"""
 utilities.py

 tested with python 3.5.2 
  
 Jim Mahoney | cs.marlboro.edu | Feb 2017 | MIT License
"""
from math import log2

def file_to_string(filename):
    """ Return the text from a file as one string, with newlines omitted. """
    return open(filename).read().replace('\n', '')

def string_to_substrings(thestring, substringsize):
    """ Return list of consecutive substrings of thestring of given size.
        If len(thestring) doesn't evenly divide substringsize, then
        the last chunk of thestring which is smaller than substringsize
        will be ignored.
        >>> string_to_substrings('1234', 2)
        ['12', '34']
        >>> string_to_substrings('12345', 2)
        ['12', '34']
    """
    substrings = []
    offset = 0
    while offset + substringsize <= len(thestring):
        substrings.append(thestring[offset : offset + substringsize])
        offset += substringsize
    return substrings

def file_to_substrings(filename, substringsize):
    """ Open file, get text, convert to chunks of given size """
    return string_to_substrings(file_to_string(filename), substringsize)

def probabilities(symbols):
    """ Given a list of strings,
        return a probability dictionary i.e. { string: p(string), ...}
        >>> sorted( probabilities(['a', 'a', 'b', 'c']).items() )
        [('a', 0.5), ('b', 0.25), ('c', 0.25)]
    """
    # No conditional probability here - just counting times for each symbol.
    probability = {}
    for symbol in symbols:                  # count 'em
        probability[symbol] = probability.get(symbol, 0) + 1
    for symbol in probability.keys():       # normalize
        probability[symbol] /= float(len(symbols))
    return probability

def mean(numbers):
    """ Return the mean of a list of numbers """
    return sum(numbers)/len(numbers)

def entropy(probability):
    """ Return the entropy of a set of probabilities
    """
    h = 0.0
    for p in probability.values():
        h += - p * log2(p)
    return h

if __name__ == '__main__':
    import doctest
    doctest.testmod()