#version 330 core

uniform sampler2D dfg_lut;
uniform samplerCube prefiltered_envmap;
uniform sampler2DShadow shadow_tex;

uniform mat4 fg_LightMatrix_csm0;
uniform mat4 fg_LightMatrix_csm1;
uniform mat4 fg_LightMatrix_csm2;
uniform mat4 fg_LightMatrix_csm3;

// Shadow mapping constants
const int sun_atlas_size = 8192;
const float DEPTH_BIAS = 2.0;
const float BAND_SIZE = 0.1;
const vec2 BAND_BOTTOM_LEFT = vec2(BAND_SIZE);
const vec2 BAND_TOP_RIGHT   = vec2(1.0 - BAND_SIZE);
// Ideally these should be passed as an uniform, but we don't support uniform
// arrays yet
const vec2 uv_shifts[4] = vec2[4](
    vec2(0.0, 0.0), vec2(0.5, 0.0),
    vec2(0.0, 0.5), vec2(0.5, 0.5));
const vec2 uv_factor = vec2(0.5, 0.5);

// BRDF constants
const float PI = 3.14159265359;
const float RECIPROCAL_PI = 0.31830988618;
const float DIELECTRIC_SPECULAR = 0.04;
const float MAX_PREFILTERED_LOD = 4.0;

//------------------------------------------------------------------------------
// Shadow mapping related stuff

float sampleOffset(vec4 pos, vec2 offset, vec2 invTexelSize)
{
    return texture(
        shadow_tex, vec3(
            pos.xy + offset * invTexelSize,
            pos.z - DEPTH_BIAS * invTexelSize));
}

// OptimizedPCF from https://github.com/TheRealMJP/Shadows
// Original by Ignacio Castaño for The Witness
// Released under The MIT License
float sampleOptimizedPCF(vec4 pos)
{
    vec2 invTexSize = vec2(1.0 / float(sun_atlas_size));

    vec2 uv = pos.xy * sun_atlas_size;
    vec2 base_uv = floor(uv + 0.5);
    float s = (uv.x + 0.5 - base_uv.x);
    float t = (uv.y + 0.5 - base_uv.y);
    base_uv -= vec2(0.5);
    base_uv *= invTexSize;
    pos.xy = base_uv.xy;

    float sum = 0.0;

    float uw0 = (4.0 - 3.0 * s);
    float uw1 = 7.0;
    float uw2 = (1.0 + 3.0 * s);

    float u0 = (3.0 - 2.0 * s) / uw0 - 2.0;
    float u1 = (3.0 + s) / uw1;
    float u2 = s / uw2 + 2.0;

    float vw0 = (4.0 - 3.0 * t);
    float vw1 = 7.0;
    float vw2 = (1.0 + 3.0 * t);

    float v0 = (3.0 - 2.0 * t) / vw0 - 2.0;
    float v1 = (3.0 + t) / vw1;
    float v2 = t / vw2 + 2.0;

    sum += uw0 * vw0 * sampleOffset(pos, vec2(u0, v0), invTexSize);
    sum += uw1 * vw0 * sampleOffset(pos, vec2(u1, v0), invTexSize);
    sum += uw2 * vw0 * sampleOffset(pos, vec2(u2, v0), invTexSize);

    sum += uw0 * vw1 * sampleOffset(pos, vec2(u0, v1), invTexSize);
    sum += uw1 * vw1 * sampleOffset(pos, vec2(u1, v1), invTexSize);
    sum += uw2 * vw1 * sampleOffset(pos, vec2(u2, v1), invTexSize);

    sum += uw0 * vw2 * sampleOffset(pos, vec2(u0, v2), invTexSize);
    sum += uw1 * vw2 * sampleOffset(pos, vec2(u1, v2), invTexSize);
    sum += uw2 * vw2 * sampleOffset(pos, vec2(u2, v2), invTexSize);

    return sum / 144.0;
}

float sampleCascade(vec4 p, vec2 shift)
{
    vec4 pos = p;
    pos.xy *= uv_factor;
    pos.xy += shift;
    return sampleOptimizedPCF(pos);
}

float sampleAndBlendBand(vec4 p1, vec4 p2, vec2 s1, vec2 s2)
{
    vec2 s = smoothstep(vec2(0.0), BAND_BOTTOM_LEFT, p1.xy)
        - smoothstep(BAND_TOP_RIGHT, vec2(1.0), p1.xy);
    float blend = 1.0 - s.x * s.y;
    return mix(sampleCascade(p1, s1),
               sampleCascade(p2, s2),
               blend);
}

bool checkWithinBounds(vec2 coords, vec2 bottomLeft, vec2 topRight)
{
    vec2 r = step(bottomLeft, coords) - step(topRight, coords);
    return bool(r.x * r.y);
}

bool isInsideCascade(vec4 p)
{
    return checkWithinBounds(p.xy, vec2(0.0), vec2(1.0)) && ((p.z / p.w) <= 1.0);
}

bool isInsideBand(vec4 p)
{
    return !checkWithinBounds(p.xy, BAND_BOTTOM_LEFT, BAND_TOP_RIGHT);
}

