text: # 6.00x Problem Set 6
  #
  # Part 1 - HAIL CAESAR!
  
  import string
  import random
  
  WORDLIST_FILENAME = "words.txt"
  
  # -----------------------------------
  # Helper code
  # (you don't need to understand this helper code)
  def loadWords():
      """
      Returns a list of valid words. Words are strings of lowercase letters.
      
      Depending on the size of the word list, this function may
      take a while to finish.
      """
      print "Loading word list from file..."
      inFile = open(WORDLIST_FILENAME, 'r')
      wordList = inFile.read().split()
      print "  ", len(wordList), "words loaded."
      return wordList
  
  def isWord(wordList, word):
      """
      Determines if word is a valid word.
  
      wordList: list of words in the dictionary.
      word: a possible word.
      returns True if word is in wordList.
  
      Example:
      >>> isWord(wordList, 'bat') returns
      True
      >>> isWord(wordList, 'asdf') returns
      False
      """
      word = word.lower()
      word = word.strip(" !@#$%^&*()-_+={}[]|\\:;'<>?,./\"")
      return word in wordList
  
  def randomWord(wordList):
      """
      Returns a random word.
  
      wordList: list of words  
      returns: a word from wordList at random
      """
      return random.choice(wordList)
  
  def randomString(wordList, n):
      """
      Returns a string containing n random words from wordList
  
      wordList: list of words
      returns: a string of random words separated by spaces.
      """
      return " ".join([randomWord(wordList) for _ in range(n)])
  
  def randomScrambled(wordList, n):
      """
      Generates a test string by generating an n-word random string
      and encrypting it with a sequence of random shifts.
  
      wordList: list of words
      n: number of random words to generate and scamble
      returns: a scrambled string of n random words
  
      NOTE:
      This function will ONLY work once you have completed your
      implementation of applyShifts!
      """
      s = randomString(wordList, n) + " "
      shifts = [(i, random.randint(0, 25)) for i in range(len(s)) if s[i-1] == ' ']
      return applyShifts(s, shifts)[:-1]
  
  def getStoryString():
      """
      Returns a story in encrypted text.
      """
      return open("story.txt", "r").read()
  
  
  # (end of helper code)
  # -----------------------------------
  
  
  #
  # Problem 1: Encryption
  #
  def buildCoder(shift):
      """
      Returns a dict that can apply a Caesar cipher to a letter.
      The cipher is defined by the shift value. Ignores non-letter characters
      like punctuation, numbers and spaces.
  
      shift: 0 <= int < 26
      returns: dict
      """
      # restrict shift value
      assert 0 <= shift < 26
      
      # original lists of upper and lowercase characters
      origU = list(string.ascii_uppercase)
      origL = list(string.ascii_lowercase)
  
      # holdU and holdL lists for wrap-around characters
      holdU = []
      for i in range(shift):
          holdU.append(origU[i])
          
      holdL = []
      for i in range(shift):
          holdL.append(origL[i])
  
      # new shifted lists
      cipherU = origU[shift:] + holdU
      cipherL = origL[shift:] + holdL
  
      # separate Caesar key dictionaries for each case
      caesarU = dict(zip(origU, cipherU))
      caesarL = dict(zip(origL, cipherL))
  
      # final key
      KEY = dict(caesarU, **caesarL)
  
      return KEY
  
  
  def applyCoder(text, coder):
      """
      Applies the coder to the text. Returns the encoded text.
  
      text: string
      coder: dict with mappings of characters to shifted characters
      returns: text after mapping coder chars to original text
      """
      encrypted = ''
      for char in text:
          if char in coder:
              encrypted += coder[char]
          else:
              encrypted += char
      return encrypted
  
  
  # This is a wrapper function
  def applyShift(text, shift):
      """
      Given a text, returns a new text Caesar shifted by the given shift
      offset. Lower case letters should remain lower case, upper case
      letters should remain upper case, and all other punctuation should
      stay as it is.
  
      text: string to apply the shift to
      shift: amount to shift the text (0 <= int < 26)
      returns: text after being shifted by specified amount.
      """
      return applyCoder(text, buildCoder(shift))
  
  #
  # Problem 2: Decryption
  #
  '''
  # My pseudocode:
  
  '''
  def findBestShift(wordList, text):
      """
      Finds a shift key that can decrypt the encoded text.
  
      text: string
      returns: 0 <= int < 26
      """
      # regular expression which splits text into a list of words
      import re
      words = re.findall(r"[\w']+", text)
  
      keydict = {}
  
      for n in range(26):
          engCount = 0
          for w in words:
              if applyShift(w, n).lower() in wordList:
                  engCount += 1
          keydict[n] = engCount
  
      bestShift = 0
  
      for k,v in keydict.items():
          if v > keydict[bestShift]:
              bestShift = k
  
      return bestShift
      
  
  def decryptStory():
      """
      Using the methods you created in this problem set,
      decrypt the story given by the function getStoryString().
      Use the functions getStoryString and loadWords to get the
      raw data you need.
  
      returns: string - story in plain text
      """
      wordList = loadWords()
      story = getStoryString()
  
      shift = findBestShift(wordList, story)
      decrypted = applyShift(story, shift)
  
      return decrypted
  
      # Story decrypted
      '''
      Jack Florey is a mythical character created on the spur of a moment
      to help cover an insufficiently planned hack. He has been registered
      for classes at MIT twice before, but has reportedly never passed a
      class. It has been the tradition of the residents of East Campus to
      become Jack Florey for a few nights each year to educate incoming
      students in the ways, means, and ethics of hacking.\n'
      '''
  
  #
  # Build data structures used for entire session and run encryption
  #
  
  if __name__ == '__main__':
      # To test findBestShift:
      wordList = loadWords()
      #s = applyShift('Hello, world!', 8)
      #bestShift = findBestShift(wordList, s)
      #assert applyShift(s, bestShift) == 'Hello, world!'
      # To test decryptStory, comment the above four lines and uncomment this line:
          #decryptStory()
  