numfmt/lib/dec2frac.js

Full Excel style number formatting

// https://web.archive.org/web/20110813042636/http://homepage.smc.edu/kennedy_john/DEC2FRAC.PDF
const PRECISION = 1e-13;

/**
 * Split a fractional number into a numerator and denominator for display as
 * vulgar fractions.
 *
 * @ignore
 * @param {number} number The value to split
 * @param {number} [numeratorMaxDigits=2] The maxdigits number
 * @param {number} [denominatorMaxDigits=2] The maxdigits de
 * @returns {Array<number>} Array of two numbers, numerator and denominator.
 */
export function dec2frac (number, numeratorMaxDigits = 2, denominatorMaxDigits = 2) {
  const sign = (number < 0) ? -1 : 1;
  const maxdigits_n = 10 ** (numeratorMaxDigits || 2);
  const maxdigits_d = 10 ** (denominatorMaxDigits || 2);
  let z = Math.abs(number);
  let last_d = 0;
  let last_n = 0;
  let curr_n = 0;
  let curr_d = 1;
  let tmp;
  let r;
  number = z;
  if (number % 1 === 0) {
    // handles exact integers including 0
    r = [ number * sign, 1 ];
  }
  else if (number < 1e-19) {
    r = [ sign, 1e+19 ];
  }
  else if (number > 1e+19) {
    r = [ 1e+19 * sign, 1 ];
  }
  else {
    do {
      z = 1 / (z - Math.floor(z));
      tmp = curr_d;
      curr_d = (curr_d * Math.floor(z)) + last_d;
      last_d = tmp;
      last_n = curr_n;
      curr_n = Math.floor(number * curr_d + 0.5); // round
      if (curr_n >= maxdigits_n || curr_d >= maxdigits_d) {
        return [ sign * last_n, last_d ];
      }
    }
    while (Math.abs(number - (curr_n / curr_d)) >= PRECISION && z !== Math.floor(z));
    r = [ sign * curr_n, curr_d ];
  }
  return r;
}