Debt Snowball Calculator

from dataclasses import dataclass, field
from typing import List, Optional

@dataclass
class Debt:
    name: str
    balance: float
    min_payment: float
    apr: float  # Annual Percentage Rate e.g. 0.20 for 20%

    @property
    def monthly_rate(self) -> float:
        return self.apr / 12

@dataclass
class PayoffResult:
    name: str
    months_to_payoff: int
    total_paid: float
    total_interest: float

def debt_snowball(debts: List[Debt], monthly_budget: float) -> List[PayoffResult]:
    """
    Simulate the debt snowball method.
    Debts are sorted by balance (smallest first).
    Extra budget is applied to the current target debt.
    """
    # Sort by balance ascending (snowball)
    queue  = sorted(debts, key=lambda d: d.balance)
    paid   = [False] * len(queue)
    bals   = [d.balance for d in queue]
    totals = [0.0] * len(queue)  # total paid per debt
    month  = 0
    results = []

    while any(not p for p in paid):
        month += 1
        # Find current target: first unpaid debt
        target_idx = next(i for i, p in enumerate(paid) if not p)

        # Calculate total minimums for non-target debts
        minimums_used = sum(
            queue[i].min_payment for i in range(len(queue))
            if not paid[i] and i != target_idx
        )
        extra = monthly_budget - minimums_used

        # Apply payments
        for i, debt in enumerate(queue):
            if paid[i]:
                continue
            payment = extra if i == target_idx else debt.min_payment
            interest = bals[i] * debt.monthly_rate
            principal = min(payment - interest, bals[i])
            bals[i] -= principal
            totals[i] += payment
            if bals[i] <= 0.01:
                paid[i] = True
                results.append(PayoffResult(
                    name=debt.name,
                    months_to_payoff=month,
                    total_paid=totals[i] + bals[i],  # adjust overpayment
                    total_interest=totals[i] - debt.balance
                ))

    return results

# Example
debts = [
    Debt("Credit Card A", balance=2500,  min_payment=75,  apr=0.22),
    Debt("Credit Card B", balance=5000,  min_payment=100, apr=0.18),
    Debt("Car Loan",      balance=8000,  min_payment=200, apr=0.06),
    Debt("Student Loan",  balance=15000, min_payment=150, apr=0.05),
]

results = debt_snowball(debts, monthly_budget=700)

total_interest = sum(r.total_interest for r in results)
print("Debt Snowball Payoff Plan:")
print(f"{'Debt':<20} {'Months':>7} {'Interest Paid':>14}")
for r in results:
    yrs = r.months_to_payoff // 12
    mos = r.months_to_payoff % 12
    print(f"{r.name:<20} {yrs}y {mos:02d}m  ${r.total_interest:>12,.2f}")
print(f"{'Total interest:':<20}        ${total_interest:>12,.2f}")