const input = "0,8,15,2,12,1,4";
// const input = "3,1,2";

const numberAt2020 = 30000000;

// const numbers = input.split(',').map(parseInt); // does not work
const numbers = input.split(',').map(a => parseInt(a));
const numberAge = new Map<number, number>();
numbers.forEach((e, i) => numberAge.set(e, i));

let currentNumber = 0; // initial numbers are unique
let nextNumber: number;
let count = numbers.length;

do {
    if (numberAge.has(currentNumber)) {
        // @ts-ignore
        nextNumber = count - numberAge.get(currentNumber);
        numberAge.set(currentNumber, count);
    } else {
        nextNumber = 0;
        numberAge.set(currentNumber, count);
    }
    count += 1;
    if (count < numberAt2020) {
        currentNumber = nextNumber;
    }
} while (count < numberAt2020);

const endTime = new Date();

console.log(currentNumber);