Calculate entropy of a password

entropy.rb

include Math

# returns the entropy in bits
def calculate_password_entropy(password)
  return unless password.is_a? String
  
  inc_digit = password[/\d/] ? 1 : 0
  inc_lower = password[/[a-z]/] ? 1 : 0
  inc_upper = password[/[A-Z]/] ? 1 : 0
  total = inc_digit * 10 + inc_upper * 26 + inc_lower * 26

  (log(total) * password.size / log(2.0)).floor
end