#version 460 core
#extension GL_ARB_bindless_texture : require

const float PI = 3.141592f;

in VS_OUT {
	vec3 normal;
	vec4 colour;
	vec3 pos;
	vec2 tex_pos;
	vec3 material;
} vin;

struct Light
{
	vec4 pos;
	vec4 colour;
};

layout(std140, binding = 1) readonly buffer LightBuffer
{
	Light lights[];
};

layout(std430, binding = 2) readonly buffer ShadowMapBuffer
{
	samplerCube shadow_maps[];
};

in flat sampler2D frag_tex;
out vec4 frag_colour;

uniform vec3 brightness;
uniform vec3 camera_pos;
uniform float far_plane;
uniform bool shadows_enabled;
uniform int lights_count;

vec3 FresnelSchlick(float cosTheta, vec3 F0)
{
	return F0 + (1.f - F0) * pow(clamp(1.f - cosTheta, 0.f, 1.f), 5.f);
}

float DistributionGGX(vec3 N, vec3 H, float roughness)
{
	float a	  = roughness*roughness;
	float a2	 = a*a;
	float NdotH  = max(dot(N, H), 0.f);
	float NdotH2 = NdotH*NdotH;
	
	float num   = a2;
	float denom = (NdotH2 * (a2 - 1.f) + 1.f);
	denom = PI * denom * denom;
	
	return num / denom;
}

float GeometrySchlickGGX(float NdotV, float roughness)
{
	float r = (roughness + 1.f);
	float k = (r*r) / 8.f;

	float num   = NdotV;
	float denom = NdotV * (1.f - k) + k;
	
	return num / denom;
}

float GeometrySmith(vec3 N, vec3 V, vec3 L, float roughness)
{
	float NdotV = max(dot(N, V), 0.f);
	float NdotL = max(dot(N, L), 0.f);
	float ggx2  = GeometrySchlickGGX(NdotV, roughness);
	float ggx1  = GeometrySchlickGGX(NdotL, roughness);
	
	return ggx1 * ggx2;
}

vec3 LinRGB_To_sRGB(vec3 c)
{
	bvec3 th = lessThan(c, vec3(0.0031308f));
	vec3 high = pow(c, vec3(1.0f / 2.4f)) * vec3(1.055f) - vec3(0.055f);
	vec3 low = c * vec3(12.92f);

	return mix(high, low, th);
}

vec3 sRGB_To_LinRGB(vec3 c)
{
	bvec3 th = lessThan(c, vec3(0.04045f));
	vec3 high = pow((c + vec3(0.055f)) * vec3(1.0f / 1.055f), vec3(2.4f));
	vec3 low = c * vec3(1.0f / 12.92f);

	return mix(high, low, th);
}

void main()
{
	vec4 albedo = texture2D(frag_tex, vin.tex_pos);
	if(albedo.a == 0.f) discard;

	vec3 albedo_lin = sRGB_To_LinRGB(albedo.rgb) * vin.colour.rgb;
	albedo *= vin.colour;
	
	float roughness = vin.material[0];
	float metalness = vin.material[1];
	float luminance = min(vin.material[2], 1.f);

	vec3 N = normalize(vin.normal);
	vec3 V = normalize(camera_pos - vin.pos.xyz);

	vec3 F0 = vec3(0.04f);
	F0 = mix(F0, albedo_lin, metalness);

	vec3 Lo = vec3(0.0f);
	for(int i = 0; i < lights_count; i++)
	{
		Light l = lights[i];

		vec3 L = normalize(l.pos.xyz - vin.pos);
		vec3 H = normalize(V + L);

		float d = length(vin.pos - l.pos.xyz);
		float atten = 1.f / (d*d);
		vec3 radiance = l.colour.rgb * atten;

		// cook-torrance brdf
		float NDF = DistributionGGX(N, H, roughness);
		float G = GeometrySmith(N, V, L, roughness);
		vec3 F = FresnelSchlick(max(dot(H, V), 0.f), F0);
		
		vec3 kS = F;
		vec3 kD = vec3(1.f) - kS;
		kD *= 1.f - metalness;
		
		vec3 numerator = NDF * G * F;
		float denominator = 4.f * max(dot(N, V), 0.f) * max(dot(N, L), 0.f) + 1e-4f;
		vec3 specular = numerator / denominator;
		
		float light_m;
		float spec_m;

		if(shadows_enabled)
		{
			float max_d = texture(shadow_maps[i], -L).r * far_plane + 1e-2f;
			spec_m = max_d > d ? 1.f : 0.f;
			light_m = spec_m * 0.25f + 0.75f;
		}

		else
		{
			light_m = 1.f;
			spec_m = 1.f;
		}

		// add to outgoing radiance Lo
		float NdotL = max(dot(N, L), 0.f);
		Lo += (kD * albedo_lin / PI + specular * spec_m) * radiance * NdotL * light_m;
	}
	
	vec3 ambient = vec3(0.03f) * albedo_lin * brightness;
	vec3 light = LinRGB_To_sRGB(ambient + Lo);

	light = mix(light, albedo.rgb, luminance);
	frag_colour = vec4(light, albedo.a);
}