# -*- coding: utf-8 -*-

import string


def __character_groups(basestring_sequence):
    """
    Returns a (character groups used,
    min. amount of characters for each of those groups)-tuple
    """
    counters = {string.ascii_lowercase: 0,
                string.ascii_uppercase: 0,
                string.digits: 0,
                string.punctuation: 0}
    for character in basestring_sequence:
        for group in counters.keys():
            if character in group:
                counters[group] += 1
    different_groups = [value for value in counters.values() if value > 0]
    return (len(different_groups), min(different_groups))


def calculate_entropy(password):
    """
    NIST 800-63

    Returns the entropy score (int)
    """
    if not isinstance(password, basestring):
        raise Exception('Password should be a basestring')
    plength = len(password)
    entropy_value = 0
    # "The entropy of the first character is four bits"
    entropy_value += len(password[0:1]) * 4
    # "The entropy of the next seven characters are two bits per character"
    entropy_value += len(password[1:8]) * 2
    # "The ninth through the twentieth character has 1.5 bits of entropy
    # per character"
    entropy_value += int(len(password[8:20]) * 1.5)  # use only the int part
    # "Characters 21 and above have one bit of entropy per character."
    if plength > 20:
        entropy_value += (plength - 20)
    different_groups, chars_per_group = __character_groups(password)
    if different_groups >= 3:
        if chars_per_group >= 2:  # 2 or more characters for each group
            entropy_value += 8
        else:
            entropy_value += 6
    return entropy_value