#include "light/light.h"
#include "scene/scene.h"
#include "shader/cooktorranceshader.h"

CookTorranceShader::CookTorranceShader(Color const &diffCol, Color const &ctCol, float IOR, float roughness,
                                       float diffCoeff, float ctCoeff) : diffuseColor(diffCol * diffCoeff),
                                                                         ctColor(ctCol * ctCoeff), F0(IOR),
                                                                         m(roughness) {}

float CookTorranceShader::D(float NdotH) const {
    // Beckmann distribution
    float const r2 = m * m;
    float const NdotH2 = NdotH * NdotH;
    return expf((NdotH2 - 1.0f) / (r2 * NdotH2)) / (4.0f * r2 * powf(NdotH, 4.0f));
}

float CookTorranceShader::F(float VdotH) const {
    // Schlicks approximation
    return F0 + (1.0f - F0) * powf(1.0f - VdotH, 5);
}

float CookTorranceShader::G(float NdotH, float NdotV, float VdotH, float NdotL) const {
    return std::min(1.0f, std::min(
            2.0f * NdotH * NdotV / VdotH, 2.0f * NdotH * NdotL / VdotH));
}

Color CookTorranceShader::shade(Scene const &scene, Ray const &ray) const {
    Color fragmentColor;

    if (m >= 0.0f) {
        // Accumulate the light over all light sources
        for (const auto &light: scene.lights()) {
            Light::Illumination illum;
            illum = light->illuminate(scene, ray);

            float const NdotL = std::max(0.0f, dotProduct(-illum.direction, ray.normal));
            if (NdotL <= 0.0f)
                continue;

            // Diffuse term
            Color const diffuse = this->diffuseColor / float(PI);
            fragmentColor += diffuse * NdotL * illum.color;

            // Cook-Torrance term
            // half angle vector
            Vector3d const H = normalized(-illum.direction - ray.direction);
            float const NdotH = std::max(0.0f, dotProduct(ray.normal, H));
            float const NdotV = std::max(0.0f, dotProduct(ray.normal, -ray.direction));
            float const VdotH = std::max(0.0f, dotProduct(-ray.direction, H));

            if (NdotV * NdotL > EPSILON) {
                Color const specular = this->ctColor * (F(VdotH) * D(NdotH) * G(NdotH, NdotV, VdotH, NdotL)) /
                                       (float(PI) * NdotV * NdotL);

                fragmentColor += specular * NdotL * illum.color;
            }
        }
    }
    return fragmentColor;
}