loc_interest_calculator/web.py

import jinja2
import os
from flask import Flask, render_template, request
from decimal import *
from datetime import *

##############################################################################
#
# This is the BQL query to put into Fava to generate the CSV that can be used
# to create the JSON file for this.  Might consider reading in the CSV directly
# and prompting for the other information?
#pip
# select date, number where year=2022 And account~".*:CreditLine"
#
##############################################################################
#
# Consider altering this program to generate the text to go into the beancount
# files directly so that it can just be a cut and paste job.
#
##############################################################################

app = Flask(__name__)
trans_template = '''{0} * "Interest Due on LoC {1} - {2}"\n\t{3} {4} USD\n\t{5}\n\n'''
loader = jinja2.FileSystemLoader(os.path.dirname(__file__))
environment = jinja2.Environment(loader=loader)
DZERO = Decimal("0.00")


######
# Flask Call Backs
######
@app.route('/', methods=['GET', 'POST'])
def hello():
    now = datetime.now()
    model = dict()
    model['year'] = now.date().strftime("%Y")
    model['rate'] = 8
    model['starting_balance'] = 0
    model['principal_account'] = "Liabilities:Current:CreditLine"
    model['interest_account'] = "Expenses:Interest:LoCInterest"
    model['int_txns'] = 'Not calculated.'
    model['txns'] = []
    model['net_balance_changes'] = {}
    model['transcript'] = []

    if request.method == 'POST':
        model['year'] = request.form['year']
        model['rate'] = request.form['rate']
        starting_balance = Decimal(request.form['starting_balance'])
        model['starting_balance'] = (DZERO + starting_balance).quantize(Decimal("0.00"))
        model['principal_account'] = request.form['principal_account']
        model['interest_account'] = request.form['interest_account']
        files = request.files

        if 'transactions' in files:
            result, model['message'], txns = validateUserInput(model, files['transactions'])

            if result is True:
                model['txns'] = txns
                model['int_txns'] = generate_interest_postings(model, txns)

    return render_template('main.html', model=model)


def validateUserInput(model: object, csv_content: object) -> (bool, str, dict):
    #TODO: What validations should I put here?

    #put the contents of the csv file into a list
    transactions = {}
    for line in csv_content:
        text = line.decode('utf-8').rstrip()
        if "date" in text:
            pass
        else:
            items = text.split(',')
            current_day = items[0]
            amount = Decimal("0.00") + Decimal(items[1])
            if current_day in transactions:
                transactions[current_day].append(amount)
            else:
                transactions[current_day] = [amount]
    return True, '', transactions


def appendToTranscript(transcript: dict, date: str, type: str, amount: Decimal, balance: Decimal):
    transcript.append({"date": date, "type": type, "amount": amount, "balance": balance})


def generate_interest_postings(model: dict, transactions: dict) -> list:
    postings = []
    transcript = []

    starting_balance = Decimal(model['starting_balance'])
    apr = Decimal(model['rate']) / 100
    daily_interest_rate = apr / 360
    year = int(model['year'])
    first_day = datetime(year, 1, 1)
    last_day = datetime(year, 12, 31)
    days_in_year = (last_day - first_day).days + 2

    current_balance = starting_balance
    accummulated_interest = DZERO
    daily_interest = calculateDailyInterest(current_balance, daily_interest_rate)

    for i in range(1, days_in_year):
        current_date = (first_day + timedelta(days=i-1)).date()

        if current_date.day == 1:
            appendToTranscript(transcript, current_date, "Balance", "", current_balance)


        #first add this day's transactions to the current balance
        if current_date.__str__() in transactions:
            for item in transactions[current_date.__str__()]:
                current_balance = current_balance - item
                appendToTranscript(transcript, current_date, "Transaction", item, current_balance)

            daily_interest = calculateDailyInterest(current_balance, daily_interest_rate)
            appendToTranscript(transcript, current_date, "Daily Interest Change", daily_interest, "")


        #accumulate this day's interest
        accummulated_interest = accummulated_interest + daily_interest

        #if this is the last day of a month, accrue the accumulated interest to the balance
        #then recalculate the new daily_interest accrual amount
        #then emit that month's interest accrual posting
        #then zero accumulated interest
        if is_last_day_of_month(current_date):
            current_balance = current_balance + accummulated_interest
            daily_interest = calculateDailyInterest(current_balance, daily_interest_rate)
            postings.append(trans_template.format(fava_date_format(current_date),
                                                   first_day_of_month(current_date), current_date,
                                                   model['principal_account'], accummulated_interest * -1,
                                                   model['interest_account']))
            appendToTranscript(transcript, current_date, "Interest Posting", accummulated_interest * -1, current_balance)
            accummulated_interest = DZERO
    model['transcript'] = transcript
    return postings


def is_last_day_of_month(some_date):
    if some_date.day < 28:
        return False

    next_day = some_date + timedelta(days=1)
    if next_day.month == some_date.month:
        return False
    return True


def first_day_of_month(some_date):
    firstDayOfMonth = date(some_date.year, some_date.month, 1)
    return firstDayOfMonth


def calculateDailyInterest(principal, rate):
    return (principal*rate).quantize(Decimal('.01'))


def fava_date_format(some_date):
    day = some_date.day
    month = some_date.month
    year = some_date.year
    return "{0}-{1}-{2}".format(year, month, day)


def main():
    app.debug = True
    app.run(host='0.0.0.0', port=8000)


if __name__ == '__main__':
    main()