Relative Strength Index (RSI)

/**
 * Calculates the Relative Strength Index (RSI) for a given dataset and period.
 *
 * @param {array} data - An array of historical stock data. Can be a single column of prices or a multi-column array from GOOGLEFINANCE.
 * @param {number} period The number of periods to calculate the RSI, e.g., 14 for a 14-day RSI.
 * @returns {array} A 2D array with headers: Date and RSI.
 * @customfunction
 */
function RSI(data, period) {
  // Argument validation
  if (arguments.length !== 2) {
    throw new Error(`Wrong number of arguments. Expected 2, but got ${arguments.length}.`);
  }
  if (typeof period !== 'number' || period <= 0 || !Number.isInteger(period)) {
    throw new Error(`Invalid period. The period must be a positive integer. Got: ${period}`);
  }

  const processedData = getData(data);

  // --- NEW: Function-level validation ---
  const columnCount = processedData[0].length;
  if (columnCount > 2 && columnCount < 5) {
      throw new Error(`Invalid data structure for RSI. For multi-column data, expected at least 5 columns (Date, O, H, L, C), but got ${columnCount}. For simple data, expected 2 columns (Date, Value).`);
  }
  // --- END of validation ---

  const dates = processedData.slice(1).map(row => row[0]);
  const values = getValues(processedData);

  if (period >= values.length) {
    throw new Error(`Invalid period. The period (${period}) cannot be greater than or equal to the number of data points (${values.length}).`);
  }

  let gainsData = []; // 2D array: [[date, gain], ...]
  let lossesData = []; // 2D array: [[date, loss], ...]

  // Note: gains and losses are calculated from the second data point (index 1)
  // So, their dates correspond to dates[i] where i starts from 1.
  for (let i = 1; i < values.length; i++) {
    let priceChange = values[i] - values[i - 1];
    // The date for this change is dates[i]
    if (priceChange > 0) {
      gainsData.push([dates[i], priceChange]);
      lossesData.push([dates[i], 0]);
    } else {
      gainsData.push([dates[i], 0]);
      lossesData.push([dates[i], Math.abs(priceChange)]);
    }
  }

  // RMA now takes 2D array data and returns 2D array data
  const avgGainsResult = RMA(gainsData, period);
  const avgLossesResult = RMA(lossesData, period);

  // Extract values from RMA results (remove headers)
  const avgGains = avgGainsResult.slice(1).map(row => row[1]);
  const avgLosses = avgLossesResult.slice(1).map(row => row[1]);

  let results = [['Date', `RSI(${period})`]];
  
  // The dates for avgGains and avgLosses are already aligned by RMA.
  // We need to use the dates from avgGainsResult (or avgLossesResult) for the final RSI output.
  // The loop should start from the first available RSI value.
  // The length of avgGains and avgLosses will be (values.length - 1) - (period - 1) = values.length - period
  // The dates for these results start from dates[period] in the original series.

  for (let i = 0; i < avgGains.length; i++) {
    const date = avgGainsResult[i + 1][0]; // +1 to skip header
    const avgGain = avgGains[i];
    const avgLoss = avgLosses[i];
    
    if (avgLoss === 0) {
        results.push([date, 100]);
        continue;
    }

    const rs = avgGain / avgLoss;
    const rsi = 100 - (100 / (1 + rs));
    results.push([date, rsi]);
  }

  return results;
}