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"""This module contains a code example related to

Think Python, 2nd Edition

by Allen Downey

http://thinkpython2.com

Copyright 2015 Allen Downey

License: http://creativecommons.org/licenses/by/4.0/

"""

from __future__ import print_function, division

import sys

import string

import random

# global variables

suffix_map = {} # map from prefixes to a list of suffixes

prefix = () # current tuple of words

def process_file(filename, order=2):

"""Reads a file and performs Markov analysis.

filename: string

order: integer number of words in the prefix

returns: map from prefix to list of possible suffixes.

"""

fp = open(filename)

skip_gutenberg_header(fp)

for line in fp:

if line.startswith('*** END OF THIS'):

break

for word in line.rstrip().split():

process_word(word, order)

def skip_gutenberg_header(fp):

"""Reads from fp until it finds the line that ends the header.

fp: open file object

"""

for line in fp:

if line.startswith('*** START OF THIS'):

break

def process_word(word, order=2):

"""Processes each word.

word: string

order: integer

During the first few iterations, all we do is store up the words;

after that we start adding entries to the dictionary.

"""

global prefix

if len(prefix) < order:

prefix += (word,)

return

try:

suffix_map[prefix].append(word)

except KeyError:

# if there is no entry for this prefix, make one

suffix_map[prefix] = [word]

prefix = shift(prefix, word)

def random_text(n=100):

"""Generates random wordsfrom the analyzed text.

Starts with a random prefix from the dictionary.

n: number of words to generate

"""

# choose a random prefix (not weighted by frequency)

start = random.choice(list(suffix_map.keys()))

for i in range(n):

suffixes = suffix_map.get(start, None)

if suffixes == None:

# if the start isn't in map, we got to the end of the

# original text, so we have to start again.

random_text(n-i)

return

# choose a random suffix

word = random.choice(suffixes)

print(word, end=' ')

start = shift(start, word)

def shift(t, word):

"""Forms a new tuple by removing the head and adding word to the tail.

t: tuple of strings

word: string

Returns: tuple of strings

"""

return t[1:] + (word,)

def main(script, filename='158-0.txt', n=100, order=2):

try:

n = int(n)

order = int(order)

except ValueError:

print('Usage: %d filename [# of words] [prefix length]' % script)

else:

process_file(filename, order)

random_text(n)

print()

if __name__ == '__main__':

main(*sys.argv)