#ifndef UNIVERSAL_BRDF_INCLUDED
#define UNIVERSAL_BRDF_INCLUDED

#include "Packages/com.unity.render-pipelines.core/ShaderLibrary/BSDF.hlsl"
#include "Packages/com.unity.render-pipelines.core/ShaderLibrary/CommonMaterial.hlsl"
#include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Deprecated.hlsl"
#include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/SurfaceData.hlsl"

#define kDielectricSpec half4(0.04, 0.04, 0.04, 1.0 - 0.04) // standard dielectric reflectivity coef at incident angle (= 4%)

struct BRDFData
{
    half3 albedo;
    half3 diffuse;
    half3 specular;
    half reflectivity;
    half perceptualRoughness;
    half roughness;
    half roughness2;
    half grazingTerm;

    // We save some light invariant BRDF terms so we don't have to recompute
    // them in the light loop. Take a look at DirectBRDF function for detailed explaination.
    half normalizationTerm;     // roughness * 4.0 + 2.0
    half roughness2MinusOne;    // roughness^2 - 1.0
};

half ReflectivitySpecular(half3 specular)
{
    return Max3(specular.r, specular.g, specular.b);
}

half OneMinusReflectivityMetallic(half metallic)
{
    // We'll need oneMinusReflectivity, so
    //   1-reflectivity = 1-lerp(dielectricSpec, 1, metallic) = lerp(1-dielectricSpec, 0, metallic)
    // store (1-dielectricSpec) in kDielectricSpec.a, then
    //   1-reflectivity = lerp(alpha, 0, metallic) = alpha + metallic*(0 - alpha) =
    //                  = alpha - metallic * alpha
    half oneMinusDielectricSpec = kDielectricSpec.a;
    return oneMinusDielectricSpec - metallic * oneMinusDielectricSpec;
}

half MetallicFromReflectivity(half reflectivity)
{
    half oneMinusDielectricSpec = kDielectricSpec.a;
    return (reflectivity - kDielectricSpec.r) / oneMinusDielectricSpec;
}

inline void InitializeBRDFDataDirect(half3 albedo, half3 diffuse, half3 specular, half reflectivity, half oneMinusReflectivity, half smoothness, inout half alpha, out BRDFData outBRDFData)
{
    outBRDFData = (BRDFData)0;
    outBRDFData.albedo = albedo;
    outBRDFData.diffuse = diffuse;
    outBRDFData.specular = specular;
    outBRDFData.reflectivity = reflectivity;

    outBRDFData.perceptualRoughness = PerceptualSmoothnessToPerceptualRoughness(smoothness);
    outBRDFData.roughness           = max(PerceptualRoughnessToRoughness(outBRDFData.perceptualRoughness), HALF_MIN_SQRT);
    outBRDFData.roughness2          = max(outBRDFData.roughness * outBRDFData.roughness, HALF_MIN);
    outBRDFData.grazingTerm         = saturate(smoothness + reflectivity);
    outBRDFData.normalizationTerm   = outBRDFData.roughness * half(4.0) + half(2.0);
    outBRDFData.roughness2MinusOne  = outBRDFData.roughness2 - half(1.0);

    // Input is expected to be non-alpha-premultiplied while ROP is set to pre-multiplied blend.
    // We use input color for specular, but (pre-)multiply the diffuse with alpha to complete the standard alpha blend equation.
    // In shader: Cs' = Cs * As, in ROP: Cs' + Cd(1-As);
    // i.e. we only alpha blend the diffuse part to background (transmittance).
    #if defined(_ALPHAPREMULTIPLY_ON)
        // TODO: would be clearer to multiply this once to accumulated diffuse lighting at end instead of the surface property.
        outBRDFData.diffuse *= alpha;
    #endif
}

// Legacy: do not call, will not correctly initialize albedo property.
inline void InitializeBRDFDataDirect(half3 diffuse, half3 specular, half reflectivity, half oneMinusReflectivity, half smoothness, inout half alpha, out BRDFData outBRDFData)
{
    InitializeBRDFDataDirect(half3(0.0, 0.0, 0.0), diffuse, specular, reflectivity, oneMinusReflectivity, smoothness, alpha, outBRDFData);
}

// Initialize BRDFData for material, managing both specular and metallic setup using shader keyword _SPECULAR_SETUP.
inline void InitializeBRDFData(half3 albedo, half metallic, half3 specular, half smoothness, inout half alpha, out BRDFData outBRDFData)
{
#ifdef _SPECULAR_SETUP
    half reflectivity = ReflectivitySpecular(specular);
    half oneMinusReflectivity = half(1.0) - reflectivity;
    half3 brdfDiffuse = albedo * oneMinusReflectivity;
    half3 brdfSpecular = specular;
#else
    half oneMinusReflectivity = OneMinusReflectivityMetallic(metallic);
    half reflectivity = half(1.0) - oneMinusReflectivity;
    half3 brdfDiffuse = albedo * oneMinusReflectivity;
    half3 brdfSpecular = lerp(kDielectricSpec.rgb, albedo, metallic);
#endif

