using System.Numerics;
using AcDream.Core.Lighting;
using Xunit;

namespace AcDream.Core.Tests.Lighting;

public sealed class LightManagerTests
{
    private static LightSource MakePoint(Vector3 pos, float range, uint ownerId = 0, bool lit = true)
        => new LightSource
        {
            Kind = LightKind.Point,
            WorldPosition = pos,
            Range = range,
            IsLit = lit,
            OwnerId = ownerId,
        };

    [Fact]
    public void Register_Unregister_TracksList()
    {
        var mgr = new LightManager();
        var a = MakePoint(Vector3.Zero, 5f);
        var b = MakePoint(new Vector3(10, 0, 0), 5f);
        mgr.Register(a);
        mgr.Register(b);
        Assert.Equal(2, mgr.RegisteredCount);

        mgr.Unregister(a);
        Assert.Equal(1, mgr.RegisteredCount);
    }

    [Fact]
    public void Register_DuplicateInstance_Idempotent()
    {
        var mgr = new LightManager();
        var light = MakePoint(Vector3.Zero, 5f);
        mgr.Register(light);
        mgr.Register(light);
        Assert.Equal(1, mgr.RegisteredCount);
    }

    [Fact]
    public void Tick_SelectsByDistance_Top8()
    {
        var mgr = new LightManager();
        // 12 lights at varying distances, all with range 100 so none filter out.
        for (int i = 0; i < 12; i++)
            mgr.Register(MakePoint(new Vector3(i, 0, 0), 100f));

        mgr.Tick(viewerWorldPos: Vector3.Zero);

        Assert.Equal(8, mgr.ActiveCount);
        // Top 8 should be the closest (i=0..7).
        foreach (var l in mgr.Active)
        {
            Assert.NotNull(l);
            Assert.True(l!.WorldPosition.X <= 7f);
        }
    }

    [Fact]
    public void Tick_SelectsByDistance_RegardlessOfViewerRange()
    {
        // Retail D3D-style: candidacy is distance-only (the nearest 8). A torch
        // lights its OWN surfaces — the shader applies the hard `d < range` cutoff
        // PER FRAGMENT (mesh_modern.frag) — so a torch the VIEWER is standing
        // outside the range of is still selected; it lights the wall it sits on.
        // Replaces the old viewer-range candidacy filter that suppressed it, which
        // left dungeon rooms (2227 registered torches) at activeLights≈1 / flat 0.2
        // ambient — the "dungeon lighting off" report (#133 A7).
        var mgr = new LightManager();
        mgr.Register(MakePoint(new Vector3(20, 0, 0), range: 5f));  // viewer outside the torch's range

        mgr.Tick(viewerWorldPos: Vector3.Zero);

        Assert.Equal(1, mgr.ActiveCount);   // selected by distance; the shader culls per-surface
    }

    [Fact]
    public void Tick_IncludesNearbyLight()
    {
        var mgr = new LightManager();
        // A nearby point light is selected (distance-only candidacy; the shader
        // applies the per-fragment range cutoff).
        mgr.Register(MakePoint(new Vector3(5, 0, 0), range: 5f));

        mgr.Tick(viewerWorldPos: Vector3.Zero);
        Assert.Equal(1, mgr.ActiveCount);
    }

    [Fact]
    public void Tick_SunSlot0_PreservedAcrossTicks()
    {
        var mgr = new LightManager();
        var sun = new LightSource { Kind = LightKind.Directional, WorldForward = -Vector3.UnitZ };
        mgr.Sun = sun;

        mgr.Register(MakePoint(Vector3.Zero, 100f));
        mgr.Tick(Vector3.Zero);

        Assert.Equal(2, mgr.ActiveCount);
        Assert.Same(sun, mgr.Active[0]);
    }

    [Fact]
    public void Tick_UnlitLight_Excluded()
    {
        var mgr = new LightManager();
        var light = MakePoint(Vector3.Zero, 100f, lit: false);
        mgr.Register(light);

        mgr.Tick(Vector3.Zero);
        Assert.Equal(0, mgr.ActiveCount);

        // Toggle lit: should now appear.
        light.IsLit = true;
        mgr.Tick(Vector3.Zero);
        Assert.Equal(1, mgr.ActiveCount);
    }

    [Fact]
    public void UnregisterByOwner_RemovesAttachedLights()
    {
        var mgr = new LightManager();
        mgr.Register(MakePoint(Vector3.Zero, 5f, ownerId: 42));
        mgr.Register(MakePoint(new Vector3(1, 0, 0), 5f, ownerId: 42));
        mgr.Register(MakePoint(new Vector3(2, 0, 0), 5f, ownerId: 99));

        mgr.UnregisterByOwner(42);
        Assert.Equal(1, mgr.RegisteredCount);
    }

    [Fact]
    public void DistSq_UpdatedEachTick()
    {
        var mgr = new LightManager();
        var light = MakePoint(new Vector3(3, 0, 4), 10f);   // dist 5
        mgr.Register(light);

        mgr.Tick(Vector3.Zero);
        Assert.Equal(25f, light.DistSq, 2);

        mgr.Tick(new Vector3(3, 0, 0));   // same x, same y, z diff 4
        Assert.Equal(16f, light.DistSq, 2);
    }

