"""
lastiter.py -- V.1.0, Jan 25 2007, by leonardo maffi

Ispired by:
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/304373
"""

class Lastiter(object):
    """Lastiter(iterable): this class is like iter(), but it starts yielding
    items from the second one. Lastiter has a "last" property that returns
    the precedent item of the iterable. Useful when you need computations that
    need two successive items of an iterable, like two lines in a file, two
    elements given by groupby, etc.

    Example usage, note the first is missing:
    >>> list(Lastiter(xrange(6)))
    [1, 2, 3, 4, 5]

    >>> d = Lastiter(xrange(6))
    >>> d.last
    0
    >>> for x in d: print x, d.last
    1 0
    2 1
    3 2
    4 3
    5 4

    >>> d = Lastiter([0, 1])
    >>> d.last
    0
    >>> for x in d: print x, d.last
    1 0

    >>> d = Lastiter([0])
    >>> d.last
    0
    >>> for x in d: print x, d.last

    >>> d = Lastiter([])
    >>> d.last
    Traceback (most recent call last):
      ...
    StopIteration
    >>> for x in d: print x, d.last

    >>> data = [1, 4, 8, 11, 11, 11, 14, 14, 15, 21, 24, 24, 25, 25, 27]
    >>> ldata = Lastiter(data)
    >>> [el-ldata.last for el in ldata] # differential
    [3, 4, 3, 0, 0, 3, 0, 1, 6, 3, 0, 1, 0, 2]
    """
    # Ispired by:
    # http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/304373
    def __init__(self, iterable):
        self._iterable = iter(iterable)
        try:
            self._last = self._iterable.next()
        except StopIteration:
            self._empty = True
        else:
            self._empty = False
        self._first = True

    def __iter__(self):
        return self

    def next(self):
        if self._first:
            self._first = False
        else:
            self._last = self._pred
        self._pred = self._iterable.next()
        return self._pred

    def get_last(self):
        if self._empty:
            raise StopIteration
        return self._last
    def set_last(self, x):
        raise TypeError("last can't be modified")
    last = property(get_last, set_last)


if __name__ == "__main__":
    import doctest
    doctest.testmod()
    print "Doctests finished.\n"

    print "A small demo, gives lesser of twin primes:"
    print "See: http://www.research.att.com/~njas/sequences/A001359"
    print
    def xsieve(): # Copied from See xsieve in primes.py, see that for better documentation
        "yields the sequence of primes via the Sieve of Eratosthenes, code by D. Eppstein."
        yield 2
        roots = xsieve()
        root = roots.next()
        square = root*root
        D = {}
        n = 3
        while True:
            if n >= square:     # Time to include another square?
                D[square] = root+root
                root = roots.next()
                square = root*root
            if n not in D:      # Not witnessed, must be prime.
                yield n
            else:               # Move witness p to next free multiple.
                p = D[n]
                q = n+p
                while q in D:
                    q += p
                del D[n]
                D[q] = p
            n += 2              # Move on to next odd number.

    from itertools import islice
    lxsieve = Lastiter(xsieve())
    twin_primes = (lxsieve.last for p in lxsieve if p-lxsieve.last == 2)
    for tp in islice(twin_primes, 0, 100):
        print tp,
    print

    primes = xsieve()
    last = primes.next()
    nout = 0
    while nout < 100:
        prime = primes.next()
        if prime - last == 2:
            nout += 1
            print last,
        last = prime
    print