using System.Numerics;
using AcDream.App.Rendering;
using Xunit;

namespace AcDream.App.Tests.Rendering;

/// <summary>
/// Phase U.3: CPU-side proof that <see cref="ClipFrame"/> packs the shared clip
/// data in the EXACT std430 (mesh SSBO) / std140 (terrain UBO) byte layout the
/// shaders read. A silent layout drift here would mis-clip at U.4 with no build
/// error — these tests are the gate that catches it.
///
/// Layout under test (mesh CellClip, std430):
///   offset  0 : uint count
///   offset  4 : uint _p0  (pad)
///   offset  8 : uint _p1  (pad)
///   offset 12 : uint _p2  (pad)
///   offset 16 : vec4 planes[0]   (16-byte vec4 stride)
///   ...
///   offset 16 + i*16 : vec4 planes[i]
///   stride 144 bytes per slot.
/// Terrain UBO (std140): int count at 0 (padded to 16), vec4 planes[8] at 16.
/// </summary>
public class ClipFrameLayoutTests
{
    private static float ReadFloat(System.ReadOnlySpan<byte> b, int offset)
        => System.BitConverter.ToSingle(b.Slice(offset, 4));

    private static uint ReadUInt(System.ReadOnlySpan<byte> b, int offset)
        => System.BitConverter.ToUInt32(b.Slice(offset, 4));

    private static int ReadInt(System.ReadOnlySpan<byte> b, int offset)
        => System.BitConverter.ToInt32(b.Slice(offset, 4));

    [Fact]
    public void LayoutConstants_MatchShaderStruct()
    {
        // CellClip: 16 (count + 3 pad uints) + 8*16 (vec4 planes) = 144.
        Assert.Equal(144, ClipFrame.CellClipStrideBytes);
        Assert.Equal(16, ClipFrame.CellClipPlanesOffset);
        Assert.Equal(8, ClipFrame.MaxPlanes);
        Assert.Equal(144, ClipFrame.TerrainUboBytes);
        // Binding contract: mesh clip regions on SSBO binding=2, terrain on UBO binding=2.
        Assert.Equal(2u, ClipFrame.MeshClipSsboBinding);
        Assert.Equal(2u, ClipFrame.TerrainClipUboBinding);
    }

    [Fact]
    public void NoClip_HasExactlyOneSlot_AllZeros_Count0()
    {
        var frame = ClipFrame.NoClip();
        Assert.Equal(1, frame.SlotCount);

        var bytes = frame.RegionBytesForTest;
        Assert.Equal(ClipFrame.CellClipStrideBytes, bytes.Length); // 144 — exactly one slot

        // count == 0 ⇒ shader passes every plane (no-clip).
        Assert.Equal(0u, ReadUInt(bytes, 0));
        // Every byte of the reserved no-clip slot is zero.
        foreach (var b in bytes)
            Assert.Equal(0, b);
    }

    [Fact]
    public void NoClip_TerrainBytes_Count0_AllZeros()
    {
        var frame = ClipFrame.NoClip();
        var t = frame.TerrainBytesForTest;
        Assert.Equal(ClipFrame.TerrainUboBytes, t.Length);
        Assert.Equal(0, ReadInt(t, 0)); // count 0 ⇒ terrain ungated
        foreach (var b in t)
            Assert.Equal(0, b);
    }

    [Fact]
    public void AppendSlot_WritesCountAndPlanes_AtStd430Offsets()
    {
        var frame = ClipFrame.NoClip();

        // Three distinct planes so each lands at a verifiable offset.
        var p0 = new Vector4(1f, 0f, 0f, 0.5f);
        var p1 = new Vector4(0f, 1f, 0f, 0.25f);
        var p2 = new Vector4(-1f, 0f, 0f, -0.75f);

        int slot = frame.AppendSlot(new[] { p0, p1, p2 });
        Assert.Equal(1, slot);              // slot 0 is the reserved no-clip; this is slot 1
        Assert.Equal(2, frame.SlotCount);

        var bytes = frame.RegionBytesForTest;
        Assert.Equal(2 * ClipFrame.CellClipStrideBytes, bytes.Length); // two slots now

        int baseOff = slot * ClipFrame.CellClipStrideBytes; // 144

        // count == 3 at offset 0 of the slot; the 3 pad uints stay zero.
        Assert.Equal(3u, ReadUInt(bytes, baseOff + 0));
        Assert.Equal(0u, ReadUInt(bytes, baseOff + 4));
        Assert.Equal(0u, ReadUInt(bytes, baseOff + 8));
        Assert.Equal(0u, ReadUInt(bytes, baseOff + 12));

