Interpolate car milage to month boundaries for use with ERP systems.

milage.py

#!/usr/bin/python

import sys
import re
import calendar
from datetime import date
from dateutil.relativedelta import relativedelta

date_format = '%Y/%m/%d'

s = re.compile('/|\s');
p = re.compile('[0-9]{4}/[0-1][0-9]/[0-3][0-9] [0-9]+')

def next_month(d):
    d += relativedelta(months = 1)
    return d.replace(day = 1)

while 1:
    start = raw_input('Enter milage at start (yyyy/mm/dd milage): ')
    if p.match(start):
        break
    print("Invalid input.")

while 1:
    end = raw_input('Enter milage at the end (yyyy/mm/dd milage): ')
    if p.match(end):
        break
    print("Invalid input.")

start = s.split(start)
end = s.split(end)

if int(start[3]) > int(end[3]):
    print("Milage at the beginning higher than at the end.")
    sys.exit(1)

start_date = date(int(start[0]), int(start[1]), int(start[2]))
end_date = date(int(end[0]), int(end[1]), int(end[2]))

if start_date > end_date:
    print("End date is earlier than the start date.")
    sys.exit(1)

delta = end_date - start_date
milage_delta = int(end[3]) - int(start[3])
milage_avg = milage_delta / float(delta.days)
print('Delta: \n - Milage: {0}\n - Days: {1}').format(milage_delta, delta.days)
print('Average milage per day: {0}\n').format(milage_avg)

d = start_date
milage = int(start[3])
while 1:
    # Determine the end of the month
    end_of_month = d.replace(day = calendar.monthrange(d.year, d.month)[1])

    # Start input on end of the month, simply print the milage and continue
    if (d == end_of_month):
        print('\t\t\t{0} - {1}').format(d.strftime(date_format), int(milage))
        d = next_month(d)
        continue;

    # Print the milage at the start of the month
    print('{0} - {1}\t').format(d.strftime(date_format), int(milage)),

    # Input ends in the middel of a month, calculate up to that specific date.
    if (end_date <= end_of_month):
        days_to_end = (end_date - d).days
        milage_at_end = milage + (milage_avg * (days_to_end + 1))
        print('{0} - {1}').format(end_date.strftime(date_format), int(milage_at_end))
        break

    # Calculate milage at end of the month and increment to next month
    days_to_eom = (end_of_month - d).days + 1
    milage_at_eom = milage + (milage_avg * days_to_eom)
    print('{0} - {1}').format(end_of_month.strftime(date_format), int(milage_at_eom))

    milage = milage_at_eom
    d = next_month(d)