/**
 * Get the light space position of point p.
 * Both p and n must be in view space. The light matrix is also assumed to
 * transform from view space to light space.
 */
vec4 getLightSpacePosition(vec3 p, vec3 n, float NdotL, float bias,
                           mat4 lightMatrix)
{
    float sinTheta = sqrt(1.0 - NdotL * NdotL);
    vec3 offset = p + n * (sinTheta * bias);
    return lightMatrix * vec4(offset, 1.0);
}

/**
 * Get shadowing factor for a given position. 1.0 corresponds to a fragment
 * being completely lit, and 0.0 to a fragment being completely in shadow.
 * Both p and n must be in view space.
 */
float getShadowing(vec3 p, vec3 n, float NdotL)
{
    // Ignore fragments that don't face the light
    if (NdotL <= 0.0)
        return 0.0;

    float shadow = 1.0;

    vec4 lightSpacePos[4];
    lightSpacePos[0] = getLightSpacePosition(p, n, NdotL, 0.05, fg_LightMatrix_csm0);
    lightSpacePos[1] = getLightSpacePosition(p, n, NdotL, 0.2, fg_LightMatrix_csm1);
    lightSpacePos[2] = getLightSpacePosition(p, n, NdotL, 1.0, fg_LightMatrix_csm2);
    lightSpacePos[3] = getLightSpacePosition(p, n, NdotL, 5.0, fg_LightMatrix_csm3);

    for (int i = 0; i < 4; ++i) {
        // Map-based cascade selection
        // We test if we are inside the cascade bounds to find the tightest
        // map that contains the fragment.
        if (isInsideCascade(lightSpacePos[i])) {
            if (isInsideBand(lightSpacePos[i]) && ((i+1) < 4)) {
                // Blend between cascades if the fragment is near the
                // next cascade to avoid abrupt transitions.
                shadow = clamp(sampleAndBlendBand(lightSpacePos[i],
                                                  lightSpacePos[i+1],
                                                  uv_shifts[i],
                                                  uv_shifts[i+1]),
                               0.0, 1.0);
            } else {
                // We are far away from the borders of the cascade, so
                // we skip the blending to avoid the performance cost
                // of sampling the shadow map twice.
                shadow = clamp(sampleCascade(lightSpacePos[i], uv_shifts[i]),
                               0.0, 1.0);
            }
            break;
        }
    }

    return shadow;
}

//------------------------------------------------------------------------------
// BRDF utility functions

/**
 * Fresnel term with included roughness to get a pleasant visual result.
 * See https://seblagarde.wordpress.com/2011/08/17/hello-world/
 */
vec3 F_SchlickRoughness(float NdotV, vec3 F0, float r)
{
    return F0 + (max(vec3(1.0 - r), F0) - F0) * pow(max(1.0 - NdotV, 0.0), 5.0);
}

/**
 * Fresnel (specular F)
 * Schlick's approximation for the Cook-Torrance BRDF.
 */
vec3 F_Schlick(float VdotH, vec3 F0)
{
    return F0 + (vec3(1.0) - F0) * pow(clamp(1.0 - VdotH, 0.0, 1.0), 5.0);
}

/**
 * Normal distribution function (NDF) (specular D)
 * Trowbridge-Reitz/GGX microfacet distribution. Includes Disney's
 * reparametrization of a=roughness*roughness
 */
float D_GGX(float NdotH, float a2)
{
    float f = (NdotH * a2 - NdotH) * NdotH + 1.0;
    return a2 / (PI * f * f);
}

/**
 * Geometric attenuation (specular G)
 * Smith-GGX formulation.
 */
float G_SmithGGX(float NdotV, float NdotL, float a2)
{
    float attV = 2.0 * NdotV / (NdotV + sqrt(a2 + (1.0 - a2) * (NdotV * NdotV)));
    float attL = 2.0 * NdotL / (NdotL + sqrt(a2 + (1.0 - a2) * (NdotL * NdotL)));
    return attV * attL;
}

/**
 * Basic Lambertian diffuse BRDF
 */
vec3 Fd_Lambert(vec3 c_diff)
{
    return c_diff * RECIPROCAL_PI;
}

