Small program for generating two permutations suggested by Hugo van der Sanden on seqfan mailing list

hugos_permutations.py

#!/usr/bin/python

import re

# Encode a natural number (incl 0) as a bijective number (string)
def enc(num, symbols="01"):
    ret = ''
    slen = len(symbols)
    while num > 0:
        num -= 1
        ret = symbols[num % slen] + ret
        num = num // slen
    return ret

# Decode a bijective number (string) as a regular natural number
def dec(s, symbols="01"):
    acc = 0
    for c in s:
        acc *= len(symbols)
        acc += (symbols.index(c) + 1)
    return acc

def hugo_a(s):
    if re.fullmatch('0+', s):
        return '1'*(len(s) - 1) + '0'
    else:
        m = re.fullmatch('0(0*)1(.*)', s)
        if m:
            return (len(m[1]) * '1') + '0' + m[2] + '0'
        else:
            m = re.fullmatch('1(0*)1(.*)', s)
            if m:
                return m[2] + '0' + '1'*(len(m[1]) + 1)
            else:
                m = re.fullmatch('1(0*)', s)
                if m: 
                    return '1' * len(m[0])
                else:
                    return "Pattern not matched?"

def hugo_b(s):
    if re.fullmatch('0+', s):
        return '1'*(len(s) - 1) + '0'
    else:
        m = re.fullmatch('0(0*)1(.*)', s)
        if m:
            return m[2] + '0' + len(m[1]) * '1' + '0'
        else:
            m = re.fullmatch('1(.*)1(0*)', s)
            if m:
                return m[1] + '0' + len(m[2]) * '1' + '1'
            else:
                m = re.fullmatch('1(0*)', s)
                if m: 
                    return '1' * len(m[0])
                else:
                    return "Pattern not matched?"

def take_n(f, n):
    return [dec(f(enc(i))) for i in range(1,n)]