AI class NLP problem 1 solution

rot.rb

#!/usr/bin/env ruby

# Data is taken from Wikipedia: http://j.mp/english-letters-frequencies
LETTERS_FREQUENCIES = {
  'a' => 8.167,
  'b' => 1.492,
  'c' => 2.782,
  'd' => 4.253,
  'e' => 12.702,
  'f' => 2.228,
  'g' => 2.015,
  'h' => 6.094,
  'i' => 6.966,
  'j' => 0.153,
  'k' => 0.772,
  'l' => 4.025,
  'm' => 2.406,
  'n' => 6.749,
  'o' => 7.507,
  'p' => 1.929,
  'q' => 0.095,
  'r' => 5.987,
  's' => 6.327,
  't' => 9.056,
  'u' => 2.758,
  'v' => 0.978,
  'w' => 2.360,
  'x' => 0.150,
  'y' => 1.974,
  'z' => 0.074
}

TEXT = "Esp qtcde nzyqpcpynp zy esp ezatn zq Lcetqtntlw Tyepwwtrpynp hld spwo le Olcexzfes Nzwwprp ty estd jplc." 
LETTERS_COUNT = TEXT.gsub(/[^a-zA-Z]/, '').size

alphs = lambda { |alph| alph.join + alph.join.upcase }

ALPH = ('a'..'z').to_a
ALPHS = alphs.call ALPH

rot = ALPH.dup
results = []

begin
  rot.push rot.shift
  rots = alphs.call rot

  original = TEXT.tr rots, ALPHS

  frequencies = ALPH.inject({}) { |m, letter| m[letter] = original.downcase.count(letter); m }
  
  diff = ALPH.inject(0) do |total, letter|
    total + (LETTERS_FREQUENCIES[letter] - 100.0 * frequencies[letter] / LETTERS_COUNT).abs
  end

  results << { original: original, diff: diff }
end until rot == ALPH

results.sort_by{ |r| r[:diff] }.each do |r|
  puts "# %06.2f - '#{r[:original]}'" % r[:diff]
end