/**
 * Get the fresnel reflectance at 0 degrees (light hitting the surface
 * perpendicularly).
 */
vec3 getF0Reflectance(vec3 baseColor, float metallic)
{
    return mix(vec3(DIELECTRIC_SPECULAR), baseColor, metallic);
}

//------------------------------------------------------------------------------
// IBL evaluation

/**
 * Indirect diffuse irradiance
 * To get better results we should be precomputing the irradiance into a cubemap
 * or calculating spherical harmonics coefficients on the CPU.
 * Sampling the roughness=1 mipmap level of the prefiltered specular map
 * works too. :)
 */
vec3 evaluateDiffuseIrradianceIBL(vec3 n)
{
    int roughnessOneLevel = int(MAX_PREFILTERED_LOD);
    ivec2 s = textureSize(prefiltered_envmap, roughnessOneLevel);
    float du = 1.0 / float(s.x);
    float dv = 1.0 / float(s.y);
    vec3 m0 = normalize(cross(n, vec3(0.0, 1.0, 0.0)));
    vec3 m1 = cross(m0, n);
    vec3 m0du = m0 * du;
    vec3 m1dv = m1 * dv;

    vec3 c;
    c  = textureLod(prefiltered_envmap, n - m0du - m1dv, roughnessOneLevel).rgb;
    c += textureLod(prefiltered_envmap, n + m0du - m1dv, roughnessOneLevel).rgb;
    c += textureLod(prefiltered_envmap, n + m0du + m1dv, roughnessOneLevel).rgb;
    c += textureLod(prefiltered_envmap, n - m0du + m1dv, roughnessOneLevel).rgb;
    return c * 0.25;
}

/**
 * Indirect specular (ambient specular)
 * Sample from the prefiltered environment map.
 */
vec3 evaluateSpecularIBL(float NdotV, vec3 reflected, float roughness, vec3 f)
{
    vec3 prefilteredColor = textureLod(prefiltered_envmap,
                                       reflected,
                                       roughness * MAX_PREFILTERED_LOD).rgb;
    vec2 envBRDF = texture(dfg_lut, vec2(NdotV, roughness)).rg;
    return prefilteredColor * (f * envBRDF.x + envBRDF.y);
}

vec3 evaluateIBL(
    vec3 baseColor,
    float metallic,
    float roughness,
    vec3 f0,                  // Use getF0Reflectance() to obtain this
    float occlusion,
    vec3 nWorldSpace,         // Normal in world space
    float NdotV,              // Must be positive and non-zero
    vec3 reflected            // Reflected vector in world space: reflect(-v, n)
    )
{
    vec3 f = F_SchlickRoughness(NdotV, f0, roughness);

    vec3 specular = evaluateSpecularIBL(NdotV, reflected, roughness, f);
    vec3 diffuse = evaluateDiffuseIrradianceIBL(nWorldSpace) * baseColor
        * (vec3(1.0) - f) * (1.0 - metallic);

    return (diffuse + specular) * occlusion;
}

//------------------------------------------------------------------------------
// Analytical light source evaluation

vec3 evaluateLight(
    vec3 baseColor,
    float metallic,
    float roughness,
    float clearcoat,
    float clearcoatRoughness,
    vec3 f0,                  // Use getF0Reflectance() to obtain this
    vec3 intensity,
    float occlusion,
    vec3 n,
    vec3 l,
    vec3 v,
    float NdotL,              // Must not be clamped to [0,1]
    float NdotV               // Must be positive and non-zero
    )
{
    // Skip fragments that are completely occluded or that are not facing the light
    if (occlusion <= 0.0 || NdotL <= 0.0)
        return vec3(0.0);

    NdotL = clamp(NdotL, 0.001, 1.0);

    vec3 h = normalize(v + l);
    float NdotH = clamp(dot(n, h), 0.0, 1.0);
    float VdotH = clamp(dot(v, h), 0.0, 1.0);

    vec3 c_diff = mix(baseColor * (1.0 - DIELECTRIC_SPECULAR), vec3(0.0), metallic);

    float a = roughness * roughness;
    float a2 = a * a;

    vec3 F = F_Schlick(VdotH, f0);
    float D = D_GGX(NdotH, a2);
    float G = G_SmithGGX(NdotV, NdotL, a2);

    // Diffuse term
    // Lambertian diffuse model
    vec3 diffuse = (vec3(1.0) - F) * Fd_Lambert(c_diff);
    // Specular term
    // Cook-Torrance specular microfacet model
    vec3 specular = (F * D * G) / (4.0 * NdotV * NdotL);

    vec3 material = diffuse + specular;

    vec3 color = material * intensity * occlusion;
    return color;
}