Access cadCAD state_history

state_history.py

import numpy as np


def p_predator_births(params, substep, state_history, previous_state):
    '''Predator Births Policy Function
    The predator birth rate (rate of predators born per unit of time) is a product of
    the prey population and the predator birth parameter plus a random variable.
    
    i.e. the larger the prey population, the higher the predator birth rate
    '''
    # Parameters   
    dt = params['dt']
    predator_birth_parameter = params['predator_birth_parameter']
    random_predator_birth = params['random_predator_birth']

    # State Variables
    predator_population = previous_state['predator_population']
    prey_population = previous_state['prey_population']

    # Calculate the predator birth rate
    birth_rate = prey_population * (predator_birth_parameter + np.random.random() * random_predator_birth)
    # Calculate change in predator population
    births = birth_rate * predator_population * dt
      
    # Example access state history
    mean_predator_population = np.mean([timestep[-1]["predator_population"] for timestep in state_history])
    print(mean_predator_population)
    
    return {'add_to_predator_population': births}