#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Created on Thu Jul 16 09:41:54 2020

@author: ashesh
Example: Simulate the parachutist's speed by ODE - Raw

"""

# Import 
import matplotlib.pyplot as plt 
import numpy as np 

# Problem parameters
mass = 70          # Mass of the parachutist 
time_deploy = 6    # Time at which the parachute is deployed 
air_drag_coeff = 12.5     # Drag coefficient for air
para_drag_coeff = 90      # Drag coefficient for parachute 
speed0 = 0  # Initial speed 
height0 = 4000 #Initial height

# Simulation parameters
time_start = 0
time_end = 600
time_increment = 0.0025

# Create a list of regularly spaced time instants
# [0,0.25,0.5,0.75,...,20] 
# 
time_array = np.arange(time_start,time_end+time_increment/2,
                       time_increment)

# Constants 
g = 9.81

# determine time increment
dt = time_array[2]-time_array[1]
    
# Simulation output
speed_array = np.zeros_like(time_array)
height_array = np.zeros_like(time_array)   

   
# Assign initial speed
speed_array[0] = speed0
height_array[0] = height0
    
# simulation loop 
for k in range(0, len(time_array)-1 ):
        
    time_now = time_array[k]  
    
    if time_now <= time_deploy: 
        drag_coeff_now = air_drag_coeff
    else:
        drag_coeff_now = para_drag_coeff    
        
    # To be completed in lecture 
    speed_array[k+1] = speed_array[k] + \
        (g - drag_coeff_now* speed_array[k]  /mass)*dt      
        
    height_array[k+1] = height_array[k] - speed_array[k] * dt 
            

landing_ht = np.min(height_array[height_array > 0])
landing_time = np.max(time_array[height_array > 0])


# Plot the figure 
fig1 = plt.figure() 
#plt.plot(time_array, speed_array, label = 'ODE (speed)')
plt.plot(time_array, height_array, label = 'height')

plt.legend()
plt.xlabel('time [seconds]')
#plt.ylabel('speed')
plt.ylabel('height')
plt.grid()
plt.show()
plt.savefig('para1.png')