freqAnalysis.py

# Frequency Finder
# https://www.nostarch.com/crackingcodes (BSD Licensed)

ETAOIN = 'ETAOINSHRDLCUMWFGYPBVKJXQZ'
LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

def getLetterCount(message):
    # Returns a dictionary with keys of single letters and values of the
    # count of how many times they appear in the message parameter:
    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(items):
    return items[0]


def getFrequencyOrder(message):
    # Returns a string of the alphabet letters arranged in order of most
    # frequently occurring in the message parameter.

    # First, get a dictionary of each letter and its frequency count:
    letterToFreq = getLetterCount(message)

    # Second, make a dictionary of each frequency count to each letter(s)
    # with that frequency:
    freqToLetter = {}
    for letter in LETTERS:
        if letterToFreq[letter] not in freqToLetter:
            freqToLetter[letterToFreq[letter]] = [letter]
        else:
            freqToLetter[letterToFreq[letter]].append(letter)

    # Third, put each list of letters in reverse "ETAOIN" order, and then
    # convert it to a string:
    for freq in freqToLetter:
        freqToLetter[freq].sort(key=ETAOIN.find, reverse=True)
        freqToLetter[freq] = ''.join(freqToLetter[freq])

    # Fourth, convert the freqToLetter dictionary to a list of
    # tuple pairs (key, value), then sort them:
    freqPairs = list(freqToLetter.items())
    freqPairs.sort(key=getItemAtIndexZero, reverse=True)

    # Fifth, now that the letters are ordered by frequency, extract all
    # the letters for the final string:
    freqOrder = []
    for freqPair in freqPairs:
        freqOrder.append(freqPair[1])

    return ''.join(freqOrder)


def englishFreqMatchScore(message):
    # Return the number of matches that the string in the message
    # parameter has when its letter frequency is compared to English
    # letter frequency. A "match" is how many of its six most frequent
    # and six least frequent letters is among the six most frequent and
    # six least frequent letters for English.
    freqOrder = getFrequencyOrder(message)

    matchScore = 0
    # Find how many matches for the six most common letters there are:
    for commonLetter in ETAOIN[:6]:
        if commonLetter in freqOrder[:6]:
            matchScore += 1
    # Find how many matches for the six least common letters there are:
    for uncommonLetter in ETAOIN[-6:]:
        if uncommonLetter in freqOrder[-6:]:
            matchScore += 1

    return matchScore