text-number.ts

const units = [
  'zero',
  'one',
  'two',
  'three',
  'four',
  'five',
  'six',
  'seven',
  'eight',
  'nine',
  'ten',
  'eleven',
  'twelve',
  'thirteen',
  'fourteen',
  'fifteen',
  'sixteen',
  'seventeen',
  'eighteen',
  'nineteen',
] as const;

const tens = [
  '',
  'ten',
  'twenty',
  'thirty',
  'fourty',
  'fifty',
  'sixty',
  'seventy',
  'eighty',
  'ninety',
] as const;

const exponents = [
  '',
  'thousand',
  'million',
  'billion',
  'trillion',
  'quadrillion',
  'quintillion',
  'sextillion',
  'septillion',
  'octillion',
  'nonillion',
  'decillion',
  'undecillion',
  'duodecillion',
  'tredecillion',
  'quattuordecillion',
  'quindecillion',
  'sexdecillion',
  'septendecillion',
  'octodecillion',
  'novemdecillion',
  'vigintillion',
] as const;

type NumberGroup = [number, number, number] | [number, number] | number[];

function renderNumberGroup(
  numberGroup: NumberGroup,
  exponent: number,
  exponentCount: number,
) {
  const unit = numberGroup.at(-1);
  const ten = numberGroup.at(-2) || 0;
  const hundred = numberGroup.at(-3) || 0;
  const unitAndTen = +`${ten}${unit}`;

  return `${
    hundred ? `${units[hundred]} hundred${unitAndTen ? ' and ' : ''}` : ''
  }${
    unitAndTen && unitAndTen < 20
      ? units[unitAndTen]
      : `${ten ? `${tens[ten]} ` : ''}${
          unit ? units[unit] : ten || exponentCount > 1 ? '' : units[0]
        }`
  }${exponent ? ` ${exponents[exponent]},` : ''}`;
}

function buildTextNumber(number: number | bigint | `${number}`) {
  const numbers = String(number).split('');
  const numberGroups: NumberGroup[] = [];

  numbers.reverse();
  numbers.forEach((number) => {
    const lastGroup = numberGroups.at(-1);
    if (!lastGroup || lastGroup.length === 3) {
      numberGroups.push([+number]);
      return;
    }

    lastGroup.push(+number);
  });
  numberGroups.forEach((group) => group.reverse());

  const namedNumberParts = numberGroups.map(
    (numberGroup, index, numberGroups) => {
      return renderNumberGroup(numberGroup, index, numberGroups.length);
    },
  );
  namedNumberParts.reverse();

  return namedNumberParts.join(' ');
}

export default buildTextNumber;