1-captcha.js

/**
 * The captcha requires you to review a sequence of digits (your puzzle input) and find the sum of all digits that match
 * the next digit in the list. The list is circular, so the digit after the last digit is the first digit in the list.
 * 
 * For example:
 * 1122 produces a sum of 3 (1 + 2) because the 1st digit (1) matches the 2nd and the 3rd digit (2) matches the fourth.
 * 1111 produces 4 because each digit (all 1) matches the next.
 * 1234 produces 0 because no digit matches the next.
 * 91212129 produces 9 because the only digit that matches the next one is the last digit, 9.
 * 
 * --- Part Two ---
 * Now, instead of considering the next digit, it wants you to consider the digit halfway around the circular list.
 * That is, if your list contains 10 items, only include a digit in your sum if the digit 10/2 = 5 steps forward matches
 * it. Fortunately, your list has an even number of elements.
 * 
 * For example:
 * 1212 produces 6: the list contains 4 items, and all four digits match the digit 2 items ahead.
 * 1221 produces 0, because every comparison is between a 1 and a 2.
 * 123425 produces 4, because both 2s match each other, but no other digit has a match.
 * 123123 produces 12.
 * 12131415 produces 4.
 */

const input = '7385764686251444473997915123782972536343732657517834671759462795461213782428342931896181695578996274321317419242359534783957372932953774336338118488967172727651862498838195317654289797558683458511126996217953322817229372373455862177844478443391835484591525235651863464891177927244954925827786799436536592561374269299474738321293575385899438446558569241236278779779983587912431395475244796538888373287186921647426866237756737342731976763959499149996315591584716122199183295277439872911371313924594486766479438544417416529743495114819825984524437367225234184772617942525954961136976875325182725754768372684531972614455134523596338355374444273522115362238734383164778129376628621497662965456761631796178353599629887665939521892447361219479646483978798392716119793282717739524897385958273726776318154977675546287789874265339688753977185129334929715486381875286278528247696464162297691698154712775589541945263574897266575996455547625537947927972497979333932115165151462742216327321116291372396585618664475715321298122335789262942284571328414569375464386446824882551918843185195829547373915482687534432942778312542752798313434628498295216692646713137244198123219531693559848915834623825919191532658735422176965451741869666714874158492556445954852299161868651448123825821775363219246244515946392686275545561989355573946924767442253465342753995764791927951158771231944177692469531494559697911176613943396258141822244578457498361352381518166587583342233816989329544415621127397996723997397219676486966684729653763525768655324443991129862129181215339947555257279592921258246646215764736698583211625887436176149251356452358211458343439374688341116529726972434697324734525114192229641464227986582845477741747787673588848439713619326889624326944553386782821633538775371915973899959295232927996742218926514374168947582441892731462993481877277714436887597223871881149693228928442427611664655772333471893735932419937832937953495929514837663883938416644387342825836673733778119481514427512453357628396666791547531814844176342696362416842993761919369994779897357348334197721735231299249116477';

const sequenceAdder = (inputSequence, stepsForward) => {
	let toAdd = [];

	for (digit of inputSequence) {
		if (digit === inputSequence[stepsForward]) {
			toAdd.push(parseInt(digit, 10));
		}

		stepsForward++;

		if (stepsForward > inputSequence.length - 1) {
			stepsForward = 0;
		}
	}

	if (toAdd.length === 0) {
		return 0;
	}

	return toAdd.reduce((a, b) => a + b);
};

// part 1
console.log(sequenceAdder(input, 1));
// part 2
console.log(sequenceAdder(input, input.length / 2));

module.exports = sequenceAdder;