class PaginationHelper:

    # The constructor takes in an array of items and an integer indicating
    # how many items fit within a single page
    def __init__(self, collection, items_per_page):
        self.collection = collection
        self.items_per_page = items_per_page

    # returns the number of items within the entire collection
    def item_count(self):
        return len(self.collection)

    # returns the number of pages
    def page_count(self):
        return int(len(self.collection) / self.items_per_page) + (
            len(self.collection) % self.items_per_page > 0
        )

    # returns the number of items on the given page. page_index is zero based
    # this method should return -1 for page_index values that are out of range
    def page_item_count(self, page_index):
        if page_index < 0 or self.page_count() - 1 < page_index:
            return -1

        if page_index == 0:
            return len(self.collection[: self.items_per_page])

        return len(
            self.collection[
                self.items_per_page * page_index : self.items_per_page * page_index
                + self.items_per_page
            ]
        )

    # determines what page an item at the given index is on. Zero based indexes.
    # this method should return -1 for item_index values that are out of range
    def page_index(self, item_index):
        try:
            self.collection[item_index]

            if item_index < 0:
                raise IndexError
        except IndexError:
            return -1

        return item_index // self.items_per_page