Discussion on: AoC Day 18: Settlers of The North Pole

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JavaScript solution

I'm gonna omit reader.js which is the same as the other solutions and jump to the point:

18-common.js

const MAP = {
    OPEN_GROUND: '.',
    TREES: '|',
    LUMBERYARD: '#'
};

const tick = originalMap => {
    const n = originalMap.length;
    const nextMap = Array.from({length: n}, row => Array.from({length: n}));

    for (let i = 0; i < n; i++) {
        for (let j = 0; j < n; j++) {
            const originalAcre = originalMap[i][j];
            const adjacents = getAdjacents(originalMap, i, j, n);

            // For an open ground acre
            if (originalAcre === MAP.OPEN_GROUND) {
                const adjacentTrees = adjacents.filter(acre => acre === MAP.TREES).length;
                nextMap[i][j] = adjacentTrees >= 3 ? MAP.TREES : MAP.OPEN_GROUND;
            }
            // For a trees acre
            else if (originalAcre === MAP.TREES) {
                const adjacentLumberyards = adjacents.filter(acre => acre === MAP.LUMBERYARD).length;
                nextMap[i][j] = adjacentLumberyards >= 3 ? MAP.LUMBERYARD : MAP.TREES;
            }
            // For a lumberyard acre
            else if (originalAcre === MAP.LUMBERYARD) {
                const adjacentLumberyards = adjacents.filter(acre => acre === MAP.LUMBERYARD).length;
                const adjacentTrees = adjacents.filter(acre => acre === MAP.TREES).length;
                nextMap[i][j] = adjacentLumberyards >= 1 && adjacentTrees >= 1 ? MAP.LUMBERYARD : MAP.OPEN_GROUND;
            }
        }
    }

    return nextMap;
};

const getAdjacents = (originalMap, i, j, n) => {
    const positions = [[i-1, j-1], [i-1, j], [i-1, j+1], [i, j-1], [i, j+1], [i+1, j-1], [i+1, j], [i+1, j+1]];

    return positions
        .filter(([i, j]) => i >= 0 && j >= 0 && i < n && j < n)
        .map(([i, j]) => originalMap[i][j])
        .filter(acre => acre !== undefined);
};

const serialize = map => map.map(row => row.join('')).join('');

const printSolution = solution => {
    const serializedMap = (Array.isArray(solution) ? serialize(solution) : solution).split('');
    const trees = count(serializedMap, MAP.TREES);
    const lumberyards = count(serializedMap, MAP.LUMBERYARD);
    console.log(`The total resource value of the lumber collection area is ${trees * lumberyards}`);
};

const count = (map, type) => {
    return map.reduce((total, acre) => total + (acre === type ? 1 : 0), 0);
};

module.exports = {
    MAP,
    tick,
    serialize,
    printSolution
};

18a.js

const { readFile } = require('./reader');

const {
    tick,
    printSolution
} = require('./18-common');

(async () => {
    let outskirts = (await readFile('18-input.txt')).map(row => row.split(''));

    for (let i = 0; i < 10; i++) {
        outskirts = tick(outskirts);
    }

    console.log(outskirts.map(row => row.join('')).join('\n'));

    printSolution(outskirts);
})();

18b.js

const { readFile } = require('./reader');

const {
    MAP,
    tick,
    serialize,
    printSolution
} = require('./18-common');

(async () => {
    let outskirts = (await readFile('18-input.txt')).map(row => row.split(''));

    const previousStates = new Map();
    let elapsedMinutes = 0;
    let serialized;
    let hasDetectedLoop = false;
    do {
        elapsedMinutes++;
        outskirts = tick(outskirts);

        serialized = serialize(outskirts);
        if (previousStates.has(serialized)) {
            hasDetectedLoop = true;
        }
        else {
            previousStates.set(serialized, elapsedMinutes);
        }
    } while (!hasDetectedLoop);

    console.log(`Loop detected at minute ${elapsedMinutes}!`);

    const firstRepetitionMinutes = previousStates.get(serialized);
    const loopDurationMinutes = elapsedMinutes - firstRepetitionMinutes;
    const equivalentMinute = ((1000000000 - firstRepetitionMinutes) % loopDurationMinutes) + firstRepetitionMinutes;

    console.log(`The minute 1000000000 is equivalent to the minute ${equivalentMinute}`);

    const solution = [...previousStates.entries()].find(([state, minute]) => minute === equivalentMinute)[0];

    printSolution(solution);
})();