/**
 * calc.js — pure calculation engine, no DOM
 */

/**
 * Run the full rent vs buy simulation month by month.
 * Returns arrays (one value per year) plus summary stats.
 *
 * @param {Object} p - input parameters
 * @returns {Object} result
 */
function run_simulation(p) {
  const months = p.years * 12;

  // --- mortgage setup ---
  const loan = p.home_price * (1 - p.down_pct / 100);
  const r_m = p.interest_rate / 100 / 12;
  const n = p.loan_years * 12;
  let monthly_payment;
  if (r_m === 0) {
    monthly_payment = loan / n;
  } else {
    monthly_payment = loan * (r_m * Math.pow(1 + r_m, n)) / (Math.pow(1 + r_m, n) - 1);
  }

  // upfront costs for buyer
  const down_payment = p.home_price * (p.down_pct / 100);
  const upfront_buy = down_payment + p.home_price * (p.closing_buy_pct / 100);
  const start_capital = p.start_capital || 0;

  // --- state ---
  let balance = loan;
  let home_value = p.home_price;
  // renter invests all startkapital; buyer spends upfront_buy and invests the remainder
  let portfolio = start_capital;
  let buyer_portfolio = Math.max(0, start_capital - upfront_buy);
  let monthly_rent = p.monthly_rent;

  let cum_buy_cost = upfront_buy; // running cash spent on buying
  let cum_rent_cost = 0;

  // per-year arrays (index 0 = end of year 1)
  const buyer_nw_arr = [];
  const renter_nw_arr = [];
  const cum_buy_arr = [];
  const cum_rent_arr = [];
  const labels = [];

  let breakeven_month = null;

  const invest_r_m = p.invest_return / 100 / 12;
  const app_r_m = p.appreciation / 100 / 12;
  const rent_inc_m = p.rent_increase / 100 / 12;

  for (let m = 1; m <= months; m++) {
    // --- buy side ---
    const interest_m = balance * r_m;
    const principal_m = monthly_payment - interest_m;
    balance = Math.max(0, balance - principal_m);
    home_value *= (1 + app_r_m);

    const prop_tax_m = home_value * (p.prop_tax_rate / 100) / 12;
    const maint_m = home_value * (p.maint_rate / 100) / 12;
    const insurance_m = p.insurance / 12;
    // German context: no mortgage interest tax deduction for private buyers
    const total_buy_m = monthly_payment + prop_tax_m + maint_m + insurance_m;

    cum_buy_cost += total_buy_m;

    buyer_portfolio *= (1 + invest_r_m);
    const selling_costs = home_value * (p.closing_sell_pct / 100);
    const buyer_nw = home_value - selling_costs - balance + buyer_portfolio;

    // --- rent side ---
    const renters_ins_m = p.renters_ins / 12;
    const total_rent_m = monthly_rent + renters_ins_m;
    cum_rent_cost += total_rent_m;

    // renter invests the difference if buying is more expensive
    const monthly_delta = Math.max(0, total_buy_m - total_rent_m);
    portfolio = portfolio * (1 + invest_r_m) + monthly_delta;

    // apply capital gains tax on portfolio gains when we "cash out" at end
    // (we track gross portfolio, deduct tax in summary)
    const renter_nw = portfolio;

    // breakeven: when buyer net worth overtakes renter
    if (breakeven_month === null && buyer_nw >= renter_nw) {
      breakeven_month = m;
    }

    // rent increases monthly
    monthly_rent *= (1 + rent_inc_m);

    // record yearly snapshots
    if (m % 12 === 0) {
      const yr = m / 12;
      labels.push(`Jahr ${yr}`);
      buyer_nw_arr.push(Math.round(buyer_nw));
      // apply Abgeltungssteuer on portfolio gains at cashout
      const portfolio_gains = portfolio - start_capital;
      const tax_on_gains = Math.max(0, portfolio_gains) * (p.tax_rate / 100);
      renter_nw_arr.push(Math.round(portfolio - tax_on_gains));
      cum_buy_arr.push(Math.round(cum_buy_cost));
      cum_rent_arr.push(Math.round(cum_rent_cost));
    }
  }

  // final net worth at end of horizon
  const final_buyer_nw = buyer_nw_arr[buyer_nw_arr.length - 1];
  const final_renter_nw = renter_nw_arr[renter_nw_arr.length - 1];

  return {
    labels,
    buyer_nw_arr,
    renter_nw_arr,
    cum_buy_arr,
    cum_rent_arr,
    monthly_payment: Math.round(monthly_payment),
    final_buyer_nw,
    final_renter_nw,
    breakeven_month,
    winner: final_buyer_nw >= final_renter_nw ? 'buy' : 'rent',
  };
}