    InitializeBRDFDataDirect(albedo, brdfDiffuse, brdfSpecular, reflectivity, oneMinusReflectivity, smoothness, alpha, outBRDFData);
}

inline void InitializeBRDFData(inout SurfaceData surfaceData, out BRDFData brdfData)
{
    InitializeBRDFData(surfaceData.albedo, surfaceData.metallic, surfaceData.specular, surfaceData.smoothness, surfaceData.alpha, brdfData);
}

half3 ConvertF0ForClearCoat15(half3 f0)
{
    return ConvertF0ForAirInterfaceToF0ForClearCoat15Fast(f0);
}

inline void InitializeBRDFDataClearCoat(half clearCoatMask, half clearCoatSmoothness, inout BRDFData baseBRDFData, out BRDFData outBRDFData)
{
    outBRDFData = (BRDFData)0;
    outBRDFData.albedo = half(1.0);

    // Calculate Roughness of Clear Coat layer
    outBRDFData.diffuse             = kDielectricSpec.aaa; // 1 - kDielectricSpec
    outBRDFData.specular            = kDielectricSpec.rgb;
    outBRDFData.reflectivity        = kDielectricSpec.r;

    outBRDFData.perceptualRoughness = PerceptualSmoothnessToPerceptualRoughness(clearCoatSmoothness);
    outBRDFData.roughness           = max(PerceptualRoughnessToRoughness(outBRDFData.perceptualRoughness), HALF_MIN_SQRT);
    outBRDFData.roughness2          = max(outBRDFData.roughness * outBRDFData.roughness, HALF_MIN);
    outBRDFData.normalizationTerm   = outBRDFData.roughness * half(4.0) + half(2.0);
    outBRDFData.roughness2MinusOne  = outBRDFData.roughness2 - half(1.0);
    outBRDFData.grazingTerm         = saturate(clearCoatSmoothness + kDielectricSpec.x);

    // Modify Roughness of base layer using coat IOR
    half ieta                        = lerp(1.0h, CLEAR_COAT_IETA, clearCoatMask);
    half coatRoughnessScale          = Sq(ieta);
    half sigma                       = RoughnessToVariance(PerceptualRoughnessToRoughness(baseBRDFData.perceptualRoughness));

    baseBRDFData.perceptualRoughness = RoughnessToPerceptualRoughness(VarianceToRoughness(sigma * coatRoughnessScale));

    // Recompute base material for new roughness, previous computation should be eliminated by the compiler (as it's unused)
    baseBRDFData.roughness          = max(PerceptualRoughnessToRoughness(baseBRDFData.perceptualRoughness), HALF_MIN_SQRT);
    baseBRDFData.roughness2         = max(baseBRDFData.roughness * baseBRDFData.roughness, HALF_MIN);
    baseBRDFData.normalizationTerm  = baseBRDFData.roughness * 4.0h + 2.0h;
    baseBRDFData.roughness2MinusOne = baseBRDFData.roughness2 - 1.0h;

    // Darken/saturate base layer using coat to surface reflectance (vs. air to surface)
    baseBRDFData.specular = lerp(baseBRDFData.specular, ConvertF0ForClearCoat15(baseBRDFData.specular), clearCoatMask);
    // TODO: what about diffuse? at least in specular workflow diffuse should be recalculated as it directly depends on it.
}

BRDFData CreateClearCoatBRDFData(SurfaceData surfaceData, inout BRDFData brdfData)
{
    BRDFData brdfDataClearCoat = (BRDFData)0;

    #if defined(_CLEARCOAT) || defined(_CLEARCOATMAP)
    // base brdfData is modified here, rely on the compiler to eliminate dead computation by InitializeBRDFData()
    InitializeBRDFDataClearCoat(surfaceData.clearCoatMask, surfaceData.clearCoatSmoothness, brdfData, brdfDataClearCoat);
    #endif

    return brdfDataClearCoat;
}

// Computes the specular term for EnvironmentBRDF
half3 EnvironmentBRDFSpecular(BRDFData brdfData, half fresnelTerm)
{
    float surfaceReduction = 1.0 / (brdfData.roughness2 + 1.0);
    return half3(surfaceReduction * lerp(brdfData.specular, brdfData.grazingTerm, fresnelTerm));
}

half3 EnvironmentBRDF(BRDFData brdfData, half3 indirectDiffuse, half3 indirectSpecular, half fresnelTerm)
{
    half3 c = indirectDiffuse * brdfData.diffuse;
    c += indirectSpecular * EnvironmentBRDFSpecular(brdfData, fresnelTerm);
    return c;
}

