Calculate compound interest

compound_interest.py

#!/usr/bin/env python3

"""
Author: Ming Wen (bitmingw@gmail.com)

This python script calculates compound interest:

1. Given (initial assets value, investment per year, growth rate, number of years),
   find out final assets value.

   Example usage:
   $ python3 compound_interest.py --initial 50000 --invest-per-year 30000 --growth-rate 7.5 --years 15 --find target

2. Given (initial assets value, growth rate, number of years, target assets value),
   find out required investment per year.

   Example usage:
   $ python3 compound_interest.py --initial 50000 --growth-rate 7.5 --years 15 --target 1000000 --find invest-per-year

3. Given (initial assets value, investment per year, number of years, target assets value),
   find out required growth rate.

   Example usage:
   $ python3 compound_interest.py --initial 50000 --invest-per-year 30000 --years 15 --target 1000000 --find growth-rate

4. Given (initial assets value, investment per year, growth rate, target assets value),
   find out required number of years.

   Example usage:
   $ python3 compound_interest.py --initial 50000 --invest-per-year 30000 --growth-rate 7.5 --target 1000000 --find years

MIT License

Copyright (c) 2020 Ming Wen

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
"""

import argparse
import math

class CompoundInterest:

    find_type_invest_per_year = ["invest", "invest per year", "invest-per-year", "invest_per_year"]
    find_type_growth_rate = ["growth", "growth rate", "growth-rate", "growth_rate"]
    find_type_years = ["years"]
    find_type_target = ["target"]
    diff_threshold = 1.0

    def __init__(self):
        self.initial = None
        self.invest_per_year = None
        self.growth_rate = None
        self.years = None
        self.target = None

    def find_invest_per_year(self) -> float:
        assert self.initial != None \
            and self.growth_rate != None \
            and self.years != None \
            and self.target != None

        # Find the value by binary search
        lo, hi = 0, self.target - self.initial
        while True:
            mi = (lo + hi) / 2
            actual_target = CompoundInterest._find_target(self.initial,
                                                          mi,
                                                          self.growth_rate,
                                                          self.years)
            diff = actual_target - self.target
            if abs(diff) < CompoundInterest.diff_threshold:
                return mi
            elif diff < 0:
                lo = mi
            else:
                hi = mi

    def find_growth_rate(self) -> float:
        assert self.initial != None \
            and self.invest_per_year != None \
            and self.years != None \
            and self.target != None

        if self.initial + self.invest_per_year * self.years >= self.target:
            return 0.0

        # Find the value by binary search
        lo, hi = 0, (self.target - self.initial) / (self.invest_per_year * self.years)
        while True:
            mi = (lo + hi) / 2
            actual_target = CompoundInterest._find_target(self.initial,
                                                          self.invest_per_year,
                                                          mi,
                                                          self.years)
            diff = actual_target - self.target
            if abs(diff) < CompoundInterest.diff_threshold:
                return mi
            elif diff < 0:
                lo = mi
            else:
                hi = mi

    def find_years(self) -> int:
        assert self.initial != None \
            and self.invest_per_year != None \
            and self.growth_rate != None \
            and self.target != None

        if self.initial >= self.target:
            return 0

        year = 1
        current = self.initial
        while True:
            current *= 1 + self.growth_rate
            current += self.invest_per_year * pow(math.e, self.growth_rate)
            if current >= self.target:
                return year
            year += 1

    def find_target(self) -> float:
        assert self.initial != None \
            and self.invest_per_year != None \
            and self.growth_rate != None \
            and self.years != None
        return CompoundInterest._find_target(self.initial,
                                             self.invest_per_year,
                                             self.growth_rate,
                                             self.years)

    @classmethod
    def _find_target(cls,
                     initial: float,
                     invest_per_year: float,
                     growth_rate: float,
                     years: int):
        current = initial
        for _ in range(years):
            current *= 1 + growth_rate
            current += invest_per_year * pow(math.e, growth_rate)
        return current

def main():
    parser = argparse.ArgumentParser()
    parser.add_argument("--initial",
                        type=float,
                        help="initial assets in the portfolio")
    parser.add_argument("--invest-per-year",
                        type=float,
                        help="contribution to portfolio per year")
    parser.add_argument("--growth-rate",
                        type=float,
                        help="percentage of portfolio growth per year")
    parser.add_argument("--years",
                        type=int,
                        help="years of investment")
    parser.add_argument("--target",
                        type=float,
                        help="target assets value")
    parser.add_argument("--find",
                        type=str,
                        help="which type of information to find",
                        required=True)
    args = parser.parse_args()

    param = CompoundInterest()
    if args.initial != None:
        if args.initial < 0:
            print("Initial assets value must be at least 0")
            exit(1)
        param.initial = args.initial
    else:
        param.initial = 0
    if args.invest_per_year != None:
        if args.invest_per_year < 0:
            print("Investment per year must be at least 0")
            exit(1)
        param.invest_per_year = args.invest_per_year
    if args.growth_rate != None:
        if args.growth_rate < 0:
            print("Growth rate must be at least 0")
            exit(1)
        param.growth_rate = args.growth_rate / 100
    if args.years != None:
        if args.years < 1:
            print("Number of years must be at least 1")
            exit(1)
        param.years = args.years
    if args.target != None:
        if args.target < param.initial:
            print("Target value can't be smaller than initial value")
            exit(1)
        param.target = args.target

    if args.find in param.find_type_invest_per_year:
        print("Investment per year: {}".format(int(param.find_invest_per_year())))
    elif args.find in param.find_type_growth_rate:
        print("Growth rate: {:3.1f}%".format(param.find_growth_rate() * 100))
    elif args.find in param.find_type_years:
        print("Number of years: {}".format(param.find_years()))
    elif args.find in param.find_type_target:
        print("Target: {}".format(int(param.find_target())))
    else:
        print("Unknown find type: {}".format(args.find))
        print("""
              Supported find types:

              invest-per-year
              growth-rate
              years
              target
              """)
        exit(1)

if __name__ == "__main__":
    main()