    // ── Fix B: per-object selection (minimize_object_lighting) ────────────────

    [Fact]
    public void BuildPointLightSnapshot_ExcludesDirectionalAndUnlit()
    {
        var mgr = new LightManager();
        mgr.Register(MakePoint(new Vector3(1, 0, 0), 5f));               // in
        mgr.Register(MakePoint(new Vector3(2, 0, 0), 5f, lit: false));   // unlit → out
        mgr.Register(new LightSource { Kind = LightKind.Directional });  // sun → out

        mgr.BuildPointLightSnapshot(Vector3.Zero);

        Assert.Single(mgr.PointSnapshot);
        Assert.Equal(1f, mgr.PointSnapshot[0].WorldPosition.X, 3);
    }

    [Fact]
    public void BuildPointLightSnapshot_IndexStable_InBudget()
    {
        var mgr = new LightManager();
        // Registration order preserved when under MaxGlobalLights (no sort).
        mgr.Register(MakePoint(new Vector3(100, 0, 0), 5f));  // far
        mgr.Register(MakePoint(new Vector3(1, 0, 0), 5f));    // near

        mgr.BuildPointLightSnapshot(Vector3.Zero);

        Assert.Equal(2, mgr.PointSnapshot.Count);
        Assert.Equal(100f, mgr.PointSnapshot[0].WorldPosition.X, 3);  // index 0 = first registered
        Assert.Equal(1f, mgr.PointSnapshot[1].WorldPosition.X, 3);
    }

    [Fact]
    public void SelectForObject_EmptySnapshot_ReturnsZero()
    {
        Span<int> idx = stackalloc int[8];
        int n = LightManager.SelectForObject(System.Array.Empty<LightSource>(), Vector3.Zero, 1f, idx);
        Assert.Equal(0, n);
    }

    [Fact]
    public void SelectForObject_InRange_Selected()
    {
        var snapshot = new[] { MakePoint(new Vector3(3, 0, 0), range: 5f) };  // dist 3 < range 5
        Span<int> idx = stackalloc int[8];
        int n = LightManager.SelectForObject(snapshot, Vector3.Zero, radius: 0f, idx);
        Assert.Equal(1, n);
        Assert.Equal(0, idx[0]);
    }

    [Fact]
    public void SelectForObject_OutOfRange_Excluded()
    {
        // dist 10, range 5, radius 0 → 10 >= 5 → excluded.
        var snapshot = new[] { MakePoint(new Vector3(10, 0, 0), range: 5f) };
        Span<int> idx = stackalloc int[8];
        int n = LightManager.SelectForObject(snapshot, Vector3.Zero, radius: 0f, idx);
        Assert.Equal(0, n);
    }

    [Fact]
    public void SelectForObject_ObjectRadiusExtendsReach()
    {
        // dist 7, range 5: out of reach at radius 0, but a radius-3 object sphere
        // overlaps (7 < 5+3). The whole object catches the light — retail uses the
        // object's bounding sphere, not its centre point.
        var snapshot = new[] { MakePoint(new Vector3(7, 0, 0), range: 5f) };
        Span<int> idx = stackalloc int[8];

        Assert.Equal(0, LightManager.SelectForObject(snapshot, Vector3.Zero, radius: 0f, idx));
        Assert.Equal(1, LightManager.SelectForObject(snapshot, Vector3.Zero, radius: 3f, idx));
    }

    [Fact]
    public void SelectForObject_MoreThan8_KeepsNearest8()
    {
        // 10 candidate lights all in range; expect the 8 nearest the object centre,
        // ascending by distance, with the two farthest dropped.
        var snapshot = new LightSource[10];
        for (int i = 0; i < 10; i++)
            snapshot[i] = MakePoint(new Vector3(i + 1, 0, 0), range: 100f);  // dist i+1, all in range

        Span<int> idx = stackalloc int[8];
        int n = LightManager.SelectForObject(snapshot, Vector3.Zero, radius: 0f, idx);

        Assert.Equal(8, n);
        // Nearest-first: index 0 (dist 1) … index 7 (dist 8). The two farthest
        // (indices 8,9 / dist 9,10) are evicted.
        for (int k = 0; k < 8; k++)
            Assert.Equal(k, idx[k]);
    }

    [Fact]
    public void SelectForObject_CameraIndependent_DependsOnlyOnObjectCentre()
    {
        // Same snapshot, same object centre → identical selection regardless of
        // where any "camera" is (the method takes no camera). This is the property
        // that kills the "lights up as I approach" popping.
        var snapshot = new[]
        {
            MakePoint(new Vector3(2, 0, 0), range: 10f),
            MakePoint(new Vector3(20, 0, 0), range: 10f),  // out of reach of centre 0
        };
        Span<int> a = stackalloc int[8];
        Span<int> b = stackalloc int[8];
        int na = LightManager.SelectForObject(snapshot, Vector3.Zero, 1f, a);
        int nb = LightManager.SelectForObject(snapshot, Vector3.Zero, 1f, b);

        Assert.Equal(1, na);
        Assert.Equal(na, nb);
        Assert.Equal(a[0], b[0]);
    }
}