class Material
  def initialize(r, g, b)
    @albedo = Colour.new(r, g, b)
  end

  def attenuation
    @albedo
  end

  def scatter(ray, record)
    nil
  end
end

class Lambertian < Material
  def scatter(ray, record)
    scat = record.normal + Vec3.random_unit
    scat = rec.normal if scat.near_zero?
    Ray.new(record.point, scat)
  end
end

class Metal < Material
  def initialize(r, g, b, fuzz)
    @fuzz = fuzz < 1 ? fuzz : 1
    super(r, g, b)
  end

  def scatter(ray, record)
    refl = ray.direction.reflect(record.normal)
    refl = refl.unit + (Vec3.random_unit * @fuzz)
    if refl.dot(record.normal) > 0
      Ray.new(record.point, refl)
    else
      nil
    end
  end
end

class Dielectric < Material
  def initialize(ref_index)
    @ref_index = ref_index
  end

  def attenuation
    Colour.new(1.0, 1.0, 1.0)
  end

  def scatter(ray, record)
    ri = record.front_face ? (1.0 / @ref_index) : @ref_index
    unit_dir = ray.direction.unit
    costheta = [(-unit_dir).dot(record.normal), 1.0].min
    sintheta = (1.0 - costheta ** 2) ** 0.5

    cannot_refract = ri * sintheta > 1.0
    maybe_reflect_anyway = Dielectric.reflectance(costheta, ri) > rand

    refr = cannot_refract || maybe_reflect_anyway ?
      unit_dir.reflect(record.normal) :
      unit_dir.refract(record.normal, ri)

    Ray.new(record.point, refr)
  end

  def self.reflectance(costheta, ri)
    r0 = ((1.0 - ri) / (1.0 + ri)) ** 2
    r0 + (1.0 - r0) * (1.0 - costheta) ** 5
  end
end