const input = "9,3,1,0,8,4";

const start = input.split(",");
const memory = {};

let turn = 1;
let lastNum = null;

for (const num of start) {
  if (lastNum !== null) {
    memory[lastNum] = turn - 1;
  }
  lastNum = num;
  console.log(`Turn ${turn}: ${num}`);
  turn += 1;
}

let num;
while (turn <= 2020) {
  if (typeof memory[lastNum] === "undefined") {
    num = 0;
  } else {
    num = turn - 1 - memory[lastNum];
  }

  console.log(
    `Turn ${turn}: ${num}${
      num !== 0 ? ` (${turn - 1} - ${memory[lastNum]})` : ""
    }`
  );
  memory[lastNum] = turn - 1;
  lastNum = num;
  turn += 1;
}

console.log("Solution:", num);