#ifndef _BRDF_GLSL_
#define _BRDF_GLSL_

// Constants
//const float PI = 3.1415926535;

// Fresnel-Schlick with optimized exponential approximation
vec3 f_schlick(const vec3 f0, const float vh) {
	return f0 + (1.0 - f0) * exp2((-5.55473 * vh - 6.98316) * vh);
}

float v_smithschlick(const float nl, const float nv, const float a) {
	return 1.0 / ((nl * (1.0 - a) + a) * (nv * (1.0 - a) + a));
}

float d_ggx(const float nh, const float a) {
	float a2 = a * a;
	float denom = nh * nh * (a2 - 1.0) + 1.0;
	denom = max(denom * denom, 2e-6);
	return a2 / (PI * denom);
}

float getMipFromRoughness(const float roughness, const float numMipmaps) {
	// First mipmap level = roughness 0, last = roughness = 1
	return roughness * numMipmaps;
}

vec3 specularBRDF(vec3 N, vec3 V, vec3 L, vec3 F0, float roughness)
{
	vec3 H = normalize(V + L);

	float NdotL = max(dot(N, L), 0.0);
	float NdotV = max(dot(N, V), 0.0);
	float NdotH = max(dot(N, H), 0.0);
	float VdotH = max(dot(V, H), 0.0);

	float alpha = roughness * roughness;

	float D = d_ggx(NdotH, alpha);
	float k = alpha / 2.0;
	float G_V = NdotV / (NdotV * (1.0 - k) + k + 1e-5);
	float G_L = NdotL / (NdotL * (1.0 - k) + k + 1e-5);
	float G = G_V * G_L;

	vec3 F = f_schlick(F0, VdotH);

	return F * D * G / max(4.0 * NdotL * NdotV, 1e-5);
}

vec3 burleyDiffuseBRDF(vec3 N, vec3 V, vec3 L, vec3 albedo, float roughness)
{
	float NdotL = max(dot(N, L), 0.0);
	float NdotV = max(dot(N, V), 0.0);
	float LdotH = max(dot(L, normalize(L + V)), 0.0);

	float FD90 = 0.5 + 2.0 * LdotH * LdotH * roughness;
	float FL = pow(1.0 - NdotL, 5.0);
	float FV = pow(1.0 - NdotV, 5.0);

	float diffuse = (1.0 + (FD90 - 1.0) * FL) * (1.0 + (FD90 - 1.0) * FV);
	return albedo * (1.0 / PI) * diffuse * NdotL;
}

// Energy-conserving material albedo (fades out for metals)
vec3 surfaceAlbedo(const vec3 baseColor, const float metalness) {
	return baseColor * (1.0 - metalness);
}

// Metal-aware F0 blending
vec3 surfaceF0(vec3 baseColor, float metalness) {
	return mix(vec3(0.04), baseColor, metalness);
}

// LUT-based approximation of IBL BRDF (Unreal-style)
vec2 EnvBRDFApprox(float Roughness, float NoV) {
	const vec4 c0 = vec4(-1, -0.0275, -0.572, 0.022);
	const vec4 c1 = vec4(1, 0.0425, 1.04, -0.04);
	vec4 r = Roughness * c0 + c1;
	float a004 = min(r.x * r.x, exp2(-9.28 * NoV)) * r.x + r.y;
	return vec2(-1.04, 1.04) * a004 + r.zw;
}

vec3 IBLSpecularApprox(vec3 specColor, float roughness, float NoV) {
	vec2 brdf = EnvBRDFApprox(roughness, NoV);
	return specColor * brdf.x + brdf.y;
}

#endif