"""
Helper for looping over sequences, particular in templates.

Often in a loop in a template it's handy to know what's next up,
previously up, if this is the first or last item in the sequence, etc.
These can be awkward to manage in a normal Python loop, but using the
looper you can get a better sense of the context.  Use like::

    >>> for loop, item in looper(['a', 'b', 'c']):
    ...     print loop.number, item
    ...     if not loop.last:
    ...         print '---'
    1 a
    ---
    2 b
    ---
    3 c

"""

__all__ = ['looper']

class looper(object):
    """
    Helper for looping (particularly in templates)
    
    Use this like::
    
        for loop, item in looper(seq):
            if loop.first:
                ...
    """

    def __init__(self, seq):
        self.seq = seq

    def __iter__(self):
        return looper_iter(self.seq)

    def __repr__(self):
        return '<%s for %r>' % (
            self.__class__.__name__, self.seq)

class looper_iter(object):

    def __init__(self, seq):
        self.seq = list(seq)
        self.pos = 0

    def __iter__(self):
        return self

    def next(self):
        if self.pos >= len(self.seq):
            raise StopIteration
        result = loop_pos(self.seq, self.pos), self.seq[self.pos]
        self.pos += 1
        return result

class loop_pos(object):

    def __init__(self, seq, pos):
        self.seq = seq
        self.pos = pos

    def __repr__(self):
        return '<loop pos=%r at %r>' % (
            self.seq[pos], pos)

    def index(self):
        return self.pos
    index = property(index)

    def number(self):
        return self.pos + 1
    number = property(number)

    def item(self):
        return self.seq[self.pos]
    item = property(item)

    def next(self):
        try:
            return self.seq[self.pos+1]
        except IndexError:
            return None
    next = property(next)

    def previous(self):
        if self.pos == 0:
            return None
        return self.seq[self.pos-1]
    previous = property(previous)

    def odd(self):
        return not self.pos % 2
    odd = property(odd)

    def even(self):
        return self.pos % 2
    even = property(even)

    def first(self):
        return self.pos == 0
    first = property(first)

    def last(self):
        return self.pos == len(self.seq)-1
    last = property(last)

    def length(self):
        return len(self.seq)
    length = property(length)

    def first_group(self, getter=None):
        """
        Returns true if this item is the start of a new group,
        where groups mean that some attribute has changed.  The getter
        can be None (the item itself changes), an attribute name like
        ``'.attr'``, a function, or a dict key or list index.
        """
        if self.first:
            return True
        return self._compare_group(self.item, self.previous, getter)

    def last_group(self, getter=None):
        """
        Returns true if this item is the end of a new group,
        where groups mean that some attribute has changed.  The getter
        can be None (the item itself changes), an attribute name like
        ``'.attr'``, a function, or a dict key or list index.
        """
        if self.last:
            return True
        return self._compare_group(self.item, self.next, getter)

    def _compare_group(self, item, other, getter):
        if getter is None:
            return item != other
        elif (isinstance(getter, basestring)
              and getter.startswith('.')):
            getter = getter[1:]
            if getter.endswith('()'):
                getter = getter[:-2]
                return getattr(item, getter)() != getattr(other, getter)()
            else:
                return getattr(item, getter) != getattr(other, getter)
        elif callable(getter):
            return getter(item) != getter(other)
        else:
            return item[getter] != other[getter]
    
