r"""
Python module for OEIS sequence number A036447.

Double and reverse digits.

Examples of use.
-----------------------------------------------------------------------
>>> from a036447 import *
>>> print a036447_list(12)
[1, 2, 4, 8, 61, 221, 244, 884, 8671, 24371, 24784, 86594]
>>> print a036447_offset
0
>>> for x in a036447_list_pairs(6):
...     print x
...
(0, 1)
(1, 2)
(2, 4)
(3, 8)
(4, 61)
(5, 221)
>>> print a036447(5)
221
-----------------------------------------------------------------------
"""

from itertools import islice, izip

__all__ = ('a036447_offset', 'a036447_list', 'a036447_list_pairs', 'a036447', 'a036447_gen')
__author__ = 'Nick Hobson <nickh@qbyte.org>'

a036447_offset = offset = 0

def a036447_gen():
    """Generator function for OEIS sequence A036447."""
    x = 1
    while True:
        yield x
        x = int(str(2 * x)[::-1])

def a036447_list(n):
    """Returns a list of the first n >= 0 terms."""
    if n < 0: raise ValueError, 'Input must be a non-negative integer'
    return list(islice(a036447_gen(), n))

def a036447_list_pairs(n):
    """Returns a list of tuples (n, a(n)) of the first n >= 0 terms."""
    if n < 0: raise ValueError, 'Input must be a non-negative integer'
    return list(izip(xrange(offset, n+offset), a036447_gen()))

def a036447(n):
    """Returns the term with index n >= 0; offset 0."""
    if n < offset: raise ValueError, 'Input must be an integer >= offset = ' + str(offset)
    return list(islice(a036447_gen(), n-offset, n-offset+1)).pop()