text: import hashlib
  
  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):
          if not isinstance(fields, list):
              raise ValueError('Index.fields must be a list.')
          if not fields:
              raise ValueError('At least one field is required to define an index.')
          self.fields = 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)
  
      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 get_sql_create_template_values(self, model, schema_editor, using):
          fields = [model._meta.get_field(field_name) for field_name, order in self.fields_orders]
          tablespace_sql = schema_editor._get_index_tablespace_sql(model, fields)
          quote_name = schema_editor.quote_name
          columns = [
              ('%s %s' % (quote_name(field.column), order)).strip()
              for field, (field_name, order) in zip(fields, self.fields_orders)
          ]
          return {
              'table': quote_name(model._meta.db_table),
              'name': quote_name(self.name),
              'columns': ', '.join(columns),
              'using': using,
              'extra': tablespace_sql,
          }
  
      def create_sql(self, model, schema_editor, using=''):
          sql_create_index = schema_editor.sql_create_index
          sql_parameters = self.get_sql_create_template_values(model, schema_editor, using)
          return sql_create_index % sql_parameters
  
      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')
          return (path, (), {'fields': self.fields, 'name': self.name})
  
      @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 = 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())
  