Problem Details located: http://www.fogcreek.com/jobs/SupportEngineer

FogCreek Application Solution

"use strict"

const buildLetterMap = (str) => {
  let letterMap = {};

  str.split("").forEach(function(letter) {
    if(letterMap[letter]) {
      letterMap[letter] = letterMap[letter] + 1;
    } else {
      letterMap[letter] = 1;
    }
  });
  
  return letterMap;
};

const descendingSort = (a, b) => (b[1] - a[1]);

const sortLettersByCount = (letterMap) => {
  let sortedLettersByCount = [];
  
  for (var letter in letterMap) {
    sortedLettersByCount.push([letter, letterMap[letter]]);
  }
   
  return sortedLettersByCount
    .sort(descendingSort)
    .map(function (letterPair) {
      return letterPair[0];
    })
    .join("");
};

const uncoverMe = (text, dropCharacter) => {
  let letterMap = buildLetterMap(text);
  let sortedLettersByCount = sortLettersByCount(letterMap);
  const underscoreIndex = sortedLettersByCount.indexOf(dropCharacter);
  
  return sortedLettersByCount.slice(0, underscoreIndex);
};

//Call the function with provided inputs
const str = "uwlcnfcejefjudkmylhmmpjfkmhveozojepfvgboboxknkfdoghkvzxrkamhaghahpnnzh_figzhelkapd_dnxsiurbjgq_bcctqgigfksdrzgtxvihiqvfvinuqfzgohvilwyxpuqjolleuecpxcbisaptnoxbglpkhsvvwdgaminprfrha_uemwdepnngtxgkqx_flerkbnnzazcvnh_sealovrpuafumrmaizidjedvovhtbadjelfrbknpioqiindxqiozzpdrv_bwooblumjc_oqtcgrfmxkwkczhhqiwuqzoznqgfmbdxrxigvkebypbsybanuccpsezgv_dvteybbrbrbldbpiyhgzkptloxfktrzkjmdhppucsaohfnehdnlnhexwj_yhulslwpdreogjsffy_fkqcdkobywhqxpjvv_sqbscknxrew_ivgvpolmqfkyxoxzuyqctriyxcncwvvqhdupkagguee_zfbcrexbtkcsbnjcfzqoq_wrdktpsclzrmeybxpzrylfdydknd_zbjjqcnubpjbfaaecuxbeprjjjfcybvcghfbcvnjicpliwzkqvwo_hloynyjrmiqvubsyobetklnsgovesswqatrcrirsywdvbpwnhtjaug_nglxamsybkop_gnkdvgzp_cmyxclrtjtoohniuszzbnakknd_ahe_enbouxvpueotcjebex_vpqbwyjgczobcirmgfvwnlrxaryoltdlozwdgcp__iwu_vkod_kzkmeklloeixrxymlvyvtlfyydwtzxugrhxwqgmmsvtshrublypevlpglldlumsbscjuv_cbtfjpenrervyxtbrxaaqsqs__boiiubqmgwtvzxlnxxyskbwquztepk_uggukayehrifcrdpcnrfhmlucqzbsvoojsfexvbzsrccyqjuufbiae_siovpbhhqzkcemm_wtzio_d_emtkxpkpqftqjrhiiuvyijtwrmj_r_nlsaqemp_yph_murlmwwibzxnerld_fracudoxwqsmjkdvbfbnmkvktrkyyoct_woymzxswtd_dvs_joviugjjvswlltkforlv_xdokzbqkzrojvevuuzkpcvvaarca_oeryyuasilqpwrzkyrsbskjnywidkcefjdekawqutrihusqtq_bjaaysidwvpmhozxcsacpehceplbihgnuaucvatj_prb_fflzfyyrndrxq__ptxkcbhhslnuuwljwxbxzbujntztebr_knnffhl_cvxowdtpxldvsonhnfxxnmwopu_tgosofhhjhmxkdnvorwlsxy_cleasbnonadrgxf_bzipudfonthlclvnhumrbjkdryoawoz__ttftltixhdhd_qoteqmyehlulpffaqmughpxxsyeevncin_ymjsnndirfg_jjcltmgphlpudrjviaixlxnpwvr_bqygvuhoskhtv_pqnyzmjymrpnphebxqfpkfhzirotxayvclcr_zcrtsakfusyyujzdjgizngmonogwknjcndlpvktgntgajmzwcnkhggmhzijyrpbkpdcribyteyqchlldzeelebqjplphcdgpaaatdelz_khxpxqclexvlcjiyssgsgmsddclgmsae_eubvmscpjwascwegqdxvvh_zuteauydayapfamxilyugubce_kibquetsvlmguecrm_uojvkjammyevgxwfcfq_fjnauajadhsjvtsduzpmqecxfuqaaq__qhtvl_uurqaquuygypqxnhrnhhiwrufntlunfqwaydimaahiihvcaycinidodqzmrnxnnxdsquwc_u_zrrkscjv_jwsp_xfkupx_rcbzovpsbwyiumeaanyiaqromomqldfprejqnytjftdcklykzsdupl_makofxndacjbbftywdxmvmfv_wogpggfeivpfiqqlzcweelzclxkgqlvysuignftxqooofgvnopvtjy_udsf_wx_wloyznfixdeyxbvqljuncfmtvjhrrrpcxowggllwcmzlp_bgwpepdntxzjbqprgdtpdbtofhwknqdarbqvryfaiq_lneqwlwpjpfxbmttidphupuhwghgafybhtulwkgpoavwbtixuvqroknoas_pvufqfanwdvzraqpxudodpifa_s_xiaef_abeawgaamorlogpmmavrwbt_bzthsnzaxzitbyuohtqswnqekujrojerffenhkna_nyioesdgaeofpuoyoybuweuswzaraanyzkaxuncumlsnzoavkmjoejygohgtrqtdouiubigjhrutk";

uncoverMe(str, '_')