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

namespace AcDream.App.Tests.Rendering;

public sealed class RetailDetailTextureContractTests
{
    private static readonly TerrainAtlas.RetailDetailTextureBinding Available = new(
        new GpuTextureSlot(7),
        Tiling: 4f,
        SurfaceTextureId: 0x05001787,
        RenderSurfaceId: 0x06006D58,
        Width: 256,
        Height: 256);

    [Fact]
    public void ExistingBuildingDetailSettingIsTheLivePassGate()
    {
        Assert.False(RetailDetailTextureContract.ShouldRender(false, Available));
        Assert.True(RetailDetailTextureContract.ShouldRender(true, Available));
        Assert.False(RetailDetailTextureContract.ShouldRender(true, default));
    }

    [Fact]
    public void OpaqueDetailUsesDepthEqualityWhileTransparentDetailDoesNot()
    {
        Assert.Equal(
            GpuCompareOp.Equal,
            RetailDetailTextureContract.DetailDepthCompare(transparent: false));
        Assert.Equal(
            GpuCompareOp.LessOrEqual,
            RetailDetailTextureContract.DetailDepthCompare(transparent: true));
    }

    [Theory]
    [InlineData(0.1f, 0.2f, 0.3f)]
    [InlineData(0.0f, 0.0f, 0.0f)]
    [InlineData(1.0f, 1.0f, 1.0f)]
    [InlineData(0.5f, 0.9f, 0.05f)]
    public void ZeroDetailAlphaIsExactNoOp(float r, float g, float b)
    {
        var baseColour = new Vector3(r, g, b);
        var transparentDetail = new Vector4(0.9f, 0.1f, 0.7f, 0f);

        Assert.Equal(
            baseColour,
            RetailDetailTextureContract.Expected(baseColour, transparentDetail, opacity: 1f));
        Assert.True(RetailDetailTextureContract.IsNeutral(transparentDetail, opacity: 1f));
    }

    [Fact]
    public void ZeroOpacityIsExactNoOp()
    {
        var baseColour = new Vector3(0.5f, 0.5f, 0.5f);
        var derethCategory = new Vector4(0.165f, 0.165f, 0.165f, 0.132f);

        // opacity 0 models a translucency fade that has finished disappearing:
        // even a fully-opaque detail texel contributes nothing.
        Assert.Equal(
            baseColour,
            RetailDetailTextureContract.Expected(baseColour, derethCategory, opacity: 0f));
        Assert.True(RetailDetailTextureContract.IsNeutral(derethCategory, opacity: 0f));
    }

    [Fact]
    public void LiveDerethCategoryTextureDarkensMidtones()
    {
        // Live category texture 0x06006D58 (VM2), opaque diffuse (opacity 1).
        var detail = new Vector4(0.165f, 0.165f, 0.161f, 0.132f);
        var baseColour = new Vector3(0.5f, 0.5f, 0.5f);

        Vector3 result = RetailDetailTextureContract.Expected(baseColour, detail, opacity: 1f);

        Assert.True(result.X < baseColour.X);
        Assert.True(result.Y < baseColour.Y);
        Assert.True(result.Z < baseColour.Z);

        // lerp(base, detail.rgb, 0.132) ~= 0.868 * base + 0.0218 per channel —
        // the mild darkening VM2 measured, not the two-pass fallback's
        // brightening. Compared as an absolute difference (not xunit's
        // decimal-rounding `precision` parameter, which would flip a channel
        // sitting on a rounding boundary) against the stated 1e-3 tolerance.
        float approx = 0.868f * baseColour.X + 0.0218f; // same for every channel: baseColour is uniform
        Assert.True(MathF.Abs(result.X - approx) < 1e-3f, $"R off by {result.X - approx}");
        Assert.True(MathF.Abs(result.Y - approx) < 1e-3f, $"G off by {result.Y - approx}");
        Assert.True(MathF.Abs(result.Z - approx) < 1e-3f, $"B off by {result.Z - approx}");
        Assert.False(RetailDetailTextureContract.IsNeutral(detail, opacity: 1f));
    }

    [Fact]
    public void FullAlphaReplacesBaseWithDetailColour()
    {
        var baseColour = new Vector3(0.9f, 0.1f, 0.4f);
        var detail = new Vector4(0.2f, 0.3f, 0.4f, 1f);

        Vector3 result = RetailDetailTextureContract.Expected(baseColour, detail, opacity: 1f);

        // Tolerance rather than bit-exact equality: Lerp's `a + (b - a) * t`
        // form is not guaranteed to cancel `a` perfectly at t=1 in floating
        // point for arbitrary inputs.
        Assert.True(MathF.Abs(result.X - detail.X) < 1e-6f);
        Assert.True(MathF.Abs(result.Y - detail.Y) < 1e-6f);
        Assert.True(MathF.Abs(result.Z - detail.Z) < 1e-6f);
    }

    [Fact]
    public void FogAppliedPerDrawMatchesRetailsFogAfterCombineOrder()
    {
        // Review fix (post-05970306): retail fogs the pixel AFTER the
        // texture-stage combine (mix(lerp(base,detail,a), fog, f)), but
        // acdream draws the combine as two separate passes, each fogging its
        // own colour before the fixed-function blend recombines them
        // (lerp(mix(base,fog,f), mix(detail,fog,f), a)). This pins that the
        // two orders are the same pixel for arbitrary inputs — see
        // ExpectedFogged's doc comment and mesh_detail.frag's header comment
        // for the algebra.
        var random = new Random(0x226226);
        for (int i = 0; i < 200; i++)
        {
            var baseColour = NextColour(random);
            var detail = new Vector4(NextColour(random), random.NextSingle());
            float opacity = random.NextSingle();
            var fog = NextColour(random);
            float fogFactor = random.NextSingle();

            Vector3 twoDrawOrder = RetailDetailTextureContract.ExpectedFogged(
                baseColour, detail, opacity, fog, fogFactor);
            Vector3 retailOrder = Vector3.Lerp(
                RetailDetailTextureContract.Expected(baseColour, detail, opacity),
                fog,
                fogFactor);

            Assert.True(
                MathF.Abs(twoDrawOrder.X - retailOrder.X) < 1e-6f,
                $"R: two-draw {twoDrawOrder.X} vs retail {retailOrder.X} (sample {i})");
            Assert.True(
                MathF.Abs(twoDrawOrder.Y - retailOrder.Y) < 1e-6f,
                $"G: two-draw {twoDrawOrder.Y} vs retail {retailOrder.Y} (sample {i})");
            Assert.True(
                MathF.Abs(twoDrawOrder.Z - retailOrder.Z) < 1e-6f,
                $"B: two-draw {twoDrawOrder.Z} vs retail {retailOrder.Z} (sample {i})");
        }
    }

    private static Vector3 NextColour(Random random) =>
        new(random.NextSingle(), random.NextSingle(), random.NextSingle());
}