export const chainPromises = <T>(
  promises: Promise<T>[],
  reducer = (p1: Promise<T>, p2: Promise<T>) => p1.then(() => p2),
  initialValue?: T
) => promises.reduce(reducer, Promise.resolve(initialValue));

const splitBigNumAt = (num: string, at: number) => num.replace(RegExp(String.raw`^([+-]?)(\d+)(\d{${at}})$`), '$1$2,$1$3')
  .replace(/^([^,]*)$/, '0,$1').split(',')
  .map(Number);

const bigNumPlus = (num1: string, num2: string) => {
  let [high, low] = [splitBigNumAt(num1, 15), splitBigNumAt(num2, 15)]
    .reduce((a, b) => [a[0] + b[0], a[1] + b[1]]);
  const [highSign, lowSign] = [high, low].map(Math.sign);
  if (highSign === 0) return low.toString();
  if (highSign + lowSign === 0) {
    [high, low] = [high - highSign, low - lowSign * 10 ** 15];
  } else {
    [high, low] = [high + Math.trunc(low / 10 ** 15), low % 10 ** 15];
  }
  if (high === 0) return low.toString();
  return `${high}${Math.abs(low).toString().padStart(15, '0')}`;
};

const bigNumCompare = (num1: string, num2: string) => Math.sign(Number(bigNumPlus(
  num1, 
  num2.replace(/^([+-]?)(\d+)/, (_, $1, $2) => `${$1 === '-' ? '' : '-'}${$2}`)
)));

const bigNumMin = (...nums: string[]) => {
  if (!nums || !nums.length) return undefined;
  let min = nums[0];
  for (let i = 1; i < nums.length; i++) {
    if (bigNumCompare(nums[i], min) < 0) min = nums[i];
  }
  return min;
};

const bigNumLShift = (num: string, by: number) => {
  if (by < 0) throw Error('cannot perform right shift');
  const at = Math.trunc((52 - by) / 10) * 3;
  const [high, low] = splitBigNumAt(num, at).map(n => n * 2 ** by);
  return bigNumPlus(`${high}${'0'.repeat(at)}`, low.toString());
};

const parseBigNum = (str: string) => (/^-?\d+$/.exec(str) || [''])[0].replace(/^(-)?0*/, '$1');

export const BigNumOps = {
  splitAt: splitBigNumAt,
  plus: bigNumPlus,
  compare: bigNumCompare,
  min: bigNumMin,
  lShift: bigNumLShift,
  parse: parseBigNum,
};