adventofcode_day3.js

const sum = (array) => array.reduce((prev, curr) => prev + curr);

const intersect = (setA, setB) => new Set(Array.from(setA).filter(i => setB.has(i)));

const charsToSet = (chars) => new Set(chars.split(''));

// Array.flat
const flatten = (array, ret) => {
    ret = ret || [];
    return array.reduce((ret, entry) => {
        if (entry instanceof Set) {
            entry = Array.from(entry);
        }
        if (Array.isArray(entry)) {
            flatten(entry, ret);
        } else {
            ret.push(entry);
        }
        return ret;
    }, ret);
}

const isUpperCase = (char) => {
    return char.toUpperCase() === char;
}

const rucksackToSets = (inputString) => {
    return inputString
        .split('\n')
        .map(line => {
            const halfLen = line.length/2;
            return {
                all: charsToSet(line),
                first: charsToSet(line.slice(0,halfLen)),
                second: charsToSet(line.slice(halfLen))
            }
        })
};

const getScore = (chars) => {
    return sum(chars.map(char => {
        return char.charCodeAt(0) - (isUpperCase(char) ?  38 : 96);
    }));
}

const findBadges = (rucksacks) => {
    const badges = [];
    for (let i=0; i < rucksacks.length;i=i+3) {
        let elfGroup = rucksacks.slice(i, i+3);
        let badge = intersect(
            intersect(
                elfGroup[0].all, 
                elfGroup[1].all
            ),
            elfGroup[2].all
        );
        badges.push(Array.from(badge)[0])
    }
    return badges;
}

const mainDayThreeA = (inputString) => {
    const rucksacks = rucksackToSets(inputString);
    const duplicatesPerElf = rucksacks.map(elf => {
        return intersect(elf.first, elf.second);
    });
    return getScore(flatten(duplicatesPerElf));
}

const mainDayThreeB = (inputString) => {
    const rucksacks = rucksackToSets(inputString);
    const badges = findBadges(rucksacks);
    return getScore(badges);
}