using UnityEngine;
using UnityEditor;

class TextureModifier : AssetPostprocessor {

    void OnPostprocessTexture (Texture2D texture){
        if (!assetPath.EndsWith ("Dither.png")) {
            return;
        }

        var texw = texture.width;
        var texh = texture.height;

        var pixels = texture.GetPixels ();
        var offs = 0;

        var k1Per15 = 1.0f / 15.0f;
        var k1Per16 = 1.0f / 16.0f;
        var k3Per16 = 3.0f / 16.0f;
        var k5Per16 = 5.0f / 16.0f;
        var k7Per16 = 7.0f / 16.0f;

        for (var y = 0; y < texh; y++) {
            for (var x = 0; x < texw; x++) {
                float a = pixels [offs].a;
                float r = pixels [offs].r;
                float g = pixels [offs].g;
                float b = pixels [offs].b;

                var a2 = Mathf.Clamp01 (Mathf.Floor (a * 16) * k1Per15);
                var r2 = Mathf.Clamp01 (Mathf.Floor (r * 16) * k1Per15);
                var g2 = Mathf.Clamp01 (Mathf.Floor (g * 16) * k1Per15);
                var b2 = Mathf.Clamp01 (Mathf.Floor (b * 16) * k1Per15);

                var ae = a - a2;
                var re = r - r2;
                var ge = g - g2;
                var be = b - b2;

                pixels [offs].a = a2;
                pixels [offs].r = r2;
                pixels [offs].g = g2;
                pixels [offs].b = b2;

                var n1 = offs + 1;
                var n2 = offs + texw - 1;
                var n3 = offs + texw;
                var n4 = offs + texw + 1;

                if (x < texw - 1) {
                    pixels [n1].a += ae * k7Per16;
                    pixels [n1].r += re * k7Per16;
                    pixels [n1].g += ge * k7Per16;
                    pixels [n1].b += be * k7Per16;
                }

                if (y < texh - 1) {
                    pixels [n3].a += ae * k5Per16;
                    pixels [n3].r += re * k5Per16;
                    pixels [n3].g += ge * k5Per16;
                    pixels [n3].b += be * k5Per16;

                    if (x > 0) {
                        pixels [n2].a += ae * k3Per16;
                        pixels [n2].r += re * k3Per16;
                        pixels [n2].g += ge * k3Per16;
                        pixels [n2].b += be * k3Per16;
                    }

                    if (x < texw - 1) {
                        pixels [n4].a += ae * k1Per16;
                        pixels [n4].r += re * k1Per16;
                        pixels [n4].g += ge * k1Per16;
                        pixels [n4].b += be * k1Per16;
                    }
                }

                offs++;
            }
        }

        texture.SetPixels (pixels);
#if UNITY_2018_1_OR_NEWER
        EditorUtility.CompressTexture (texture, TextureFormat.RGBA4444, UnityEditor.TextureCompressionQuality.Best);
#else
        EditorUtility.CompressTexture (texture, TextureFormat.RGBA4444, TextureCompressionQuality.Best);
#endif
    }
}