using System.Globalization;

/// <summary>
/// The single author of Assets/Shaders/Gravity.cginc, the GPU lowering of the zone's potential. The law is
/// GravityField's (its constants are read here, not respelled) and the height is Zone.GetHeight: each well's
/// potential minus its reference, summed, divided by the exaggeration. Unity does not run C# when it imports a
/// shader, so the committed cginc is this output; GravityShaderTests fails unless it equals Source() exactly.
/// To change the shader, change this file and regenerate: AETHERIA_REGENERATE_GRAVITY_CGINC=1 dotnet test --filter GravityShaderTests.
/// </summary>
public static class GravityShader
{
    public const string Path = "Assets/Shaders/Gravity.cginc";

    /// <summary>The whole file, with LF line ends.</summary>
    public static string Source() => (Template
        .Replace("@TAIL@", GravityField.TailOffset.ToString("R", CultureInfo.InvariantCulture)))
        .Replace("\r\n", "\n");

    private const string Template = @"#ifndef AETHERIA_GRAVITY_INCLUDED
#define AETHERIA_GRAVITY_INCLUDED

// GENERATED by ServerShared/GravityShader.cs from GravityField; do not edit (GravityShaderTests fails on any difference).
// The lowering of the zone's potential (aetheria-gravity, cut grid-lowering). The law is ServerShared's GravityField
// and the height is Zone.GetHeight; this file decides nothing. GravityLowering.Lower fills the buffer and the globals
// from the zone (positions from Orbit.Position, reference from Zone.WellReference) each frame.
// GravityFieldParityVerify renders it and compares every texel with -Zone.GetHeight.

struct GravityWell
{
    float2 position;
    float strength;
    float coreRadius;
    float reference;
};

StructuredBuffer<GravityWell> _GravityWells;
int _GravityWellCount;
float _HeightExaggeration;

// GravityField.Potential: c is the well's strength, R its core radius, r the distance.
float gravityPotential(float c, float R, float r)
{
    return c == 0 ? 0 :
        r < R
            ? c * r * r / (2 * R * R)
            : c * (log(r / R) + @TAIL@);
}

// Zone.GetHeight at world xz: each well's potential minus its own reference, summed, divided by the exaggeration.
float gravityHeight(float2 p)
{
    float result = 0;
    for (int i = 0; i < _GravityWellCount; i++)
    {
        GravityWell well = _GravityWells[i];
        result += gravityPotential(well.strength, well.coreRadius, length(p - well.position)) - well.reference;
    }
    return result / _HeightExaggeration;
}

#endif
";
}