from lib.irt.models.three_parameter_logistic import ThreeParameterLogistic

class ItemInformationFunction():
  def __init__(self, irt_model):
    self.model_data = irt_model

  # determines the amount of information for a given question at a given theta (ability level)
  # further detailed on page 161, equation 4 here:
  #  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5978482/pdf/10.1177_0146621615613308.pdf
  def calculate(self, **kwargs):
    if self.model_data.model == '3PL':
      p = ThreeParameterLogistic(self.model_data, kwargs).result()
      q = 1 - p
      return (self.model_data.a_param * q * (p - self.model_data.c_param)**2) / (p * ((1 - self.model_data.c_param)**2))
    else:
      # potentially error out
      return None