Add amortization calculator to financial folder

financial/amortization.py

@@ -0,0 +1,76 @@
+"""
+Wiki: https://en.wikipedia.org/wiki/Amortization_calculator
+"""
+
+
+def level_payment(principal, annual_rate_pct, years, payments_per_year=12):
+    if principal <= 0:
+        raise ValueError("principal must be > 0")
+    if years <= 0 or payments_per_year <= 0:
+        raise ValueError("years and payments_per_year must be > 0")
+    r = (annual_rate_pct / 100.0) / payments_per_year
+    n = years * payments_per_year
+    if r == 0:
+        return principal / n
+    factor = (1 + r) ** n
+    return principal * (r * factor) / (factor - 1)
+
+
+def amortization_schedule(principal, annual_rate_pct, years, payments_per_year=12, print_annual_summary=False, eps=1e-9):
+    pmt = level_payment(principal, annual_rate_pct, years, payments_per_year)
+    r = (annual_rate_pct / 100.0) / payments_per_year
+    n = years * payments_per_year
+
+    balance = float(principal)
+    schedule = []
+
+    if print_annual_summary:
+        print(
+            (
+                f"{'Year':<6}{'Months Pd':<12}{'Tenure Left':<13}"
+                f"{'Payment/Period':<16}{'Outstanding':<14}"
+            )
+        )
+
+
+    for period in range(1, n + 1):
+        interest = balance * r
+        principal_component = pmt - interest
+
+        # shortpay on the last period if the scheduled principal would overshoot
+        if principal_component > balance - eps:
+            principal_component = balance
+            payment_made = interest + principal_component
+        else:
+            payment_made = pmt
+
+        if principal_component < 0 and principal_component > -eps:  # clamp tiny negatives
+            principal_component = 0.0
+
+        balance = max(0.0, balance - principal_component)
+        schedule.append([period, payment_made, interest, principal_component, balance])
+
+        # streamline for all time periods (monthly/quarterly/biweekly/weekly)
+        months_elapsed = (round((period * 12) / payments_per_year))
+
+        if print_annual_summary and (months_elapsed % 12 == 0 or balance <= eps):
+            tenure_left_periods = n - period
+            print(
+            (
+                f"{months_elapsed // 12:<6}{months_elapsed:<12}{tenure_left_periods:<13}"
+                f"{pmt:<16.2f}{balance:<14.2f}"
+            )
+        )
+
+        if balance <= eps:
+            break
+
+    # normalize any tiny residual
+    if schedule and schedule[-1][4] <= eps:
+        schedule[-1][4] = 0.0
+
+    return round(pmt, 4), schedule
+
+
+pmt, sched = amortization_schedule(10000, 5.5, 15, payments_per_year=12, print_annual_summary=True)
+print(pmt)