function addTradingDays(date, days) {
  const value = new Date(`${date}T00:00:00Z`);
  let remaining = Math.max(0, Number(days) || 0);
  while (remaining > 0) {
    value.setUTCDate(value.getUTCDate() + 1);
    const weekday = value.getUTCDay();
    if (weekday !== 0 && weekday !== 6) remaining -= 1;
  }
  return value.toISOString().slice(0, 10);
}

function nearestOnOrAfter(history, date) {
  return history.find((row) => row.date >= date)?.close ?? null;
}

function calculateOutcome(prediction, instrumentHistory, benchmarkHistory) {
  const eventDate = String(prediction.information_cutoff).slice(0, 10);
  const horizonDate = addTradingDays(eventDate, prediction.horizon_days);
  const price0 = nearestOnOrAfter(instrumentHistory, eventDate);
  const priceHorizon = nearestOnOrAfter(instrumentHistory, horizonDate);
  const benchmark0 = nearestOnOrAfter(benchmarkHistory, eventDate);
  const benchmarkHorizon = nearestOnOrAfter(benchmarkHistory, horizonDate);
  if (![price0, priceHorizon, benchmark0, benchmarkHorizon].every(Number.isFinite)) return null;
  const instrumentReturn = (priceHorizon - price0) / price0;
  const benchmarkReturn = (benchmarkHorizon - benchmark0) / benchmark0;
  const excessReturn = instrumentReturn - benchmarkReturn;
  const directionCorrect = prediction.direction === 'positive' ? excessReturn > 0 : excessReturn < 0;
  return {
    price0, priceHorizon, benchmark0, benchmarkHorizon,
    excessReturn, directionCorrect: directionCorrect ? 1 : 0,
    eventDate, horizonDate,
  };
}

module.exports = { addTradingDays, nearestOnOrAfter, calculateOutcome };