mcmc.py

# Takes as input a text to decrypt and runs a MCMC algorithm for n_iter. Returns the state having maximum score and also
# the last few states 
def MCMC_decrypt(n_iter,cipher_text,scoring_params):
    current_cipher = string.ascii_uppercase # Generate a random cipher to start
    state_keeper = set()
    best_state = ''
    score = 0
    for i in range(n_iter):
        state_keeper.add(current_cipher)
        proposed_cipher = generate_cipher(current_cipher)
        score_current_cipher = get_cipher_score(cipher_text,current_cipher,scoring_params)
        score_proposed_cipher = get_cipher_score(cipher_text,proposed_cipher,scoring_params)
        acceptance_probability = min(1,math.exp(score_proposed_cipher-score_current_cipher))
        if score_current_cipher>score:
            best_state = current_cipher
        if random_coin(acceptance_probability):
            current_cipher = proposed_cipher
        if i%500==0:
            print("iter",i,":",apply_cipher_on_text(cipher_text,current_cipher)[0:99])
    return state_keeper,best_state