'use strict';

/**
 * Computes a suggested expected amount for a bill based on the rolling median
 * of the last 6 months of actual data. Prefers monthly_bill_state.actual_amount
 * (user-corrected values) over raw payment sums.
 */
function computeAmountSuggestion(db, billId, year, month) {
  const amounts = [];
  let y = year;
  let m = month;

  for (let i = 0; i < 6; i++) {
    m -= 1;
    if (m === 0) { m = 12; y -= 1; }

    const mbs = db.prepare(
      'SELECT actual_amount FROM monthly_bill_state WHERE bill_id = ? AND year = ? AND month = ?'
    ).get(billId, y, m);

    if (mbs?.actual_amount != null) {
      amounts.push(mbs.actual_amount);
      continue;
    }

    const result = db.prepare(`
      SELECT COALESCE(SUM(amount), 0) AS total
      FROM payments
      WHERE bill_id = ?
        AND deleted_at IS NULL
        AND strftime('%Y', paid_date) = ?
        AND strftime('%m', paid_date) = ?
    `).get(billId, String(y), String(m).padStart(2, '0'));

    if (result.total > 0) amounts.push(result.total);
  }

  if (amounts.length === 0) return null;

  const sorted = [...amounts].sort((a, b) => a - b);
  const mid = Math.floor(sorted.length / 2);
  const median = sorted.length % 2 === 0
    ? (sorted[mid - 1] + sorted[mid]) / 2
    : sorted[mid];

  return {
    suggestion: Math.round(median * 100) / 100,
    months_used: amounts.length,
    confidence: amounts.length >= 3 ? 'high' : 'low',
  };
}

module.exports = { computeAmountSuggestion };