// Environment BRDF without diffuse for clear coat
half3 EnvironmentBRDFClearCoat(BRDFData brdfData, half clearCoatMask, half3 indirectSpecular, half fresnelTerm)
{
    float surfaceReduction = 1.0 / (brdfData.roughness2 + 1.0);
    return indirectSpecular * EnvironmentBRDFSpecular(brdfData, fresnelTerm) * clearCoatMask;
}

// Computes the scalar specular term for Minimalist CookTorrance BRDF
// NOTE: needs to be multiplied with reflectance f0, i.e. specular color to complete
half DirectBRDFSpecular(BRDFData brdfData, half3 normalWS, half3 lightDirectionWS, half3 viewDirectionWS)
{
    float3 lightDirectionWSFloat3 = float3(lightDirectionWS);
    float3 halfDir = SafeNormalize(lightDirectionWSFloat3 + float3(viewDirectionWS));

    float NoH = saturate(dot(float3(normalWS), halfDir));
    half LoH = half(saturate(dot(lightDirectionWSFloat3, halfDir)));

    // GGX Distribution multiplied by combined approximation of Visibility and Fresnel
    // BRDFspec = (D * V * F) / 4.0
    // D = roughness^2 / ( NoH^2 * (roughness^2 - 1) + 1 )^2
    // V * F = 1.0 / ( LoH^2 * (roughness + 0.5) )
    // See "Optimizing PBR for Mobile" from Siggraph 2015 moving mobile graphics course
    // https://community.arm.com/events/1155

    // Final BRDFspec = roughness^2 / ( NoH^2 * (roughness^2 - 1) + 1 )^2 * (LoH^2 * (roughness + 0.5) * 4.0)
    // We further optimize a few light invariant terms
    // brdfData.normalizationTerm = (roughness + 0.5) * 4.0 rewritten as roughness * 4.0 + 2.0 to a fit a MAD.
    float d = NoH * NoH * brdfData.roughness2MinusOne + 1.00001f;

    half LoH2 = LoH * LoH;
    half specularTerm = brdfData.roughness2 / ((d * d) * max(0.1h, LoH2) * brdfData.normalizationTerm);

    // On platforms where half actually means something, the denominator has a risk of overflow
    // clamp below was added specifically to "fix" that, but dx compiler (we convert bytecode to metal/gles)
    // sees that specularTerm have only non-negative terms, so it skips max(0,..) in clamp (leaving only min(100,...))
#if REAL_IS_HALF
    specularTerm = specularTerm - HALF_MIN;
    // Update: Conservative bump from 100.0 to 1000.0 to better match the full float specular look.
    // Roughly 65504.0 / 32*2 == 1023.5,
    // or HALF_MAX / ((mobile) MAX_VISIBLE_LIGHTS * 2),
    // to reserve half of the per light range for specular and half for diffuse + indirect + emissive.
    specularTerm = clamp(specularTerm, 0.0, 1000.0); // Prevent FP16 overflow on mobiles
#endif

    return specularTerm;
}

// Based on Minimalist CookTorrance BRDF
// Implementation is slightly different from original derivation: http://www.thetenthplanet.de/archives/255
//
// * NDF [Modified] GGX
// * Modified Kelemen and Szirmay-Kalos for Visibility term
// * Fresnel approximated with 1/LdotH
half3 DirectBDRF(BRDFData brdfData, half3 normalWS, half3 lightDirectionWS, half3 viewDirectionWS, bool specularHighlightsOff)
{
    // Can still do compile-time optimisation.
    // If no compile-time optimized, extra overhead if branch taken is around +2.5% on some untethered platforms, -10% if not taken.
    [branch] if (!specularHighlightsOff)
    {
        half specularTerm = DirectBRDFSpecular(brdfData, normalWS, lightDirectionWS, viewDirectionWS);
        half3 color = brdfData.diffuse + specularTerm * brdfData.specular;
        return color;
    }
    else
        return brdfData.diffuse;
}

// Based on Minimalist CookTorrance BRDF
// Implementation is slightly different from original derivation: http://www.thetenthplanet.de/archives/255
//
// * NDF [Modified] GGX
// * Modified Kelemen and Szirmay-Kalos for Visibility term
// * Fresnel approximated with 1/LdotH
half3 DirectBRDF(BRDFData brdfData, half3 normalWS, half3 lightDirectionWS, half3 viewDirectionWS)
{
#ifndef _SPECULARHIGHLIGHTS_OFF
    return brdfData.diffuse + DirectBRDFSpecular(brdfData, normalWS, lightDirectionWS, viewDirectionWS) * brdfData.specular;
#else
    return brdfData.diffuse;
#endif
}

#endif