// Blinn-Phong model
// apply lighting in vertex shader instead of fragment shader
// @see https://learnopengl.com/Advanced-Lighting/Advanced-Lighting
uniform float u_Ambient : 1.0;
uniform float u_Diffuse : 1.0;
uniform float u_Specular : 1.0;
uniform int u_NumOfDirectionalLights : 1;
uniform int u_NumOfSpotLights : 0;

#define SHININESS 32.0
#define MAX_NUM_OF_DIRECTIONAL_LIGHTS 3
#define MAX_NUM_OF_SPOT_LIGHTS 3

struct DirectionalLight {
  vec3 direction;
  vec3 ambient;
  vec3 diffuse;
  vec3 specular;
};

struct SpotLight {
  vec3 position;
  vec3 direction;
  vec3 ambient;
  vec3 diffuse;
  vec3 specular;
  float constant;
  float linear;
  float quadratic;
  float angle;
  float blur;
  float exponent;
};

uniform DirectionalLight u_DirectionalLights[MAX_NUM_OF_DIRECTIONAL_LIGHTS];
uniform SpotLight u_SpotLights[MAX_NUM_OF_SPOT_LIGHTS];

vec3 calc_directional_light(DirectionalLight light, vec3 normal, vec3 viewDir) {
  vec3 lightDir = normalize(light.direction);
  // diffuse shading
  float diff = max(dot(normal, lightDir), 0.0);
  // Blinn-Phong specular shading
  vec3 halfwayDir = normalize(lightDir + viewDir);
  float spec = pow(max(dot(normal, halfwayDir), 0.0), SHININESS);

  vec3 ambient = light.ambient * u_Ambient;
  vec3 diffuse = light.diffuse * diff * u_Diffuse;
  vec3 specular = light.specular * spec * u_Specular;

  return ambient + diffuse + specular;
}


vec3 calc_lighting(vec3 position, vec3 normal, vec3 viewDir) {
  vec3 weight = vec3(0.0);
  for (int i = 0; i < MAX_NUM_OF_DIRECTIONAL_LIGHTS; i++) {
    if (i >= u_NumOfDirectionalLights) {
      break;
    }
    weight += calc_directional_light(u_DirectionalLights[i], normal, viewDir);
  }
  return weight;
}