Solution to Advent of Code 2020 Day 15 in JavaScript

AOC_2020_Day15.js

const input = [ 6, 4, 12, 1, 20, 0, 16 ];

function part1(debug) {
    const memory = Array.from(input);

    for (let i = input.length; i < 2020; i++) {
        const lastNumber = memory[memory.length - 1];
        const lastNumberIndex = memory.lastIndexOf(lastNumber, memory.length - 1);
        const lastNumberLastIndex = memory.lastIndexOf(lastNumber, lastNumberIndex - 1);

        if (lastNumberLastIndex >= 0) {
            // number has been seen before
            memory.push(lastNumberIndex - lastNumberLastIndex);

        } else {
            // number has never been seen before
            memory.push(0);

        }

        if (debug !== undefined) {
            console.log(`Iteration ${ i + 1 - input.length }: index=${ i } this=${ memory[i] } last=${ lastNumber } index(last)=${ lastNumberIndex } index-1(last)=${ lastNumberLastIndex }`);
        }
    }

    return memory[memory.length - 1];
}

function part2(debug) {
    /** @type {Map<number, number>} */
    const memory = new Map();
    input.forEach((n, i) => memory.set(n, i));

    let lastNumber = input[input.length - 1];
    let lastNumberIndex = input.length - 1;
    let lastNumberLastIndex = undefined;

    for (let i = input.length; i < 30000000; i++) {
        let thisNumber;
        if (lastNumberLastIndex !== undefined) {
            // number has been seen before
            thisNumber = lastNumberIndex - lastNumberLastIndex;

        } else {
            // number has never been seen before
            thisNumber = 0;

        }

        if (debug !== undefined) {
            console.log(`Iteration ${ i + 1 - input.length }: index=${ i } this=${ memory[i] } last=${ lastNumber } index(last)=${ lastNumberIndex } index-1(last)=${ lastNumberLastIndex }`);
        }

        lastNumber = thisNumber;
        lastNumberIndex = i;
        lastNumberLastIndex = memory.get(thisNumber);

        memory.set(thisNumber, i);
    }

    return lastNumber;
}

const part1answer = part1();
console.log(`Part 1: The 2020th number spoken is ${ part1answer }`);

const part2answer = part2();
console.log(`Part 2: The 30000000th number spoken is ${ part2answer }`);