#version 330 core
out vec4 FragColor;

in vec2 TexCoord;

struct DirLight {
    vec3 direction;
    vec3 color;
    float intensity;
};

struct PointLight {
    vec3 position;
    vec3 color;
    float intensity;
    float radius;
    int shadowTile;
};

struct SpotLight {
    vec3 position;
    vec3 direction;
    vec3 color;
    float intensity;
    float radius;
    float innerCos;
    float outerCos;
    int shadowTile;
};

// The array sizes have to match Light.h
#define MAX_DIRECTIONAL_LIGHTS 4
#define MAX_POINT_LIGHTS 16
#define MAX_SPOT_LIGHTS 8
#define SHADOW_ATLAS_COLUMNS 6
#define SHADOW_ATLAS_TILES (SHADOW_ATLAS_COLUMNS * SHADOW_ATLAS_COLUMNS)

uniform DirLight dirLight;   // the sun, the only light with a shadow map
uniform DirLight directionalLights[MAX_DIRECTIONAL_LIGHTS];
uniform int directionalLightCount = 0;
uniform PointLight pointLights[MAX_POINT_LIGHTS];
uniform int pointLightCount = 0;
uniform SpotLight spotLights[MAX_SPOT_LIGHTS];
uniform int spotLightCount = 0;
uniform sampler2DShadow localShadowMap;
uniform mat4 shadowTileMatrices[SHADOW_ATLAS_TILES];

uniform vec3 viewPos;
uniform mat4 lightSpaceMatrix;

uniform sampler2D gPosition;
uniform sampler2D gNormal;
uniform sampler2D gAlbedo;
uniform sampler2D gMaterial;
uniform sampler2DShadow shadowMap;
uniform samplerCube environmentMap;
uniform float envIntensity = 0.8;

const float PI = 3.14159265359;

// Plain Blinn-Phong: ambient, Lambert diffuse and a half vector highlight.
// Roughness sets how tight and strong the highlight is, metallic tints it and takes away the diffuse.

// Close enough to the 2.2 curve and much cheaper than pow
vec3 ToLinear(vec3 c)
{
    return c * c;
}

// Light from one direction, L points from the surface toward the light.
// diffuse and specular are already scaled for this pixel, see main.
vec3 ShadeLight(vec3 L, vec3 radiance, vec3 N, vec3 V, vec3 diffuse, vec3 specular, float power)
{
    float NdotL = max(dot(N, L), 0.0);
    vec3 H = normalize(V + L);
    float highlight = pow(max(dot(N, H), 0.0), power);
    return (diffuse + specular * highlight) * radiance * NdotL;
}

// Inverse square falloff, windowed so it reaches exactly zero at the radius
float PointAttenuation(float distanceSquared, float radius)
{
    float ratio = distanceSquared / (radius * radius);
    float window = clamp(1.0 - ratio * ratio, 0.0, 1.0);
    return window * window / max(distanceSquared, 0.01);
}

// Which of a point light's 6 atlas tiles covers offset, the order is set in RenderLocalShadows
int PointShadowFace(vec3 offset)
{
    vec3 size = abs(offset);
    if (size.x >= size.y && size.x >= size.z) return offset.x > 0.0 ? 0 : 1;
    if (size.y >= size.z) return offset.y > 0.0 ? 2 : 3;
    return offset.z > 0.0 ? 4 : 5;
}

// 1 = lit, 0 = shadowed. Stays inside the tile so the filter can't pick up its neighbours.
float LocalShadow(int tile, vec3 worldPos, vec3 N)
{
    vec4 clipPos = shadowTileMatrices[tile] * vec4(worldPos + N * 0.04, 1.0);
    if (clipPos.w <= 0.0)
        return 1.0;

    vec3 projCoords = clipPos.xyz / clipPos.w * 0.5 + 0.5;
    if (any(lessThan(projCoords, vec3(0.0))) || any(greaterThan(projCoords, vec3(1.0))))
        return 1.0;

    float tileSize = 1.0 / float(SHADOW_ATLAS_COLUMNS);
    vec2 tileOrigin = vec2(float(tile % SHADOW_ATLAS_COLUMNS), float(tile / SHADOW_ATLAS_COLUMNS)) * tileSize;
    vec2 texelSize = 1.0 / vec2(textureSize(localShadowMap, 0));
    vec2 uv = tileOrigin + projCoords.xy * tileSize;

    float shadow = 0.0;
    for (int x = -1; x <= 1; x++) {
        for (int y = -1; y <= 1; y++) {
            vec2 sampleUV = clamp(uv + vec2(x, y) * texelSize, tileOrigin + texelSize, tileOrigin + tileSize - texelSize);
            shadow += texture(localShadowMap, vec3(sampleUV, projCoords.z - 0.0002));
        }
    }
    return shadow / 9.0;
}

