Spending calculator

spending_calc.py

#Everything is monthly

G_INITIAL_AMOUNT = 0.0
G_INVESTMENT = 1200.0
G_INTEREST_RATE = 1.07 ** (1.0/12.0)
G_INFLATION_RATE = 1.0228 ** (1.0/12.0)
G_CURRENT_SPENDING = 1000.0

def compound(principal, investment, rate, time):
    "Compound interest calculator"
    return float(principal) * float(rate ** time) + float(investment) * float(rate ** time - 1.0) / float(rate - 1.0)

def profit(initial_amount, investment, interest_rate, time):
    "Profit calculator"
    return compound(initial_amount, investment, interest_rate, time)

def loss(spending, inflation_rate, time):
    "Loss calculator"
    return compound(0, spending, inflation_rate, time)

def burn_time(current_amount, current_spending, investment, after_time):
    "Calculates amount of burn time available"
    amount = profit(current_amount, investment, G_INTEREST_RATE, after_time)
    spending = current_spending * (G_INFLATION_RATE ** after_time)
    time = 0.0
    while True:
        amount -= spending
        if amount < 0.0:
            break
        amount *= G_INTEREST_RATE
        spending *= G_INFLATION_RATE
        time += 1.0
    return max(time - 7.0, 0.0)

def expected_to_reach_amount(current_amount, expected_amount):
    "Calculates expected time to reach a given amount"
    time = 0.0
    while True:
        if current_amount >= expected_amount:
            return time
        time += 1.0
        current_amount = compound(current_amount, G_INVESTMENT, G_INTEREST_RATE, 1.0)

def expected_to_reach_burn_time(current_amount, needed_burn_time):
    "Calculates expected time to reach burn"
    after_time = 1.0
    while True:
        calculated_burn_time = burn_time(current_amount, G_CURRENT_SPENDING, G_INVESTMENT, after_time)
        if calculated_burn_time >= needed_burn_time:
            return after_time
        after_time += 1.0

def print_wait_time_for_burn(required_time):
    "Prints wait time to reach a given burn time in human readable format"
    needed_work_time = expected_to_reach_burn_time(G_INITIAL_AMOUNT, required_time)
    received_burn_time = burn_time(G_INITIAL_AMOUNT, G_CURRENT_SPENDING, G_INVESTMENT, needed_work_time)
    print("You'll have %d months of burn time after %d months" % (received_burn_time, needed_work_time))

def iterate_wait_time_for_burn(required_burn_time):
    "Iterates over and prints different working periods for different burn times"
    for required_time in range(1, required_burn_time + 1):
        print_wait_time_for_burn(float(required_time))

G_INITIAL_AMOUNT = 0.0
iterate_wait_time_for_burn(12)