#version 330 core

in vert_t {
	vec3 position;
} vert;

out vec4 frag_color;

uniform samplerCube cubemap;
uniform float roughness;

const float PI = 3.14159265359;

float distribution_ggx(vec3 n, vec3 h, float roughness)
{
	float a = roughness * roughness;
	float a2 = a * a;
	float ndoth = max(dot(n, h), 0.0);
	float ndoth2 = ndoth * ndoth;
	float nom = a2;
	float denom = (ndoth2 * (a2 - 1.0) + 1.0);
	denom = PI * denom * denom;
	return nom / denom;
}

float radical_inverse_vdc(uint bits)
{
	bits = (bits << 16u) | (bits >> 16u);
	bits = ((bits & 0x55555555u) << 1u) | ((bits & 0xAAAAAAAAu) >> 1u);
	bits = ((bits & 0x33333333u) << 2u) | ((bits & 0xCCCCCCCCu) >> 2u);
	bits = ((bits & 0x0F0F0F0Fu) << 4u) | ((bits & 0xF0F0F0F0u) >> 4u);
	bits = ((bits & 0x00FF00FFu) << 8u) | ((bits & 0xFF00FF00u) >> 8u);
	return float(bits) * 2.3283064365386963e-10; // / 0x100000000
}

vec2 hammersley(uint i, uint n)
{
	return vec2(float(i) / float(n), radical_inverse_vdc(i));
}

vec3 importance_sample_ggx(vec2 xi, vec3 n, float roughness)
{
	float a = roughness * roughness;
	
	float phi = 2.0 * PI * xi.x;
	float cos_theta = sqrt((1.0 - xi.y) / (1.0 + (a * a - 1.0) * xi.y));
	float sin_theta = sqrt(1.0 - cos_theta * cos_theta);
	
	vec3 h;
	h.x = cos(phi) * sin_theta;
	h.y = sin(phi) * sin_theta;
	h.z = cos_theta;
	
	vec3 up = abs(n.z) < 0.999 ? vec3(0.0, 0.0, 1.0) : vec3(1.0, 0.0, 0.0);
	vec3 tangent = normalize(cross(up, n));
	vec3 bitangent = cross(n, tangent);
	
	vec3 samplevec = tangent * h.x + bitangent * h.y + n * h.z;
	return normalize(samplevec);
}

void main(void)
{
	vec3 n = normalize(vert.position);
	vec3 r = n;
	vec3 v = r;

	uint count = 1024u;
	float total = 0.0;
	vec3 prefiltered = vec3(0.0);
	for (uint i = 0u; i < count; i++) {
		vec2 xi = hammersley(i, count);
		vec3 h = importance_sample_ggx(xi, n, roughness);
		vec3 l = normalize(2.0 * dot(v, h) * h - v);
		float ndotl = max(dot(n, l), 0.0);
		if (ndotl > 0.0) {
			float d = distribution_ggx(n, h, roughness);
			float ndoth = max(dot(n, h), 0.0);
			float hdotv = max(dot(h, v), 0.0); 
			float pdf = d * ndoth / (4.0 * hdotv) + 0.0001;
			float resolution = 512.0;
			float sa_texel = 4.0 * PI / (6.0 * resolution * resolution);
			float sa_sample = 1.0 / (float(count) * pdf + 0.0001);
			float mip_level = roughness == 0.0 ? 0.0 : 0.5 * log2(sa_sample / sa_texel);
			prefiltered += textureLod(cubemap, l, mip_level).rgb * ndotl;
			total += ndotl;
		}
	}
	prefiltered /= total;
	frag_color = vec4(prefiltered, 1.0);
}