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1 files changed, 55 insertions, 0 deletions

diff --git a/mortgage.py b/mortgage.py
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+++ b/mortgage.py
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+import argparse
+import math
+import sys
+from tabulate import tabulate, SEPARATING_LINE
+
+
+def calc_payments(principal, annual_interest, installments):
+    p = principal
+    i = annual_interest / 100 / 12 # Monthly equivalent.
+    n = installments
+
+    def fmt(x):
+        return f"{math.ceil(x*100)/100:.2f}"
+
+    table = [["Time", "Outstanding Debt", "Monthly Installment", "Interest Paid", "Amount Amortised"]]
+
+    monthly_installment = p * (i * (1 + i)**n) / ((1 + i)**n - 1)
+    debt = principal
+    total_interest = 0
+    total_amortised = 0
+
+    for k in range(n):
+        interest = i * debt
+        amortised = monthly_installment - interest
+        table.append([k] + [fmt(x) for x in [debt, monthly_installment, interest, amortised]])
+
+        total_interest += interest
+        total_amortised += amortised
+        debt -= amortised
+
+    total_interest_pct = total_interest / principal * 100
+
+    table.append(SEPARATING_LINE)
+    table.append(["Total", "", "Amount Paid", "Interest Paid", "Amount Amortised"])
+    table.append(["", ""] + [
+        fmt(monthly_installment * n),
+        fmt(total_interest) + f" ({fmt(total_interest_pct)}%)",
+        fmt(total_amortised)])
+    return table
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument("-p", "--principal", type=float, default=100000, help="Principal")
+    parser.add_argument("-i", "--interest", type=float, default=5, help="Annual interest %")
+    parser.add_argument("-n", "--installments", type=int, default=300, help="Number of installments")
+    args = parser.parse_args()
+
+    table = calc_payments(args.principal, args.interest, args.installments)
+    print(tabulate(table))
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())