using Godot;
using System;

using Rokojori.Extensions;
namespace Rokojori
{
  /** Blend modes for a TextureStackLayer.

      The first four are *native* modes: BlitMaterial carries them as a runtime
      blend_mode, so they need no shader at all and no ping-pong buffer - the
      hardware blends the layer straight onto the accumulation. The remaining
      six have no hardware equivalent and are computed in
      TextureStackComposite.gdshader, which needs the accumulation handed in as
      a source texture (see TextureStackCompositor.NeedsCompositeShader).

      The formulas mirror the CPU predecessor in
      Procedural/Textures/TextureCombiner ( TextureCombinerBlendMode /
      TextureCombinerBlendModeAlgorithm / ColorX.Blend* ). Two copies of the
      same math exist on purpose - see the note in TextureStack.gdshaderinc -
      so a fix to one belongs in the other as well.

      Order matches the int values the composite shader switches on. Do not
      renumber without updating TextureStack.gdshaderinc.
  */
  public enum TextureStackBlendMode
  {
    // Native - BlitMaterial.BlendModeEnum, no shader needed.
    Mix       = 0,
    Add       = 1,
    Sub       = 2,
    Multiply  = 3,

    // In-shader only, require the accumulation as a source.
    Screen    = 4,
    Overlay   = 5,
    HardLight = 6,
    SoftLight = 7,
    Color     = 8,
    Hue       = 9
  }

  public static class TextureStackBlendModeX
  {
    /** True for the four modes BlitMaterial can do in hardware.

        Note this says nothing about whether the *layer* can take the native
        path - a mask, clip-to-below or a non-trivial alpha requirement still
        forces the composite shader. TextureStackCompositor decides that.
    */
    public static bool IsNative( this TextureStackBlendMode mode )
    {
      return mode == TextureStackBlendMode.Mix
          || mode == TextureStackBlendMode.Add
          || mode == TextureStackBlendMode.Sub
          || mode == TextureStackBlendMode.Multiply;
    }

    /** Spike finding G5/constraint 2: the hardware blend applies to alpha too,
        so Add and Sub drive the target's alpha somewhere unintended even with a
        fully opaque source. Only safe where the target's alpha does not matter.
    */
    public static bool NativePreservesAlpha( this TextureStackBlendMode mode )
    {
      return mode == TextureStackBlendMode.Mix;
    }

    public static BlitMaterial.BlendModeEnum ToBlitBlendMode( this TextureStackBlendMode mode )
    {
      switch ( mode )
      {
        case TextureStackBlendMode.Mix:      return BlitMaterial.BlendModeEnum.Mix;
        case TextureStackBlendMode.Add:      return BlitMaterial.BlendModeEnum.Add;
        case TextureStackBlendMode.Sub:      return BlitMaterial.BlendModeEnum.Sub;
        case TextureStackBlendMode.Multiply: return BlitMaterial.BlendModeEnum.Mul;
      }

      return BlitMaterial.BlendModeEnum.Mix;
    }
  }
}