! @brief Compute the number of erased pages.

        Determine how many pages in the new data are already erased.

def _compute_chip_erase_pages_and_weight(self):
        """! @brief Compute the number of erased pages.

        Determine how many pages in the new data are already erased.
        """
        chip_erase_count = 0
        chip_erase_weight = 0
        chip_erase_weight += self.flash.get_flash_info().erase_weight
        for page in self.page_list:
            if page.erased is None:
                page.erased = self.flash.region.is_erased(page.data)
            if not page.erased:
                chip_erase_count += 1
                chip_erase_weight += page.get_program_weight()
        self.chip_erase_count = chip_erase_count
        self.chip_erase_weight = chip_erase_weight
        return chip_erase_count, chip_erase_weight