from django.core.paginator import Paginator
from django.db.models import Index


class PgPartialIndex(Index):
    suffix = "part"
    max_name_length = 31

    def __init__(self, fields=[], name=None, where=None):
        if not where:
            raise ValueError("partial index requires WHERE clause")
        self.where = where

        super().__init__(fields, name)

    def set_name_with_model(self, model):
        table_name = model._meta.db_table

        column_names = sorted(self.where.keys())
        where_items = []
        for key in sorted(self.where.keys()):
            where_items.append("%s:%s" % (key, repr(self.where[key])))

        # The length of the parts of the name is based on the default max
        # length of 30 characters.
        hash_data = [table_name] + self.fields + where_items + [self.suffix]
        self.name = "%s_%s_%s" % (
            table_name[:11],
            column_names[0][:7],
            "%s_%s" % (self._hash_generator(*hash_data), self.suffix),
        )

        assert len(self.name) <= self.max_name_length, (
            "Index too long for multiple database support. Is self.suffix "
            "longer than 3 characters?"
        )
        self.check_name()

    def __repr__(self):
        if self.where is not None:
            where_items = []
            for key in sorted(self.where.keys()):
                where_items.append("=".join([key, repr(self.where[key])]))
            return "<%(name)s: fields=%(fields)s, where=%(where)s>" % {
                "name": self.__class__.__name__,
                "fields": "'%s'" % (", ".join(self.fields)),
                "where": "'%s'" % (", ".join(where_items)),
            }
        else:
            return super().__repr__()

    def deconstruct(self):
        path, args, kwargs = super().deconstruct()
        kwargs["where"] = self.where
        return path, args, kwargs

    def get_sql_create_template_values(self, model, schema_editor, using):
        parameters = super().get_sql_create_template_values(model, schema_editor, "")
        parameters["extra"] = self.get_sql_extra(model, schema_editor)
        return parameters

    def get_sql_extra(self, model, schema_editor):
        quote_name = schema_editor.quote_name
        quote_value = schema_editor.quote_value

        clauses = []
        for field, condition in self.where.items():
            field_name = None
            compr = None
            if field.endswith("__lt"):
                field_name = field[:-4]
                compr = "<"
            elif field.endswith("__gt"):
                field_name = field[:-4]
                compr = ">"
            elif field.endswith("__lte"):
                field_name = field[:-5]
                compr = "<="
            elif field.endswith("__gte"):
                field_name = field[:-5]
                compr = ">="
            else:
                field_name = field
                compr = "="

            column = model._meta.get_field(field_name).column
            clauses.append(
                "%s %s %s" % (quote_name(column), compr, quote_value(condition))
            )
        # sort clauses for their order to be determined and testable
        return " WHERE %s" % (" AND ".join(sorted(clauses)))


def chunk_queryset(queryset, chunk_size=20):
    ordered_queryset = queryset.order_by("-pk")  # bias to newest items first
    chunk = ordered_queryset[:chunk_size]
    while chunk:
        last_pk = None
        for item in chunk:
            last_pk = item.pk
            yield item
        chunk = ordered_queryset.filter(pk__lt=last_pk)[:chunk_size]