### RESULTS ###

# 033.39 - 'The first conference on the topic of Artificial Intelligence was held at Dartmouth College in this year.'
# 083.83 - 'Esp qtcde nzyqpcpynp zy esp ezatn zq Lcetqtntlw Tyepwwtrpynp hld spwo le Olcexzfes Nzwwprp ty estd jplc.'
# 086.74 - 'Iwt uxghi rdcutgtcrt dc iwt idexr du Pgixuxrxpa Xcitaaxvtcrt lph wtas pi Spgibdjiw Rdaatvt xc iwxh ntpg.'
# 092.40 - 'Pda benop ykjbanajya kj pda pkley kb Wnpebeyewh Ejpahhecajya swo dahz wp Zwnpikqpd Ykhhaca ej pdeo uawn.'
# 094.34 - 'Znk loxyz iutlkxktik ut znk zuvoi ul Gxzoloiogr Otzkrromktik cgy nkrj gz Jgxzsuazn Iurrkmk ot znoy ekgx.'
# 101.10 - 'Aol mpyza jvumlylujl vu aol avwpj vm Hyapmpjphs Pualsspnlujl dhz olsk ha Khyatvbao Jvsslnl pu aopz flhy.'
# 101.16 - 'Gur svefg pbasrerapr ba gur gbcvp bs Negvsvpvny Vagryyvtrapr jnf uryq ng Qnegzbhgu Pbyyrtr va guvf lrne.'
# 101.70 - 'Ftq rudef oazrqdqzoq az ftq fabuo ar Mdfuruoumx Uzfqxxusqzoq ime tqxp mf Pmdfyagft Oaxxqsq uz ftue kqmd.'
# 104.09 - 'Xli jmvwx gsrjivirgi sr xli xstmg sj Evxmjmgmep Mrxippmkirgi aew liph ex Hevxqsyxl Gsppiki mr xlmw ciev.'
# 105.15 - 'Sgd ehqrs bnmedqdmbd nm sgd snohb ne Zqshehbhzk Hmsdkkhfdmbd vzr gdkc zs Czqslntsg Bnkkdfd hm sghr xdzq.'
# 106.02 - 'Max ybklm vhgyxkxgvx hg max mhibv hy Tkmbybvbte Bgmxeebzxgvx ptl axew tm Wtkmfhnma Vheexzx bg mabl rxtk.'
# 106.23 - 'Wkh iluvw frqihuhqfh rq wkh wrslf ri Duwlilfldo Lqwhooljhqfh zdv khog dw Gduwprxwk Froohjh lq wklv bhdu.'
# 106.60 - 'Cqn orabc lxwonanwln xw cqn cxyrl xo Jacrorlrju Rwcnuurpnwln fjb qnum jc Mjacvxdcq Lxuunpn rw cqrb hnja.'
# 107.76 - 'Uif gjstu dpogfsfodf po uif upqjd pg Bsujgjdjbm Joufmmjhfodf xbt ifme bu Ebsunpvui Dpmmfhf jo uijt zfbs.'
# 108.03 - 'Nby zclmn wihzylyhwy ih nby nijcw iz Ulnczcwcuf Chnyffcayhwy qum byfx un Xulngionb Wiffyay ch nbcm syul.'
# 108.19 - 'Hvs twfgh qcbtsfsbqs cb hvs hcdwq ct Ofhwtwqwoz Wbhszzwusbqs kog vszr oh Rofhacihv Qczzsus wb hvwg msof.'
# 108.51 - 'Vjg hktuv eqphgtgpeg qp vjg vqrke qh Ctvkhkekcn Kpvgnnkigpeg ycu jgnf cv Fctvoqwvj Eqnngig kp vjku agct.'
# 109.05 - 'Ymj knwxy htskjwjshj ts ymj ytunh tk Fwynknhnfq Nsyjqqnljshj bfx mjqi fy Ifwyrtzym Htqqjlj ns ymnx djfw.'
# 109.91 - 'Kyv wzijk tfewvivetv fe kyv kfgzt fw Rikzwztzrc Zekvcczxvetv nrj yvcu rk Urikdflky Tfccvxv ze kyzj pvri.'
# 111.52 - 'Jxu vyhij sedvuhudsu ed jxu jefys ev Qhjyvysyqb Ydjubbywudsu mqi xubt qj Tqhjcekjx Sebbuwu yd jxyi ouqh.'
# 111.81 - 'Ocz admno xjiazmzixz ji ocz ojkdx ja Vmodadxdvg Diozggdbzixz rvn czgy vo Yvmohjpoc Xjggzbz di ocdn tzvm.'
# 112.19 - 'Rfc dgpqr amldcpclac ml rfc rmnga md Yprgdgagyj Glrcjjgeclac uyq fcjb yr Byprkmsrf Amjjcec gl rfgq wcyp.'
# 113.23 - 'Dro psbcd myxpoboxmo yx dro dyzsm yp Kbdspsmskv Sxdovvsqoxmo gkc rovn kd Nkbdwyedr Myvvoqo sx drsc iokb.'
# 117.42 - 'Bpm nqzab kwvnmzmvkm wv bpm bwxqk wn Izbqnqkqit Qvbmttqomvkm eia pmtl ib Lizbuwcbp Kwttmom qv bpqa gmiz.'
# 117.77 - 'Qeb cfopq zlkcbobkzb lk qeb qlmfz lc Xoqfcfzfxi Fkqbiifdbkzb txp ebia xq Axoqjlrqe Zliibdb fk qefp vbxo.'
# 118.15 - 'Lzw xajkl ugfxwjwfuw gf lzw lghau gx Sjlaxauasd Aflwddaywfuw osk zwdv sl Vsjlegmlz Ugddwyw af lzak qwsj.'