#functions from the Vigenere notebook

#Both key and plaintext are strings of lower-case letters
#The function returns the ciphertext.

def vigencrypt(key,plaintext):
    keylength=len(key)
    ciphertext=''
    for j in range(len(plaintext)):
        base=ord(plaintext[j])-ord('a')
        shift=ord(key[j%keylength])-ord('a')
        ciphertext+=chr(ord('a')+(base+shift)%26)
    return ciphertext

#This computes the inverse of the key:  To decrypt, call vigencrypt
#with the inverse key.
def inverse_key(key):
    inv=''
    for ch in key:
        shift=ord(ch)-ord('a')
        newshift=(-shift)%26
        newch=chr(ord('a')+newshift)
        inv+=newch
    return inv

#Return the number of characters in the ciphertext that are equal to the character
#m places further on, where m is the value of shift.  (Here 'shift' means we
#are literally moving the text, not shifting in the Caesar cipher sense.) We only
#do this for the first len-11 characters of the ciphertext, where
#len is the length (so we assume that the key is no more than ten
#characters long)

def count_coincidences(ciphertext,shift):
    count = 0
    for j in range(len(ciphertext)-11):
        if ciphertext[j]==ciphertext[j+shift]:
            count += 1
    return count

#Return a list of keylength different subtexts
#extracted from the ciphertext.  The first subtext
#consists of characters 0,keylength, 2*keylength, etc.,from the ciphertext
#the next of characters 1, 1_keylength, 1+2*keylength, etc.

def extract_subtexts(ciphertext,keylength):
    subtexts=['']*keylength
    for j in range(len(ciphertext)):
        subtexts[j%keylength]+=ciphertext[j]
    return subtexts

#Return a score that measures how closely the letter distribution of the text matches that of
#English. For idealized English text the score is about 0.0644; for random text about
#1/26 = 0.0385
def score(text):
    freq=[0.0817, 0.0153, 0.0224, 0.0470, 0.121, 0.0217, 0.0210, 0.0606, 0.0724, 0.0010, 0.0090, 0.0379, 0.0315, 0.0684, 0.0773, 0.0170, 0.0009, 0.0575, 0.0605, 0.0885, 0.0283, 0.0092, 0.0260, 0.0013, 0.0226, 0.0002]
    dist=[0]*26
    for ch in text:
        dist[ord(ch)-ord('a')]+=1
    result=0
    for j in range(26):
        result+=dist[j]*freq[j]
    return result/len(text)

#Find the score for all 26 Caesar shifts of the text, and return both the keyletter that
#leads to the maximum score, and the maximum score itself.

def maxscore(text):
    maxval=0
    maxletter='?'
    for keyletter in 'abcdefghijklmnopqrstuvwxyz':
        newscore=score(vigencrypt(inverse_key(keyletter),text))
        if newscore>maxval:
            maxval=newscore
            maxletter=keyletter
    return(maxletter,maxval)