Shader "ShaderChallenge/CustomPBR"
{
    Properties
    {
        _Albedo("Albedo", 2D) = "white" {}
        _Roughness("Roughness", Range(0, 1)) = 1.0
        _Metallic("Metallic", Range(0, 1)) = 0.0
    }
    SubShader
    {
        Pass
        {
            Tags { "LightMode" = "ForwardBase" }

            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityStandardBRDF.cginc"
            #include "UnityStandardUtils.cginc"

            #define pi 3.14159265359

            sampler2D _Albedo;
            float _Roughness;
            float _Metallic;

            struct v2f
            {
                float4 pos : SV_POSITION;
                float3 worldPos : TEXCOORD0;
                float3 normal : TEXCOORD1;
                float2 uv : TEXCOORD2;
            };

            v2f vert(appdata_base v)
            {
                v2f o;
                o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
                // Convert vertex position to world space
                o.worldPos = mul(unity_ObjectToWorld, v.vertex);
                // Normal vector
                o.normal = UnityObjectToWorldNormal(v.normal);
                o.uv = v.texcoord;
                return o;
            }

            // Oren-Nayar
            float OrenNayar(float3 n, float3 v, float3 l)
            {
                float nl = dot(n, l);
                float nv = dot(n, v);

                float anglenl = acos(nl);
                float anglenv = acos(nv);

                float alpha = max(anglenv, anglenl);
                float beta = min(anglenv, anglenl);
                float gamma = dot(v - n * nv, l - n * nl);

                float a2 = pow(_Roughness, 2.0);

                float A = 1.0 - 0.5 * (a2 / (a2 + 0.57));

                float B = 0.45 * (a2 / (a2 + 0.09));

                float C = sin(alpha) * tan(beta);

                float result = max(0.0, nl) * (A + B * max(0.0, gamma) * C);

                return result;
            }

            float DistributionGGX(float3 n, float3 h)
            {
                float a2 = pow(_Roughness, 2.0);
                float nh = dot(n, h);
                float nh2 = pow(nh, 2.0);

                float num = a2;
                float den = (nh2 * (a2 - 1.0) + 1.0);
                den = pi * pow(den, 2.0);

                return num / den;
            }

            float GeometrySchlickGGX(float dot, float k)
            {
                float num   = dot;
                float den = dot * (1.0 - k) + k;

                return num / den;
            }

            float GeometrySmith(float3 n, float3 v, float3 l)
            {
                float k = pow(_Roughness + 1.0, 2.0) / 8.0;

                float nv = DotClamped(n, v);
                float nl = DotClamped(n, l);
                float ggx1 = GeometrySchlickGGX(nv, k);
                float ggx2 = GeometrySchlickGGX(nl, k);

                return ggx1 * ggx2;
            }

            float FresnelSchlick(float3 n, float3 v, float3 albedo)
            {
                float cosTheta = dot(n, v);
                float3 F0 = 0.04;
                F0 = lerp(F0, albedo, _Metallic);

                return F0 + (1.0 - F0) * pow(1.0 - cosTheta, 5.0);
            }

            float Microfacet(float3 n, float3 h, float3 v, float3 l, float3 albedo)
            {
                float d = DistributionGGX(h, n);
                float g = GeometrySmith(n, v, l);
                float f = FresnelSchlick(n, v, albedo);

                return (d * g * f) / 4.0 * DotClamped(n, l) * DotClamped(n, v) + 0.001;
            }

            float4 frag(v2f i) : SV_TARGET
            {
                float3 lightCol = _LightColor0.rgb;

                // Normalize the normal
                float3 n = normalize(i.normal);
                // Light vector from mesh's surface
                float3 l = normalize(_WorldSpaceLightPos0.xyz);
                // Viewport(camera) vector from mesh's surface
                float3 v = normalize(_WorldSpaceCameraPos - i.worldPos.xyz);
                // Halfway vector
                float3 h = normalize(l + v);

                float3 albedo = tex2D(_Albedo, i.uv).rgb;
                float3 ambient = unity_AmbientSky;

                float3 specular = Microfacet(n, h, v, l, albedo);

                float3 diffuse = ambient + OrenNayar(n, v, l);
                diffuse *= 1.0 - _Metallic;

                float3 color = albedo * (diffuse * lightCol) + (specular * lightCol);

                return float4(color, 1.0);
            }
            ENDCG
        }
    }
}