using System;
using System.Collections.Generic;
using System.Numerics;
using AcDream.Core.Physics;
using DatReaderWriter.Enums;
using DatReaderWriter.Types;
using Xunit;

namespace AcDream.Core.Tests.Physics;

/// <summary>
/// Render Residual A — verbatim port of <c>CEnvCell::find_visible_child_cell</c>
/// (<c>acclient_2013_pseudo_c.txt:311397</c>): given a start cell, a world point,
/// and a mode, return the cell whose cell-BSP <c>point_in_cell</c> contains the
/// point — checking the start cell itself, then (stab-list mode) the start's
/// <c>VisibleCellIds</c> or (portal mode) its direct portal neighbours.
///
/// <para>
/// This is the sibling of <see cref="CellTransit.FindCellList"/> (retail
/// <c>find_cell_list</c>); both resolve cell membership from the cell graph. The
/// camera's <c>SmartBox::update_viewer</c> start-cell uses the stab-list mode
/// (<c>AdjustPosition</c> at pc:280028 passes <c>arg5=1</c>) to seat the sweep at
/// the PIVOT's cell, which differs from the feet cell at a low connector (the
/// cellar lip), where the pivot is up at floor level in a different cell.
/// </para>
///
/// <para>
/// Geometry is identity-transform (cell-local == world) so the synthetic CellBSP
/// splitting planes read directly: cell A is the half-space Y≤3, cell B (in A's
/// stab list) is the half-space Y≥7, and Y∈(3,7) belongs to neither.
/// </para>
/// </summary>
public class CellTransitFindVisibleChildCellTests
{
    private const uint StartCellId   = 0xA9B40174u;  // low 0x0174 ≥ 0x0100 → indoor
    private const uint SiblingCellId = 0xA9B40171u;  // the "room above" in StartCell's stab list

    [Fact]
    public void PointInsideStartCell_ReturnsStartCell()
    {
        var cache = new PhysicsDataCache();
        cache.RegisterCellStructForTest(StartCellId,  MakeCell(InteriorYAtMost(3f), new uint[] { SiblingCellId }));
        cache.RegisterCellStructForTest(SiblingCellId, MakeCell(InteriorYAtLeast(7f), Array.Empty<uint>()));

        // P at Y=1 is inside A (Y≤3) → the "this" branch returns the start cell.
        uint result = CellTransit.FindVisibleChildCell(cache, StartCellId, new Vector3(0f, 1f, 0f), useStabList: true);

        Assert.Equal(StartCellId, result);
    }

    [Fact]
    public void PointInStabListSibling_ReturnsSibling()
    {
        var cache = new PhysicsDataCache();
        cache.RegisterCellStructForTest(StartCellId,  MakeCell(InteriorYAtMost(3f), new uint[] { SiblingCellId }));
        cache.RegisterCellStructForTest(SiblingCellId, MakeCell(InteriorYAtLeast(7f), Array.Empty<uint>()));

        // P at Y=8 is outside A (Y≤3) but inside B (Y≥7), and B is in A's stab list.
        uint result = CellTransit.FindVisibleChildCell(cache, StartCellId, new Vector3(0f, 8f, 0f), useStabList: true);

        Assert.Equal(SiblingCellId, result);
    }

    [Fact]
    public void PointInNoCell_ReturnsZero()
    {
        var cache = new PhysicsDataCache();
        cache.RegisterCellStructForTest(StartCellId,  MakeCell(InteriorYAtMost(3f), new uint[] { SiblingCellId }));
        cache.RegisterCellStructForTest(SiblingCellId, MakeCell(InteriorYAtLeast(7f), Array.Empty<uint>()));

        // P at Y=5 is in the gap: outside A (Y≤3) and outside B (Y≥7).
        uint result = CellTransit.FindVisibleChildCell(cache, StartCellId, new Vector3(0f, 5f, 0f), useStabList: true);

        Assert.Equal(0u, result);
    }

    [Fact]
    public void UnknownStartCell_ReturnsZero()
    {
        var cache = new PhysicsDataCache();
        uint result = CellTransit.FindVisibleChildCell(cache, 0xDEADBEEFu, new Vector3(0f, 1f, 0f), useStabList: true);
        Assert.Equal(0u, result);
    }

    // ── helpers ─────────────────────────────────────────────────────────────

    /// <summary>CellBSP root for the half-space Y ≤ <paramref name="boundary"/>
    /// (interior on the −Y side; <c>point_in_cell</c> true when Y ≤ boundary).</summary>
    private static CellBSPNode InteriorYAtMost(float boundary) => new()
    {
        SplittingPlane = new Plane(new Vector3(0f, -1f, 0f), boundary),  // dist = boundary − Y ≥ 0 ⇔ Y ≤ boundary
        PosNode        = new CellBSPNode { Type = BSPNodeType.Leaf },
    };

    /// <summary>CellBSP root for the half-space Y ≥ <paramref name="boundary"/>.</summary>
    private static CellBSPNode InteriorYAtLeast(float boundary) => new()
    {
        SplittingPlane = new Plane(new Vector3(0f, 1f, 0f), -boundary),  // dist = Y − boundary ≥ 0 ⇔ Y ≥ boundary
        PosNode        = new CellBSPNode { Type = BSPNodeType.Leaf },
    };

    private static CellPhysics MakeCell(CellBSPNode cellBspRoot, uint[] visibleCellIds) => new()
    {
        BSP                   = new PhysicsBSPTree { Root = new PhysicsBSPNode { Type = BSPNodeType.Leaf } },
        WorldTransform        = Matrix4x4.Identity,
        InverseWorldTransform = Matrix4x4.Identity,
        Resolved              = new Dictionary<ushort, ResolvedPolygon>(),
        CellBSP               = new CellBSPTree { Root = cellBspRoot },
        Portals               = Array.Empty<PortalInfo>(),
        PortalPolygons        = new Dictionary<ushort, ResolvedPolygon>(),
        VisibleCellIds        = new HashSet<uint>(visibleCellIds),
    };
}