const PRICE_DECIMALS = 2; // Number of decimal places kept in each returned price.

/**
 * Returns a price after a discount. Accepts one price or a range of prices.
 *
 * @param {number|Array<Array<*>>} price A price, or a range of prices such as B2:B5.
 * @param {number} discountRate The discount as a rate from 0 to 1, such as 10% or 0.1.
 * @return The discounted price, or one discounted price per cell of the range.
 * @customfunction
 */
function DISCOUNTED_PRICE(price, discountRate) {
  const rate = requireRate_(discountRate, 'discountRate');
  return mapValueOrRange_(price, 'price', function (amount) {
    return roundPrice_(amount * (1 - rate));
  });
}

/**
 * Returns a price with sales tax added. Accepts one price or a range of prices.
 *
 * @param {number|Array<Array<*>>} price A price, or a range of prices such as B2:B5.
 * @param {number} taxRate The tax as a rate from 0 to 1, such as 8% or 0.08.
 * @return The price including tax, or one taxed price per cell of the range.
 * @customfunction
 */
function PRICE_WITH_TAX(price, taxRate) {
  const rate = requireRate_(taxRate, 'taxRate');
  return mapValueOrRange_(price, 'price', function (amount) {
    return roundPrice_(amount * (1 + rate));
  });
}

/**
 * Logs sample results so you can check the calculations from the editor.
 *
 * @returns {void}
 */
function testPriceFunctions() {
  Logger.log(DISCOUNTED_PRICE(40, 0.1));
  Logger.log(JSON.stringify(DISCOUNTED_PRICE([[40], [12.5], ['']], 0.1)));
  Logger.log(JSON.stringify(PRICE_WITH_TAX([[36], [11.25]], 0.08)));
}

/**
 * Applies a calculation to one value, or to every cell of a two-dimensional range.
 * Blank cells stay blank so the result has the same shape as the input.
 *
 * @param {*} input One cell value, or a two-dimensional array of cell values.
 * @param {string} argumentName The argument name used in error messages.
 * @param {function(number): number} calculate The calculation for one number.
 * @returns {*} One result, or a two-dimensional array of results.
 */
function mapValueOrRange_(input, argumentName, calculate) {
  if (!Array.isArray(input)) {
    return calculateCell_(input, argumentName, calculate, '');
  }
  const rows = /** @type {Array<Array<*>>} */ (input);
  return rows.map(function (row, rowIndex) {
    return row.map(function (cell, columnIndex) {
      const position = ' (range row ' + (rowIndex + 1) + ', column ' + (columnIndex + 1) + ')';
      return calculateCell_(cell, argumentName, calculate, position);
    });
  });
}

/**
 * Calculates one cell, keeping a blank cell blank and rejecting text or dates.
 *
 * @param {*} value The cell value.
 * @param {string} argumentName The argument name used in error messages.
 * @param {function(number): number} calculate The calculation for one number.
 * @param {string} position Where the value sits in a range, or an empty string.
 * @returns {number|string} The result, or an empty string for a blank cell.
 */
function calculateCell_(value, argumentName, calculate, position) {
  if (value === '' || value === null) {
    return '';
  }
  if (typeof value !== 'number' || !isFinite(value)) {
    throw new Error(argumentName + ' must be a number' + position + '. Received: ' + String(value));
  }
  return calculate(value);
}

/**
 * Checks that a rate is a single number from 0 to 1.
 *
 * @param {*} rate The rate argument from the formula.
 * @param {string} argumentName The argument name used in error messages.
 * @returns {number} The checked rate.
 */
function requireRate_(rate, argumentName) {
  if (typeof rate !== 'number' || !isFinite(rate) || rate < 0 || rate > 1) {
    throw new Error(argumentName + ' must be one number from 0 to 1, such as 10% or 0.1.');
  }
  return rate;
}

/**
 * Rounds a calculated price to PRICE_DECIMALS decimal places.
 *
 * @param {number} amount The unrounded price.
 * @returns {number} The rounded price.
 */
function roundPrice_(amount) {
  const factor = Math.pow(10, PRICE_DECIMALS);
  return Math.round(amount * factor) / factor;
}