using System.Diagnostics.CodeAnalysis;

namespace AcDream.App.Rendering.Walk;

/// <summary>
/// Campaign FW3.1 — the walk's per-landblock <see cref="WalkBuilding"/>
/// registry: the production sibling of
/// <see cref="AcDream.App.Rendering.Wb.BuildingRegistry"/>. Publish/retire
/// mirror that registry's landblock lifecycle exactly — both are committed
/// together in <c>LandblockRenderPublisher.AdvanceCompleteOne</c>'s
/// <c>BuildingRegistryCommitted</c> step and retired together from
/// <c>LandblockRenderPublisher.RemoveBuildingRegistry</c> — because a walk
/// building and its BFS-derived <c>Wb.Building</c> counterpart come from the
/// SAME <c>BuildingInfo</c> array at the SAME landblock commit.
///
/// The reverse index (<see cref="TryGetEntry"/>) exists because
/// <see cref="WalkProductionFrameContext"/> resolves a building's placement
/// by REFERENCE during the portal pass — <c>ViewpointInBuilding</c>,
/// <c>ViewerDistanceTo</c>, and <c>ClipBuildingPolygon</c> are called many
/// times per building per frame once FW3.2 wires the walk into the render
/// loop, so this must be O(1), not a per-landblock scan.
/// </summary>
public sealed class WalkBuildingRegistry
{
    private readonly Dictionary<uint, IReadOnlyList<WalkBuildingFactory.Entry>> _byLandblock = new();
    private readonly Dictionary<WalkBuilding, WalkBuildingFactory.Entry> _byBuilding = new();

    /// <summary>Atomically replaces one landblock's complete building set.
    /// Mirrors <c>Wb.BuildingRegistry</c>'s "no partial landblock" commit
    /// discipline — <paramref name="entries"/> is the immutable, fully-built
    /// list carried by the streaming worker's
    /// <c>Wb.EnvCellLandblockBuild.WalkBuildings</c>.</summary>
    public void Publish(uint landblockId, IReadOnlyList<WalkBuildingFactory.Entry> entries)
    {
        uint key = landblockId & 0xFFFF0000u;
        if (_byLandblock.TryGetValue(key, out IReadOnlyList<WalkBuildingFactory.Entry>? previous))
        {
            foreach (WalkBuildingFactory.Entry entry in previous)
                _byBuilding.Remove(entry.Building);
        }
        _byLandblock[key] = entries;
        foreach (WalkBuildingFactory.Entry entry in entries)
            _byBuilding[entry.Building] = entry;
    }

    /// <summary>Removes every building of one landblock. Safe to call on a
    /// landblock that never published (no-op).</summary>
    public void Retire(uint landblockId)
    {
        uint key = landblockId & 0xFFFF0000u;
        if (_byLandblock.Remove(key, out IReadOnlyList<WalkBuildingFactory.Entry>? previous))
        {
            foreach (WalkBuildingFactory.Entry entry in previous)
                _byBuilding.Remove(entry.Building);
        }
    }

    /// <summary>The buildings of one landblock, or empty when none are
    /// published (unloaded, far-tier, or no <c>BuildingInfo</c> entries).</summary>
    public IReadOnlyList<WalkBuildingFactory.Entry> GetBuildings(uint landblockId) =>
        _byLandblock.TryGetValue(landblockId & 0xFFFF0000u, out IReadOnlyList<WalkBuildingFactory.Entry>? list)
            ? list
            : Array.Empty<WalkBuildingFactory.Entry>();

    /// <summary>O(1) reverse lookup: this building's committed placement.
    /// False means the building is not (or no longer) committed — the walk
    /// must treat that as a hard desync, never a silent skip (the FW
    /// fail-loud rule).</summary>
    public bool TryGetEntry(
        WalkBuilding building, [MaybeNullWhen(false)] out WalkBuildingFactory.Entry entry) =>
        _byBuilding.TryGetValue(building, out entry);

    /// <summary>Number of landblocks with committed buildings (diagnostics).</summary>
    public int LandblockCount => _byLandblock.Count;
}