'''A program that allows the user to interactively enter tuition costs and
display the expected year that tuition will reach that level.

Author: Sean Barker
Computer Science 1101'''

CURRENT_YEAR = 2016

# tuition cost in the current year
CURRENT_COST = 65590 

# annual percentage tuition increase
ANNUAL_INCREASE = 0.0386

# the prompt shown to the user when entering a future tuition cost
USER_PROMPT = 'Input a target cost: '

# controls how the output is displayed to the user
OUTPUT_FORMAT_STRING = 'Tuition cost is expected to exceed {0} in {1}\n'


def years_until_cost(target_cost):
    '''Return the number of years until tuition costs are expected
    to exceed target_cost.'''

    years_passed = 0
    cost = CURRENT_COST

    while cost < target_cost:
        cost += cost * ANNUAL_INCREASE
        years_passed += 1
    
    return years_passed


def run_prompt_loop():
    '''Repeatedly prompt the user for a target cost and displays
    the number of years until tuition reaches that level.'''
    
    finished = False
    while not finished:
        desired_cost = input(USER_PROMPT)
        if desired_cost == 'quit':
            finished = True
        else:
            year = CURRENT_YEAR + years_until_cost(int(desired_cost))
            print(OUTPUT_FORMAT_STRING.format(desired_cost, year))


def main():
    '''Greet the user, run the tuition forecaster, then print an exit message.'''
    print('Welcome to the Bowdoin tuition forecaster!')
    run_prompt_loop()
    print('Goodbye!')

main() # start the program