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const input = '818275977931166178424892653779931342156567268946849597948944469863818248114327524824136924486891794739281668741616818614613222585132742386168687517939432911753846817997473555693821316918473474459788714917665794336753628836231159578734813485687247273288926216976992516314415836985611354682821892793983922755395577592859959966574329787693934242233159947846757279523939217844194346599494858459582798326799512571365294673978955928416955127211624234143497546729348687844317864243859238665326784414349618985832259224761857371389133635711819476969854584123589566163491796442167815899539788237118339218699137497532932492226948892362554937381497389469981346971998271644362944839883953967698665427314592438958181697639594631142991156327257413186621923369632466918836951277519421695264986942261781256412377711245825379412978876134267384793694756732246799739464721215446477972737883445615664755923441441781128933369585655925615257548499628878242122434979197969569971961379367756499884537433839217835728263798431874654317137955175565253555735968376115749641527957935691487965161211853476747758982854811367422656321836839326818976668191525884763294465366151349347633968321457954152621175837754723675485348339261288195865348545793575843874731785852718281311481217515834822185477982342271937155479432673815629144664144538221768992733498856934255518875381672342521819499939835919827166318715849161715775427981485233467222586764392783699273452228728667175488552924399518855743923659815483988899924199449721321589476864161778841352853573584489497263216627369841455165476954483715112127465311353411346132671561568444626828453687183385215975319858714144975174516356117245993696521941589168394574287785233685284294357548156487538175462176268162852746996633977948755296869616778577327951858348313582783675149343562362974553976147259225311183729415381527435926224781181987111454447371894645359797229493458443522549386769845742557644349554641538488252581267341635761715674381775778868374988451463624332123361576518411234438681171864923916896987836734129295354684962897616358722633724198278552339794629939574841672355699222747886785616814449297817352118452284785694551841431869545321438468118'; | |
const output = input.split('') // turn input string into an array | |
// turn it all into actual numbers (it's still strings so far) | |
.map(n=>parseInt(n,10)) | |
// add the first number again at the end to effect circular..ness? circularitude? ..ity? | |
.concat(n[0]) | |
// now reduce everything by summing up numbers that are the same as their successor | |
// if the current number `n` is the last one in the array, then `a[i+1]` will be `undefined`, | |
// which is fine. It just means, summing another `0`. Yay Javascript! | |
.reduce((result, n, i, a) => (n === a[i+1]) ? res + n : res, 0); | |
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