123_math_black_hole.js

// http://www.pleacher.com/handley/puzzles/blackhol.html

const randBI = () => {
  const maxLen = 10_000;
	return BigInt(new Array(Math.round(Math.random() * maxLen))
  	.fill(0)
  	.map(_ => Math.round(Math.random() * 10) % 10)
  	.join(''))
};

const test123 = () => {
  let bi = randBI();
  let last;
  while (bi !== last) {
    const arr = [...bi + ''];
    const even = arr.reduce((cnt, n) => {
      return cnt + (+n % 2 === 0 ? 1 : 0);
    }, 0);
    const odd = arr.length - even;
    last = bi
    bi = BigInt([even, odd, arr.length].join(''));
  }
  return bi;
}

const map = new Map();

for (let i = 0; i < 1e6; ++i) {
  const bi = test123();
  const cnt = map.get(bi) || 0;
  map.set(bi, cnt + 1);
  
  if (i % 1e3 === 0) {
    console.log(i, map);
  }
}