from math import ceil

class InvalidPage(Exception):

    pass

class PageNotAnInteger(InvalidPage):

    pass

class EmptyPage(InvalidPage):

    pass

class Paginator(object):

    def __init__(self, object_list, per_page, orphans=0, allow_empty_first_page=True):

        self.object_list = object_list

        self.per_page = per_page

        self.orphans = orphans

        self.allow_empty_first_page = allow_empty_first_page

        self._num_pages = self._count = None

    def validate_number(self, number):

        "Validates the given 1-based page number."

        try:

            number = int(number)

        except ValueError:

            raise PageNotAnInteger('That page number is not an integer')

        if number < 1:

            raise EmptyPage('That page number is less than 1')

        if number > self.num_pages:

            if number == 1 and self.allow_empty_first_page:

                pass

            else:

                raise EmptyPage('That page contains no results')

        return number

    def page(self, number):

        "Returns a Page object for the given 1-based page number."

        number = self.validate_number(number)

        bottom = (number - 1) * self.per_page

        top = bottom + self.per_page

        if top + self.orphans >= self.count:

            top = self.count

        return Page(self.object_list[bottom:top], number, self)

    def _get_count(self):

        "Returns the total number of objects, across all pages."

        if self._count is None:

            try:

                self._count = self.object_list.count()

            except (AttributeError, TypeError):

                # AttributeError if object_list has no count() method.

                # TypeError if object_list.count() requires arguments

                # (i.e. is of type list).

                self._count = len(self.object_list)

        return self._count

    count = property(_get_count)

    def _get_num_pages(self):

        "Returns the total number of pages."

        if self._num_pages is None:

            if self.count == 0 and not self.allow_empty_first_page:

                self._num_pages = 0

            else:

                hits = max(1, self.count - self.orphans)

                self._num_pages = int(ceil(hits / float(self.per_page)))

        return self._num_pages

    num_pages = property(_get_num_pages)

    def _get_page_range(self):

        """

        Returns a 1-based range of pages for iterating through within

        a template for loop.

        """

        return range(1, self.num_pages + 1)

    page_range = property(_get_page_range)

QuerySetPaginator = Paginator # For backwards-compatibility.

class Page(object):

    def __init__(self, object_list, number, paginator):

        self.object_list = object_list

        self.number = number

        self.paginator = paginator

    def __repr__(self):

        return '<Page %s of %s>' % (self.number, self.paginator.num_pages)

    def has_next(self):

        return self.number < self.paginator.num_pages

    def has_previous(self):

        return self.number > 1

    def has_other_pages(self):

        return self.has_previous() or self.has_next()

    def next_page_number(self):

        return self.number + 1

    def previous_page_number(self):

        return self.number - 1

    def start_index(self):

        """

        Returns the 1-based index of the first object on this page,

        relative to total objects in the paginator.

        """

        # Special case, return zero if no items.

        if self.paginator.count == 0:

            return 0

        return (self.paginator.per_page * (self.number - 1)) + 1

    def end_index(self):

        """

        Returns the 1-based index of the last object on this page,

        relative to total objects found (hits).

        """

        # Special case for the last page because there can be orphans.

        if self.number == self.paginator.num_pages:

            return self.paginator.count

        return self.number * self.paginator.per_page