first commit

app/simulator.py

@@ -0,0 +1,129 @@
+from __future__ import annotations
+
+from math import pow
+from .models import SimulationRequest, ScheduleRow, Summary, SimulationResponse, InstallmentType, OverpaymentEffect
+
+
+def _round_money(value: float) -> float:
+    return round(max(value, 0.0) + 1e-9, 2)
+
+
+def _rate_for_month(req: SimulationRequest, month: int) -> float:
+    changes = sorted(req.rate_changes, key=lambda x: x.month)
+    rate = req.base_rate + req.margin
+    for change in changes:
+        if month >= change.month:
+            rate = change.annual_rate
+        else:
+            break
+    return rate
+
+
+def _monthly_payment(balance: float, monthly_rate: float, months_left: int) -> float:
+    if months_left <= 0:
+        return balance
+    if monthly_rate == 0:
+        return balance / months_left
+    factor = pow(1 + monthly_rate, months_left)
+    return balance * monthly_rate * factor / (factor - 1)
+
+
+def _overpayment_for_month(req: SimulationRequest, month: int) -> float:
+    total = 0.0
+    for op in req.overpayments:
+        until = op.until_month or month
+        if op.repeat == "once" and month == op.month:
+            total += op.amount
+        elif op.repeat == "monthly" and month >= op.month and month <= until:
+            total += op.amount
+        elif op.repeat == "yearly" and month >= op.month and month <= until and (month - op.month) % 12 == 0:
+            total += op.amount
+    return total
+
+
+def _simulate_raw(req: SimulationRequest, include_overpayments: bool = True) -> list[ScheduleRow]:
+    balance = float(req.principal)
+    total_months = int(req.years * 12)
+    rows: list[ScheduleRow] = []
+    fixed_payment = None
+    recalculation_month = 1
+    month = 1
+
+    while balance > 0.005 and month <= total_months + 600:
+        months_left = max(total_months - month + 1, 1)
+        annual_rate = _rate_for_month(req, month)
+        monthly_rate = annual_rate / 100 / 12
+        interest = balance * monthly_rate
+
+        if req.installment_type == InstallmentType.equal:
+            if fixed_payment is None or req.overpayment_effect == OverpaymentEffect.lower_payment:
+                fixed_payment = _monthly_payment(balance, monthly_rate, months_left)
+            payment = fixed_payment
+            principal_part = max(payment - interest, 0.0)
+        else:
+            principal_base = req.principal / total_months
+            if req.overpayment_effect == OverpaymentEffect.lower_payment:
+                active_months_left = max(total_months - month + 1, 1)
+                principal_base = balance / active_months_left
+            payment = principal_base + interest
+            principal_part = principal_base
+
+        if principal_part > balance:
+            principal_part = balance
+            payment = interest + principal_part
+
+        balance -= principal_part
+
+        overpayment = _overpayment_for_month(req, month) if include_overpayments else 0.0
+        overpayment = min(overpayment, balance)
+        balance -= overpayment
+
+        rows.append(ScheduleRow(
+            month=month,
+            rate=round(annual_rate, 4),
+            payment=_round_money(payment),
+            principal_part=_round_money(principal_part),
+            interest_part=_round_money(interest),
+            overpayment=_round_money(overpayment),
+            remaining=_round_money(balance),
+        ))
+
+        if include_overpayments and overpayment > 0 and req.overpayment_effect == OverpaymentEffect.lower_payment:
+            fixed_payment = None
+            recalculation_month = month + 1
+
+        if include_overpayments and overpayment > 0 and req.overpayment_effect == OverpaymentEffect.shorten:
+            # przy skroceniu okresu rata zostaje z grubsza taka sama; petla zakonczy sie szybciej
+            pass
+
+        month += 1
+        if month - recalculation_month > 1200:
+            break
+
+    return rows
+
+
+def simulate(req: SimulationRequest) -> SimulationResponse:
+    actual = _simulate_raw(req, include_overpayments=True)
+    baseline_req = req.model_copy(update={"overpayments": []})
+    baseline = _simulate_raw(baseline_req, include_overpayments=False)
+
+    total_interest = sum(r.interest_part for r in actual)
+    total_overpayment = sum(r.overpayment for r in actual)
+    total_paid = sum(r.payment + r.overpayment for r in actual)
+    baseline_interest = sum(r.interest_part for r in baseline)
+    payments = [r.payment for r in actual]
+
+    summary = Summary(
+        months=len(actual),
+        total_paid=_round_money(total_paid),
+        total_interest=_round_money(total_interest),
+        total_overpayment=_round_money(total_overpayment),
+        interest_saved=_round_money(baseline_interest - total_interest),
+        months_saved=max(len(baseline) - len(actual), 0),
+        baseline_interest=_round_money(baseline_interest),
+        baseline_months=len(baseline),
+        average_payment=_round_money(sum(payments) / len(payments)) if payments else 0,
+        max_payment=_round_money(max(payments)) if payments else 0,
+    )
+    return SimulationResponse(schedule=actual, summary=summary)