#version 410 core
out vec4 FragColor;

in VS_OUT {
    vec3 FragPos;
    vec3 Normal;
    vec2 TexCoords;
} fs_in;

uniform sampler2D diffuseTexture;
uniform sampler2DArray shadowMap;

uniform vec3 lightDir;
uniform vec3 viewPos;
uniform float farPlane;

uniform mat4 view;

layout (std140) uniform LightSpaceMatrices
{
    mat4 lightSpaceMatrices[16];
};
uniform float cascadePlaneDistances[16];
uniform int cascadeCount;   // number of frusta - 1

float ShadowCalculation(vec3 fragPosWorldSpace)
{
    // select cascade layer
    vec4 fragPosViewSpace = view * vec4(fragPosWorldSpace, 1.0);
    float depthValue = abs(fragPosViewSpace.z);

    int layer = -1;
    for (int i = 0; i < cascadeCount; ++i)
    {
        if (depthValue < cascadePlaneDistances[i])
        {
            layer = i;
            break;
        }
    }
    if (layer == -1)
    {
        layer = cascadeCount;
    }

    vec4 fragPosLightSpace = lightSpaceMatrices[layer] * vec4(fragPosWorldSpace, 1.0);
    // perform perspective divide
    vec3 projCoords = fragPosLightSpace.xyz / fragPosLightSpace.w;
    // transform to [0,1] range
    projCoords = projCoords * 0.5 + 0.5;

    // get depth of current fragment from light's perspective
    float currentDepth = projCoords.z;

    // keep the shadow at 0.0 when outside the far_plane region of the light's frustum.
    if (currentDepth > 1.0)
    {
        return 0.0;
    }
    // calculate bias (based on depth map resolution and slope)
    vec3 normal = normalize(fs_in.Normal);
    float bias = max(0.05 * (1.0 - dot(normal, lightDir)), 0.005);
    const float biasModifier = 0.5f;
    if (layer == cascadeCount)
    {
        bias *= 1 / (farPlane * biasModifier);
    }
    else
    {
        bias *= 1 / (cascadePlaneDistances[layer] * biasModifier);
    }

    // PCF
    float shadow = 0.0;
    vec2 texelSize = 1.0 / vec2(textureSize(shadowMap, 0));
    for(int x = -1; x <= 1; ++x)
    {
        for(int y = -1; y <= 1; ++y)
        {
            float pcfDepth = texture(shadowMap, vec3(projCoords.xy + vec2(x, y) * texelSize, layer)).r;
            shadow += (currentDepth - bias) > pcfDepth ? 1.0 : 0.0;        
        }    
    }
    shadow /= 9.0;
        
    return shadow;
}

void main()
{           
    vec3 color = texture(diffuseTexture, fs_in.TexCoords).rgb;
    vec3 normal = normalize(fs_in.Normal);
    vec3 lightColor = vec3(0.3);
    // ambient
    vec3 ambient = 0.3 * color;
    // diffuse
    float diff = max(dot(lightDir, normal), 0.0);
    vec3 diffuse = diff * lightColor;
    // specular
    vec3 viewDir = normalize(viewPos - fs_in.FragPos);
    vec3 reflectDir = reflect(-lightDir, normal);
    float spec = 0.0;
    vec3 halfwayDir = normalize(lightDir + viewDir);  
    spec = pow(max(dot(normal, halfwayDir), 0.0), 64.0);
    vec3 specular = spec * lightColor;    
    // calculate shadow
    float shadow = ShadowCalculation(fs_in.FragPos);                      
    vec3 lighting = (ambient + (1.0 - shadow) * (diffuse + specular)) * color;    
    
    FragColor = vec4(lighting, 1.0);
}