#version 450

const int MAX_LIGHTS = 10;

layout(location = 0) in vec3 v_Normal;
layout(location = 1) in vec4 v_Position;

layout(location = 0) out vec4 o_Target;

struct Light {
    mat4 proj;
    vec4 pos;
    vec4 color;
};

layout(set = 0, binding = 0) uniform Globals {
    mat4 u_ViewProj;
    uvec4 u_NumLights;
};
layout(set = 0, binding = 1) uniform Lights {
    Light u_Lights[MAX_LIGHTS];
};
layout(set = 0, binding = 2) uniform texture2DArray t_Shadow;
layout(set = 0, binding = 3) uniform samplerShadow s_Shadow;

layout(set = 1, binding = 0) uniform Entity {
    mat4 u_World;
    vec4 u_Color;
};

float fetch_shadow(int light_id, vec4 homogeneous_coords) {
    if (homogeneous_coords.w <= 0.0) {
        return 1.0;
    }
    // compensate for the Y-flip difference between the NDC and texture coordinates
    const vec2 flip_correction = vec2(0.5, -0.5);
    // compute texture coordinates for shadow lookup
    vec4 light_local = vec4(
        homogeneous_coords.xy * flip_correction/homogeneous_coords.w + 0.5,
        light_id,
        homogeneous_coords.z / homogeneous_coords.w
    );
    // do the lookup, using HW PCF and comparison
    return texture(sampler2DArrayShadow(t_Shadow, s_Shadow), light_local);
}

void main() {
    vec3 normal = normalize(v_Normal);
    vec3 ambient = vec3(0.05, 0.05, 0.05);
    // accumulate color
    vec3 color = ambient;
    for (int i=0; i<int(u_NumLights.x) && i<MAX_LIGHTS; ++i) {
        Light light = u_Lights[i];
        // project into the light space
        float shadow = fetch_shadow(i, light.proj * v_Position);
        // compute Lambertian diffuse term
        vec3 light_dir = normalize(light.pos.xyz - v_Position.xyz);
        float diffuse = max(0.0, dot(normal, light_dir));
        // add light contribution
        color += shadow * diffuse * light.color.xyz;
    }
    // multiply the light by material color
    o_Target = vec4(color, 1.0) * u_Color;
}