# Django 2.1.8.dev20190330215650 documentation

# Source code for django.db.models.indexes

    import hashlib

    from django.db.backends.utils import split_identifier
    from django.utils.encoding import force_bytes

    __all__ = ['Index']

    class Index:
        suffix = 'idx'
        # The max length of the name of the index (restricted to 30 for
        # cross-database compatibility with Oracle)
        max_name_length = 30

        def __init__(self, *, fields=(), name=None, db_tablespace=None):
            if not isinstance(fields, (list, tuple)):
                raise ValueError('Index.fields must be a list or tuple.')
            if not fields:
                raise ValueError('At least one field is required to define an index.')
            self.fields = list(fields)
            # A list of 2-tuple with the field name and ordering ('' or 'DESC').
            self.fields_orders = [
                (field_name[1:], 'DESC') if field_name.startswith('-') else (field_name, '')
                for field_name in self.fields
            ]
            self.name = name or ''
            if self.name:
                errors = self.check_name()
                if len(self.name) > self.max_name_length:
                    errors.append('Index names cannot be longer than %s characters.' % self.max_name_length)
                if errors:
                    raise ValueError(errors)
            self.db_tablespace = db_tablespace

        def check_name(self):
            errors = []
            # Name can't start with an underscore on Oracle; prepend D if needed.
            if self.name[0] == '_':
                errors.append('Index names cannot start with an underscore (_).')
                self.name = 'D%s' % self.name[1:]
            # Name can't start with a number on Oracle; prepend D if needed.
            elif self.name[0].isdigit():
                errors.append('Index names cannot start with a number (0-9).')
                self.name = 'D%s' % self.name[1:]
            return errors

        def create_sql(self, model, schema_editor, using=''):
            fields = [model._meta.get_field(field_name) for field_name, _ in self.fields_orders]
            col_suffixes = [order[1] for order in self.fields_orders]
            return schema_editor._create_index_sql(
                model, fields, name=self.name, using=using, db_tablespace=self.db_tablespace,
                col_suffixes=col_suffixes,
            )

        def remove_sql(self, model, schema_editor):
            quote_name = schema_editor.quote_name
            return schema_editor.sql_delete_index % {
                'table': quote_name(model._meta.db_table),
                'name': quote_name(self.name),
            }

        def deconstruct(self):
            path = '%s.%s' % (self.__class__.__module__, self.__class__.__name__)
            path = path.replace('django.db.models.indexes', 'django.db.models')
            kwargs = {'fields': self.fields, 'name': self.name}
            if self.db_tablespace is not None:
                kwargs['db_tablespace'] = self.db_tablespace
            return (path, (), kwargs)

        def clone(self):
            """Create a copy of this Index."""
            _, _, kwargs = self.deconstruct()
            return self.__class__(**kwargs)

        @staticmethod
        def _hash_generator(*args):
            """
            Generate a 32-bit digest of a set of arguments that can be used to
            shorten identifying names.
            """
            h = hashlib.md5()
            for arg in args:
                h.update(force_bytes(arg))
            return h.hexdigest()[:6]

        def set_name_with_model(self, model):
            """
            Generate a unique name for the index.

            The name is divided into 3 parts - table name (12 chars), field name
            (8 chars) and unique hash + suffix (10 chars). Each part is made to
            fit its size by truncating the excess length.
            """
            _, table_name = split_identifier(model._meta.db_table)
            column_names = [model._meta.get_field(field_name).column for field_name, order in self.fields_orders]
            column_names_with_order = [
                (('-%s' if order else '%s') % column_name)
                for column_name, (field_name, order) in zip(column_names, self.fields_orders)
            ]
            # The length of the parts of the name is based on the default max
            # length of 30 characters.
            hash_data = [table_name] + column_names_with_order + [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):
            return "<%s: fields='%s'>" % (self.__class__.__name__, ', '.join(self.fields))

        def __eq__(self, other):
            return (self.__class__ == other.__class__) and (self.deconstruct() == other.deconstruct())