AoC - Day 1:

inverseArray.js

const captcha = "29917128875332952564321392569634257121244516819997569284938677239676779378822158323549832814412597817651244117851771257438674567254146559419528411463781241159837576747416543451994579655175322397355255587935456185669334559882554936642122347526466965746273596321419312386992922582836979771421518356285534285825212798113159911272923448284681544657616654285632235958355867722479252256292311384799669645293812691169936746744856227797779513997329663235176153745581296191298956836998758194274865327383988992499115472925731787228592624911829221985925935268785757854569131538763133427434848767475989173579655375125972435359317237712667658828722623837448758528395981635746922144957695238318954845799697142491972626942976788997427135797297649149849739186827185775786254552866371729489943881272817466129271912247236569141713377483469323737384967871876982476485658337183881519295728697121462266226452265259877781881868585356333494916519693683238733823362353424927852348119426673294798416314637799636344448941782774113142925315947664869341363354235389597893211532745789957591898692253157726576488811769461354938575527273474399545366389515353657644736458182565245181653996192644851687269744491856672563885457872883368415631469696994757636288575816146927747179133188841148212825453859269643736199836818121559198563122442483528316837885842696283932779475955796132242682934853291737434482287486978566652161245555856779844813283979453489221189332412315117573259531352875384444264457373153263878999332444178577127433891164266387721116357278222665798584824336957648454426665495982221179382794158366894875864761266695773155813823291684611617853255857774422185987921219618596814446229556938354417164971795294741898631698578989231245376826359179266783767935932788845143542293569863998773276365886375624694329228686284863341465994571635379257258559894197638117333711626435669415976255967412994139131385751822134927578932521461677534945328228131973291962134523589491173343648964449149716696761218423314765168285342711137126239639867897341514131244859826663281981251614843274762372382114258543828157464392";

// PART 1:
// const captchaSplit = captcha.split('');
// var result = 0;
// const indexZero = parseInt(captchaSplit[0]);
// for(var i = 0; i < captchaSplit.length - 1; i++) {
// 	if(parseInt(captchaSplit[i]) === parseInt(captchaSplit[i + 1])) result += parseInt(captchaSplit[i]);
// }
// if(indexZero === parseInt(captchaSplit[captchaSplit.length - 1])) result += indexZero;
// console.log(result);


// PART 2:
const captchaSplit = captcha.split('');
var result = 0;
/* Notice that you're essentially adding the first half of the array to the second half
	of the array, if and only if, the indexs of the 1st and 2nd half have matching
	values. Also, you need to double the final sum, since the direction is transitive,
	meaning that if you tranverse forward and get 2 matches on the first half, then you
	will also get 2 more matches on the last half transversal, which gives you 4 matches,
	or double the first set of matches. 
*/

// Find the half index
var halfIndex = (captcha.length / 2);
const firstHalf = captchaSplit.slice(0, halfIndex); // do not want halfIndex included
const secondHalf = captchaSplit.slice(halfIndex);
for(var i = 0; i < firstHalf.length; i++)
	if(parseInt(firstHalf[i]) === parseInt(secondHalf[i])) result += (parseInt(firstHalf[i]) * 2);
console.log(result);