float ShadowCalculation(vec4 fragPosLightSpace, vec3 N, vec3 L)
{
    vec3 projCoords = fragPosLightSpace.xyz / fragPosLightSpace.w;
    projCoords = projCoords * 0.5 + 0.5;

    if (projCoords.z > 1.0 || any(lessThan(projCoords, vec3(0.0))) ||
        any(greaterThan(projCoords, vec3(1.0))))
        return 1.0;

    float bias = max(0.004 * (1.0 - dot(N, L)), 0.001);
    vec2 texelSize = 1.0 / vec2(textureSize(shadowMap, 0));
    float shadow = 0.0;

    for (int x = -1; x <= 1; x++) {
        for (int y = -1; y <= 1; y++) {
            vec2 offset = vec2(x, y) * texelSize;
            shadow += texture(shadowMap, vec3(projCoords.xy + offset, projCoords.z - bias));
        }
    }
    return shadow / 9.0;
}

void main()
{
    vec3 normal = texture(gNormal, TexCoord).rgb;

    // Nothing was written here, keep the skybox underneath
    if (dot(normal, normal) < 0.01)
        discard;

    vec3 FragPos = texture(gPosition, TexCoord).rgb;
    vec4 albedoSample = texture(gAlbedo, TexCoord);
    vec4 material = texture(gMaterial, TexCoord);

    vec3 albedo = albedoSample.rgb;
    float metal = material.r;
    float rough = material.g;
    float ao = material.b;

    vec3 N = normalize(normal);
    vec3 V = normalize(viewPos - FragPos);

    float smoothness = 1.0 - clamp(rough, 0.0, 1.0);
    float power = exp2(10.0 * smoothness + 1.0);   // 2 when fully rough, 2048 when mirror smooth
    vec3 specularColor = mix(vec3(0.5 * smoothness * smoothness), albedo * (0.25 + 0.75 * smoothness), metal);
    vec3 diffuseBRDF = albedo * (1.0 - metal) / PI;   // the light intensities were tuned with the 1/PI in
    vec3 specularBRDF = specularColor / PI;
    vec3 direct = vec3(0.0);

    // Facing away from the sun has no direct light, so skip the BRDF and the 9 shadow samples
    vec3 sunL = normalize(-dirLight.direction);
    if (dot(N, sunL) > 0.0) {
        float shadow = ShadowCalculation(lightSpaceMatrix * vec4(FragPos, 1.0), N, sunL);
        direct += ShadeLight(sunL, dirLight.color * dirLight.intensity, N, V, diffuseBRDF, specularBRDF, power) * shadow;
    }

    for (int i = 0; i < directionalLightCount; i++) {
        vec3 L = normalize(-directionalLights[i].direction);
        vec3 radiance = directionalLights[i].color * directionalLights[i].intensity;
        direct += ShadeLight(L, radiance, N, V, diffuseBRDF, specularBRDF, power);
    }

    for (int i = 0; i < pointLightCount; i++) {
        vec3 toLight = pointLights[i].position - FragPos;
        float distanceSquared = dot(toLight, toLight);
        float radius = pointLights[i].radius;
        if (distanceSquared >= radius * radius)
            continue;

        vec3 radiance = pointLights[i].color * pointLights[i].intensity * PointAttenuation(distanceSquared, radius);
        vec3 L = toLight / sqrt(distanceSquared);
        if (dot(N, L) <= 0.0)
            continue;

        vec3 lit = ShadeLight(L, radiance, N, V, diffuseBRDF, specularBRDF, power);
        if (pointLights[i].shadowTile >= 0)
            lit *= LocalShadow(pointLights[i].shadowTile + PointShadowFace(-toLight), FragPos, N);
        direct += lit;
    }

    for (int i = 0; i < spotLightCount; i++) {
        vec3 toLight = spotLights[i].position - FragPos;
        float distanceSquared = dot(toLight, toLight);
        float radius = spotLights[i].radius;
        if (distanceSquared >= radius * radius)
            continue;

        vec3 L = toLight / sqrt(distanceSquared);
        float cone = smoothstep(spotLights[i].outerCos, spotLights[i].innerCos, dot(-L, spotLights[i].direction));
        if (cone <= 0.0)
            continue;

        if (dot(N, L) <= 0.0)
            continue;

        vec3 radiance = spotLights[i].color * spotLights[i].intensity * PointAttenuation(distanceSquared, radius) * cone;
        vec3 lit = ShadeLight(L, radiance, N, V, diffuseBRDF, specularBRDF, power);
        if (spotLights[i].shadowTile >= 0)
            lit *= LocalShadow(spotLights[i].shadowTile, FragPos, N);
        direct += lit;
    }

    // Ambient is the sky's blurred colour in the direction the surface faces
    vec3 ambient = ToLinear(textureLod(environmentMap, N, 7.0).rgb) * envIntensity * albedo * ao;

    vec3 emission = albedo * material.a * 4.0;

    FragColor = vec4(direct + ambient + emission, albedoSample.a);
}