using System;
using System.Collections.Generic;

namespace AcDream.App.Rendering.Wb;

/// <summary>
/// Phase A8 (2026-05-26): per-landblock registry of <see cref="Building"/>s.
/// Two-way indexed for O(1) cell→building and building-id→building lookups.
/// Built once per landblock at load time by <see cref="BuildingLoader"/>;
/// no mutations occur after initial population.
///
/// <para>The cell→building index uses a <c>List&lt;Building&gt;</c> value type
/// to handle the (rare but valid) case where two buildings share an EnvCell —
/// each building performs its own BFS so a shared boundary cell ends up in both
/// <c>EnvCellIds</c> sets. <see cref="GetBuildingsContainingCell"/> returns all
/// owners so RR7's render path can pick the correct one.</para>
///
/// <para>WB reference: <c>WorldBuilder.Shared/Services/PortalService.cs</c>
/// (<c>BuildingPortalGroup</c>). Design:
/// <c>docs/superpowers/specs/2026-05-26-phase-a8-wb-full-port-design.md</c>.</para>
/// </summary>
public sealed class BuildingRegistry
{
    // Index 1: cell-id → list of buildings containing that cell.
    // Cells may belong to multiple buildings (rare; handled via List<Building>).
    private readonly Dictionary<uint, List<Building>> _byCellId = new();

    // Index 2: building-id → Building.
    private readonly Dictionary<uint, Building> _byBuildingId = new();

    /// <summary>
    /// Adds a building to both indexes. Idempotent if the exact same
    /// <see cref="Building"/> instance is added twice with the same
    /// <see cref="Building.BuildingId"/>.
    /// </summary>
    /// <param name="b">The building to register.</param>
    public void Add(Building b)
    {
        if (_byBuildingId.TryGetValue(b.BuildingId, out var existing) && ReferenceEquals(existing, b))
            return;
        _byBuildingId[b.BuildingId] = b;
        foreach (var cellId in b.EnvCellIds)
        {
            if (!_byCellId.TryGetValue(cellId, out var list))
            {
                list = new List<Building>();
                _byCellId[cellId] = list;
            }
            if (!list.Contains(b)) list.Add(b);
        }
    }

    /// <summary>
    /// Returns the buildings containing <paramref name="cellId"/>.
    /// Returns an empty read-only list when the cell isn't part of any
    /// building (outdoor cells, dungeon cells not tagged by
    /// <c>LandBlockInfo.Buildings</c>).
    /// </summary>
    /// <param name="cellId">Full 32-bit cell id.</param>
    public IReadOnlyList<Building> GetBuildingsContainingCell(uint cellId) =>
        _byCellId.TryGetValue(cellId, out var list) ? list : Array.Empty<Building>();

    /// <summary>Returns the building with the given id, or <c>null</c> if not found.</summary>
    /// <param name="buildingId">The building id as allocated by <see cref="BuildingLoader"/>.</param>
    public Building? GetById(uint buildingId) =>
        _byBuildingId.TryGetValue(buildingId, out var b) ? b : null;

    /// <summary>Enumerates every registered building in unspecified order.</summary>
    public IEnumerable<Building> All() => _byBuildingId.Values;

    /// <summary>Number of registered buildings.</summary>
    public int Count => _byBuildingId.Count;
}