PixiEditor/src/PixiEditor.DrawingApi.Core/Shaders/Generation/BuiltInFunctions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using PixiEditor.DrawingApi.Core.Shaders.Generation.Expressions;

namespace PixiEditor.DrawingApi.Core.Shaders.Generation;

public class BuiltInFunctions
{
    private readonly List<BuiltInFunctionType> usedFunctions = new(Enum.GetValues(typeof(BuiltInFunctionType)).Length);

    public Expression GetHslToRgb(Expression hsla)
    {
        Require(BuiltInFunctionType.HslToRgb);
        
        return new Expression($"{nameof(HslToRgb)}({hsla.ExpressionValue})");
    }

    public Expression GetRgbToHsl(Expression rgba)
    {
        Require(BuiltInFunctionType.RgbToHsl);

        return new Expression($"{nameof(RgbToHsl)}({rgba.ExpressionValue})");
    }

    public Expression GetHslToRgb(Expression h, Expression s, Expression l, Expression a) =>
        GetHslToRgb(Half4.Constructor(h, s, l, a));
    
    public string BuildFunctions()
    {
        var builder = new StringBuilder();

        AppendIf(BuiltInFunctionType.HueToRgb, HueToRgb);
        AppendIf(BuiltInFunctionType.RgbToHcv, RgbToHcv);
        
        AppendIf(BuiltInFunctionType.HslToRgb, HslToRgb);
        AppendIf(BuiltInFunctionType.RgbToHsl, RgbToHsl);
        
        return builder.ToString();

        void AppendIf(BuiltInFunctionType type, string source)
        {
            if (usedFunctions.Contains(type))
            {
                builder.AppendLine(source);
            }
        }
    }

    private void Require(BuiltInFunctionType type)
    {
        if (usedFunctions.Contains(type))
        {
            return;
        }

        switch (type)
        {
            case BuiltInFunctionType.HslToRgb:
                Require(BuiltInFunctionType.HueToRgb);
                break;
            case BuiltInFunctionType.RgbToHsl:
                Require(BuiltInFunctionType.RgbToHcv);
                break;
        }

        usedFunctions.Add(type);
    }

    // Taken from here https://www.shadertoy.com/view/4dKcWK
    private const string HueToRgb =
        $$"""
        half3 {{nameof(HueToRgb)}}(float hue)
        {
            vec3 rgb = abs(hue * 6. - vec3(3, 2, 4)) * vec3(1, -1, -1) + vec3(-1, 2, 2);
            return clamp(rgb, 0., 1.);
        }
        """;
    
    private const string HslToRgb = 
        $$"""
        half4 {{nameof(HslToRgb)}}(half4 hsla)
        {
            half3 rgb = {{nameof(HueToRgb)}}(hsla.x);
            float c = (1. - abs(2. * hsla.z - 1.)) * hsla.y;
            return half4((rgb - 0.5) * c + hsla.z, hsla.w);
        }
        """;
    
    private const string RgbToHcv = 
        $$"""
        half3 {{nameof(RgbToHcv)}}(half3 rgba)
        {
            half4 p = (rgba.g < rgba.b) ? half4(rgba.bg, -1., 2. / 3.) : half4(rgba.gb, 0., -1. / 3.);
            half4 q = (rgba.r < p.x) ? half4(p.xyw, rgba.r) : half4(rgba.r, p.yzx);
            float c = q.x - min(q.w, q.y);
            float h = abs((q.w - q.y) / (6. * c) + q.z);
            return half3(h, c, q.x);
        }
        """;

    private const string RgbToHsl = 
        $$"""
        half4 {{nameof(RgbToHsl)}}(half4 rgba)
        {
            half3 hcv = {{nameof(RgbToHcv)}}(rgba.xyz);
            half z = hcv.z - hcv.y * 0.5;
            half s = hcv.y / (1. - abs(z * 2. - 1.));
            return half4(hcv.x, s, z, rgba.w);
        }
        """;
    
    enum BuiltInFunctionType
    {
        HueToRgb,
        RgbToHcv,

        RgbToHsl,
        HslToRgb,
        RgbToHsv,
        HsvToRgb
    }
}