Williams %R (WPR)

/**
 * Calculates Williams %R, a momentum indicator that measures overbought and oversold levels.
 *
 * @param {array} data - The input range. Must include at least 5 columns: Date, Open, High, Low, Close.
 * @param {number} period - The lookback period (e.g., 14).
 * @returns {array} A two-column array with headers "Date" and "Williams %R".
 * @customfunction
 */
function WPR(data, period) {
    checkPremium();

  // 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 < 5) {
    throw new Error(`Invalid data structure for Williams %R. Expected at least 5 columns (Date, Open, High, Low, Close), but got ${columnCount}.`);
  }
  // --- END of validation ---

  const dates = processedData.slice(1).map(row => row[0]);
  const highs = processedData.slice(1).map(row => row[2]);
  const lows = processedData.slice(1).map(row => row[3]);
  const closes = processedData.slice(1).map(row => row[4]);

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

  const results = [["Date", `Williams %R (${period})`]];

  for (let i = period - 1; i < closes.length; i++) {
    const periodHigh = Math.max(...highs.slice(i - period + 1, i + 1));
    const periodLow = Math.min(...lows.slice(i - period + 1, i + 1));
    const currentClose = closes[i];

    let williamsR;
    if (periodHigh === periodLow) {
      williamsR = 0; // Avoid division by zero
    } else {
      williamsR = ((periodHigh - currentClose) / (periodHigh - periodLow)) * -100;
    }
    results.push([dates[i], williamsR]);
  }

  return results;
}