        // planes[0..2] at offset 16, 32, 48 (vec4 stride 16).
        AssertPlaneAt(bytes, baseOff + 16, p0);
        AssertPlaneAt(bytes, baseOff + 32, p1);
        AssertPlaneAt(bytes, baseOff + 48, p2);

        // Slot 0 (the reserved no-clip) is untouched: still count 0.
        Assert.Equal(0u, ReadUInt(bytes, 0));
    }

    [Fact]
    public void AppendSlot_EmptyPlaneList_PacksNoClipSlot_Count0()
    {
        var frame = ClipFrame.NoClip();
        int slot = frame.AppendSlot(System.ReadOnlySpan<Vector4>.Empty);
        Assert.Equal(1, slot);

        var bytes = frame.RegionBytesForTest;
        Assert.Equal(0u, ReadUInt(bytes, slot * ClipFrame.CellClipStrideBytes)); // count 0
    }

    [Fact]
    public void AppendSlot_ClampsToEightPlanes()
    {
        var frame = ClipFrame.NoClip();
        var planes = new Vector4[12];
        for (int i = 0; i < planes.Length; i++)
            planes[i] = new Vector4(i, 0f, 0f, 0f);

        int slot = frame.AppendSlot(planes);
        var bytes = frame.RegionBytesForTest;
        // Only MaxPlanes (8) are recorded in the count.
        Assert.Equal((uint)ClipFrame.MaxPlanes, ReadUInt(bytes, slot * ClipFrame.CellClipStrideBytes));
    }

    [Fact]
    public void AppendSlot_FromClipPlaneSet_AxisAlignedSquare_PacksFourPlanes()
    {
        // A unit square in NDC → ClipPlaneSet with 4 convex planes.
        var cv = new CellView();
        cv.Add(new ViewPolygon(new[]
        {
            new Vector2(-0.5f, -0.5f), new Vector2(0.5f, -0.5f),
            new Vector2(0.5f, 0.5f),   new Vector2(-0.5f, 0.5f),
        }));
        var cps = ClipPlaneSet.From(cv);
        Assert.Equal(4, cps.Count);

        var frame = ClipFrame.NoClip();
        int slot = frame.AppendSlot(cps);

        var bytes = frame.RegionBytesForTest;
        int baseOff = slot * ClipFrame.CellClipStrideBytes;
        Assert.Equal(4u, ReadUInt(bytes, baseOff + 0));

        // Each packed plane must match the ClipPlaneSet's plane bit-for-bit.
        for (int i = 0; i < 4; i++)
            AssertPlaneAt(bytes, baseOff + ClipFrame.CellClipPlanesOffset + i * 16, cps.Planes[i]);
    }

    [Fact]
    public void SetTerrainClip_WritesCountAndPlanes_AtStd140Offsets()
    {
        var frame = ClipFrame.NoClip();
        var p0 = new Vector4(0.3f, -0.4f, 0f, 0.1f);
        var p1 = new Vector4(-0.6f, 0.8f, 0f, -0.2f);

        frame.SetTerrainClip(new[] { p0, p1 });

        var t = frame.TerrainBytesForTest;
        Assert.Equal(2, ReadInt(t, 0)); // int count at offset 0
        AssertPlaneAt(t, ClipFrame.CellClipPlanesOffset + 0, p0);  // planes start at 16 under std140 too
        AssertPlaneAt(t, ClipFrame.CellClipPlanesOffset + 16, p1);
    }

    private static void AssertPlaneAt(System.ReadOnlySpan<byte> bytes, int offset, Vector4 expected)
    {
        Assert.Equal(expected.X, ReadFloat(bytes, offset + 0), 6);
        Assert.Equal(expected.Y, ReadFloat(bytes, offset + 4), 6);
        Assert.Equal(expected.Z, ReadFloat(bytes, offset + 8), 6);
        Assert.Equal(expected.W, ReadFloat(bytes, offset + 12), 6);
    }
}