feat: core financial engine (PMT, amortization, simulation calculations)

src/lib/types.ts

@@ -0,0 +1,156 @@
+// ─── Property Type ───────────────────────────────────────────────────────────
+
+export type PropertyType = 'neuf' | 'ancien';
+
+// ─── Loan Input ──────────────────────────────────────────────────────────────
+
+export interface LoanInput {
+  id: string;
+  name: string;
+  amount: number;
+  rate: number;            // Annual rate as decimal (3.25% → 0.0325)
+  duration: number;        // Years
+  deferral: number;        // Deferred period in years
+  insuranceRate: number;   // Annual rate on initial capital (0.3% → 0.003)
+  isMainLoan: boolean;     // If true, amount = totalCredit - sum(others)
+  enabled: boolean;
+}
+
+// ─── Simulation State (all inputs) ──────────────────────────────────────────
+
+export interface SimulationState {
+  // Project
+  propertyType: PropertyType;
+  propertyPrice: number;
+  notaryFeesRate: number;
+  guaranteeRate: number;
+  bankFees: number;
+  worksCost: number;
+  downPayment: number;
+
+  // Current costs (tenant)
+  currentRent: number;
+  rentEvolutionRate: number;   // Annual evolution (1.2% → 0.012)
+  currentUtilities: number;    // EDF / fibre / eau
+  currentCharges: number;
+  currentHomeInsurance: number;
+
+  // Projected costs (owner)
+  projectedCharges: number;    // Copropriété
+  propertyTax: number;         // Taxe foncière (monthly)
+  projectedUtilities: number;  // EDF / fibre / eau
+  projectedHomeInsurance: number;
+
+  // Loans
+  loans: LoanInput[];
+
+  // Resale parameters
+  firstYearDepreciation: number;  // e.g., -0.05 (5% loss first year)
+  annualAppreciation: number;     // e.g., 0.01 (1% per year)
+  saleFees: number;               // e.g., 0.07 (7% agency + notary)
+}
+
+// ─── Monthly Amortization (per loan) ────────────────────────────────────────
+
+export interface MonthlyRow {
+  month: number;        // 1-based global month
+  monthInYear: number;  // 1-12
+  year: number;         // 1-based year
+  interest: number;
+  principal: number;
+  insurance: number;
+  remaining: number;
+  payment: number;      // interest + principal + insurance
+}
+
+// ─── Yearly Amortization (per loan) ─────────────────────────────────────────
+
+export interface YearlyRow {
+  year: number;
+  interest: number;
+  principal: number;
+  insurance: number;
+  remaining: number;
+  payment: number;
+}
+
+// ─── Loan Computation Result ────────────────────────────────────────────────
+
+export interface LoanResult {
+  loan: LoanInput;
+  effectiveAmount: number;     // Actual amount (auto-calculated for main)
+  monthlyPayment: number;     // Monthly principal + interest (excl. insurance)
+  monthlyInsurance: number;
+  monthly: MonthlyRow[];
+  yearly: YearlyRow[];
+  totalInterest: number;
+  totalInsurance: number;
+  totalCost: number;           // amount + totalInterest + totalInsurance
+}
+
+// ─── Yearly Summary (aggregated across all loans) ───────────────────────────
+
+export interface YearlySummary {
+  year: number;
+
+  // Credits aggregated
+  totalInterest: number;
+  totalPrincipal: number;
+  totalInsurance: number;
+  totalRemaining: number;
+  totalPayment: number;
+
+  // Projected monthly costs (as owner)
+  monthlyInterest: number;
+  monthlyPrincipal: number;
+  monthlyCharges: number;     // charges + tax + utilities + home insurance
+  monthlyLoanInsurance: number;
+  monthlyTotal: number;
+  capitalRatio: number;       // Part of payment building equity
+  yearlyTotal: number;
+
+  // Current monthly costs (as tenant)
+  currentRent: number;
+  currentCharges: number;     // utilities + charges + insurance
+  currentMonthlyTotal: number;
+  currentYearlyTotal: number;
+
+  // Comparison
+  difference: number;         // projected - current (negative = savings)
+
+  // Resale projection
+  resalePrice: number;
+  netResale: number;          // After fees - remaining debt - down payment
+}
+
+// ─── Final Simulation Results ───────────────────────────────────────────────
+
+export interface SimulationResults {
+  // Financing
+  notaryFees: number;
+  guaranteeFees: number;
+  totalFinancing: number;
+  totalCredit: number;
+  mainLoanAmount: number;
+
+  // Per-loan results
+  loanResults: LoanResult[];
+
+  // Yearly summaries (up to 30 years)
+  yearlySummaries: YearlySummary[];
+
+  // Global totals
+  totalCapital: number;
+  totalInterest: number;
+  totalInsurance: number;
+  grandTotal: number;
+
+  // Year 1 monthly snapshot
+  monthlyPaymentYear1: number;
+  projectedMonthlyCharges: number;
+  projectedMonthlyTotal: number;
+  currentMonthlyTotal: number;
+
+  // Max duration across all loans
+  maxDuration: number;
+}