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()