14.ts

let recipeScores: number[], i1: number, i2: number, serialized: string;

function reset() {
  recipeScores = [3, 7];
  i1 = 0;
  i2 = 1;
  serialized = '37';
}

function updateScores() {
  const sum = recipeScores[i1] + recipeScores[i2];
  const sumStr = String(sum);
  sumStr
    .split('')
    .map(Number)
    .forEach(newRecipeScore => {
      recipeScores.push(newRecipeScore);
      serialized += String(newRecipeScore);
    });

  i1 += 1 + recipeScores[i1];
  i1 = i1 % recipeScores.length;
  i2 += 1 + recipeScores[i2];
  i2 = i2 % recipeScores.length;
}

reset();
const after = 890691;

while (recipeScores.length < after + 10 + 1) {
  updateScores();
}

console.log(recipeScores.slice(after, after + 10).join(''));

// Part 2
const search = '890691';
console.log(serialized.indexOf(search));
reset();

while (!serialized.includes(search)) {
  // Above check is expensive, do 1,000,000 updates before checking each time
  for(let i = 0; i < 1000000; ++i) {
    updateScores();
  }
  console.log(recipeScores.length, serialized.indexOf(search));
}

console.log(serialized.indexOf(search));