Normal Markov algorithm

from __future__ import print_function

class Rule:
    def __init__(self, f, t, last=0):
        self.f = f  # from
        self.t = t  # to
        self.len = len(self.f)
        self.last = last  # last rule?

    def __str__(self):
        return "<Rule%s %s -> %s>" % ('.' if self.last else '', self.f, self.t)

    def sub(self, text):
        """ Substitution:
        >>> Rule('a', 'b').sub('qweaaannn')
        (1, 'qwebaannn')
        >>> Rule('a', 'b').sub('aaa')
        (1, 'baa')
        >>> Rule('a', '').sub('qweaba')
        (1, 'qweba')
        >>> Rule('', 'xyz').sub('qweaba')
        (1, 'xyzqweaba')
        >>> Rule('', '').sub('qweaba')
        (0, 'qweaba')
        """
        if not self.f and not self.t: return (0, text)
        for i in range(0, len(text)):
            end = i + self.len
            if self.f == text[i:end]:
                return (1, text[0:i] + self.t + text[end:])
        return (0, text)



class NM:
    """ Normal Markov's Algorithm
    >>> nm = NM([Rule("ba", "ab"),])
    >>> nm('aababbbabababbaaa')
    'aaaaaaaaabbbbbbbb'
    >>> nm = NM([Rule("c", ""), Rule("bb", "ddd", last=1)])
    >>> nm('abbcabbca')
    'adddabba'
    >>> nm('dcacb')
    'dab'
    >>> nm = NM([Rule("*a", "", last=1),
    ...          Rule("*b", "", last=1),
    ...          Rule("*", "", last=1),
    ...          Rule("", "*"),
    ... ])
    >>> nm('bbaba')
    'baba'
    >>> nm = NM([Rule("*0", "00*"),
    ...          Rule("*1", "01*"),
    ...          Rule("*2", "10*"),
    ...          Rule("*3", "11*"),
    ...          Rule("*", "", last=1),
    ...          Rule("", "*"),
    ... ])
    >>> nm('0123')
    '00011011'
    >>> nm = NM([Rule("*a", "a*"),
    ...          Rule("*b", "b*"),
    ...          Rule("*", "a", last=1),
    ...          Rule("", "*"),
    ... ])
    >>> nm('bbab')
    'bbaba'
    """
    def __init__(self, rules):
        self.rules = rules

    def eval(self, text):
        while 1:
            changed = 0
            for rule in self.rules:
                oldtext = text
                applied, text = rule.sub(text)
                #print("%s -> %s [by %s]" % (oldtext, text, rule))
                if applied:
                    changed = 1
                    if rule.last: return text
                    else: break
            if not changed:
                return text
    __call__ = eval

import doctest
doctest.testmod()