r"""
Python module for OEIS sequence number A070864.

a(1) = a(2) = 1; for n > 2, a(n) = 2 + a(n-a(n-1)).

Examples of use.
-----------------------------------------------------------------------
>>> from a070864 import *
>>> print a070864_list(18)
[1, 1, 3, 3, 3, 5, 3, 5, 5, 5, 7, 5, 7, 5, 7, 7, 7, 9]
>>> print a070864_offset
1
>>> for x in a070864_list_pairs(6):
...     print x
...
(1, 1)
(2, 1)
(3, 3)
(4, 3)
(5, 3)
(6, 5)
>>> print a070864(100)
19
-----------------------------------------------------------------------
"""

from itertools import islice, izip, count

__all__ = ('a070864_offset', 'a070864_list', 'a070864_list_pairs', 'a070864', 'a070864_gen')
__author__ = 'Nick Hobson <nickh@qbyte.org>'

a070864_offset = offset = 1

def a070864_gen():
    """Generator function for OEIS sequence A070864."""
    a = {1:1, 2:1}
    yield a[1]
    yield a[2]
    for n in count(3):
        a[n] = 2 + a[n - a[n-1]]
        yield a[n]

def a070864_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(a070864_gen(), n))

def a070864_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), a070864_gen()))

def a070864(n):
    """Returns the term with index n >= 1; offset 1."""
    if n < offset: raise ValueError, 'Input must be an integer >= offset = ' + str(offset)
    return list(islice(a070864_gen(), n-offset, n-offset+1)).pop()