Python/other/frequency_finder.py

# Frequency Finder

# frequency taken from http://en.wikipedia.org/wiki/Letter_frequency
englishLetterFreq = {'E': 12.70, 'T': 9.06, 'A': 8.17, 'O': 7.51, 'I': 6.97,
                     'N': 6.75, 'S': 6.33, 'H': 6.09, 'R': 5.99, 'D': 4.25,
                     'L': 4.03, 'C': 2.78, 'U': 2.76, 'M': 2.41, 'W': 2.36,
                     'F': 2.23, 'G': 2.02, 'Y': 1.97, 'P': 1.93, 'B': 1.29,
                     'V': 0.98, 'K': 0.77, 'J': 0.15, 'X': 0.15, 'Q': 0.10,
                     'Z': 0.07}
ETAOIN = 'ETAOINSHRDLCUMWFGYPBVKJXQZ'
LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

def getLetterCount(message):
    letterCount = {'A': 0, 'B': 0, 'C': 0, 'D': 0, 'E': 0, 'F': 0, 'G': 0, 'H': 0,
                   'I': 0, 'J': 0, 'K': 0, 'L': 0, 'M': 0, 'N': 0, 'O': 0, 'P': 0,
                   'Q': 0, 'R': 0, 'S': 0, 'T': 0, 'U': 0, 'V': 0, 'W': 0, 'X': 0,
                   'Y': 0, 'Z': 0}
    for letter in message.upper():
        if letter in LETTERS:
            letterCount[letter] += 1

    return letterCount

def getItemAtIndexZero(x):
    return x[0]

def getFrequencyOrder(message):
    letterToFreq = getLetterCount(message)
    freqToLetter = {}
    for letter in LETTERS:
        if letterToFreq[letter] not in freqToLetter:
            freqToLetter[letterToFreq[letter]] = [letter]
        else:
            freqToLetter[letterToFreq[letter]].append(letter)

    for freq in freqToLetter:
        freqToLetter[freq].sort(key = ETAOIN.find, reverse = True)
        freqToLetter[freq] = ''.join(freqToLetter[freq])

    freqPairs = list(freqToLetter.items())
    freqPairs.sort(key = getItemAtIndexZero, reverse = True)

    freqOrder = []
    for freqPair in freqPairs:
        freqOrder.append(freqPair[1])

    return ''.join(freqOrder)

def englishFreqMatchScore(message):
    '''
    >>> englishFreqMatchScore('Hello World')
    1
    '''
    freqOrder = getFrequencyOrder(message)
    matchScore = 0
    for commonLetter in ETAOIN[:6]:
        if commonLetter in freqOrder[:6]:
            matchScore += 1

    for uncommonLetter in ETAOIN[-6:]:
        if uncommonLetter in freqOrder[-6:]:
            matchScore += 1

    return matchScore

if __name__ == '__main__':
    import doctest
    doctest.testmod()