#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
ENGG1811 lecture 7

Library for simulation 
"""
# import the math library
import math

# Constants
g = 9.81

# %% Functions for simulation by formula
#


def parachute(t, mass, drag_air, time_deploy, drag_para):
    if t < time_deploy:
        speed = freefall(t, mass, drag_air)            
    else:
        # Compute speed when parachute is deployed
        speed_at_deployment = freefall(time_deploy, mass, drag_air) 
        # Calculate terminal speed 
        terminal_speed_para = g*mass/drag_para
        # Speed at time t
        decay_now = decay(t-time_deploy,mass,drag_para)
        speed = speed_at_deployment * decay_now + terminal_speed_para * (1 - decay_now)
    return speed            

def decay(t,mass,drag):
    decay_from_drag = math.exp(-drag/mass*t)
    return decay_from_drag


# Calculte the freefall speed 
def freefall(t, mass, drag_air):
    # decay = CalcDecay(currentTime,mass,drag_air);
    # Compute velocity of freefall
    terminal_speed_air  = g * mass / drag_air
    speed = terminal_speed_air * (1 - decay(t, mass, drag_air))
    return speed