the new entity appended. We accept the
        // allocation here because live spawns are rare compared to frame
        // iteration.
        var newEntities = new List<WorldEntity>(lb.Entities.Count + 1);
        newEntities.AddRange(lb.Entities);
        newEntities.Add(entity);
        _loaded[landblockId] = new LoadedLandblock(lb.LandblockId, lb.Heightmap, newEntities);
        RebuildFlatView();
    }

    private void RebuildFlatView()
    {
        _flatEntities = _loaded.Values.SelectMany(lb => lb.Entities).ToArray();
    }
}