import hashlib
import logging

from django.db.backends.utils import truncate_name
from django.db.transaction import atomic
from django.utils import six
from django.utils.encoding import force_bytes

logger = logging.getLogger('django.db.backends.schema')


def _related_non_m2m_objects(old_field, new_field):
    # Filters out m2m objects from reverse relations.
    # Returns (old_relation, new_relation) tuples.
    return zip(
        (obj for obj in old_field.model._meta.related_objects if not obj.field.many_to_many),
        (obj for obj in new_field.model._meta.related_objects if not obj.field.many_to_many)
    )


class BaseDatabaseSchemaEditor(object):
    """
    This class (and its subclasses) are responsible for emitting schema-changing
    statements to the databases - model creation/removal/alteration, field
    renaming, index fiddling, and so on.

    It is intended to eventually completely replace DatabaseCreation.

    This class should be used by creating an instance for each set of schema
    changes (e.g. a migration file), and by first calling start(),
    then the relevant actions, and then commit(). This is necessary to allow
    things like circular foreign key references - FKs will only be created once
    commit() is called.
    """

    # Overrideable SQL templates
    sql_create_table = "CREATE TABLE %(table)s (%(definition)s)"
    sql_rename_table = "ALTER TABLE %(old_table)s RENAME TO %(new_table)s"
    sql_retablespace_table = "ALTER TABLE %(table)s SET TABLESPACE %(new_tablespace)s"
    sql_delete_table = "DROP TABLE %(table)s CASCADE"

    sql_create_column = "ALTER TABLE %(table)s ADD COLUMN %(column)s %(definition)s"
    sql_alter_column = "ALTER TABLE %(table)s %(changes)s"
    sql_alter_column_type = "ALTER COLUMN %(column)s TYPE %(type)s"
    sql_alter_column_null = "ALTER COLUMN %(column)s DROP NOT NULL"
    sql_alter_column_not_null = "ALTER COLUMN %(column)s SET NOT NULL"
    sql_alter_column_default = "ALTER COLUMN %(column)s SET DEFAULT %(default)s"
    sql_alter_column_no_default = "ALTER COLUMN %(column)s DROP DEFAULT"
    sql_delete_column = "ALTER TABLE %(table)s DROP COLUMN %(column)s CASCADE"
    sql_rename_column = "ALTER TABLE %(table)s RENAME COLUMN %(old_column)s TO %(new_column)s"
    sql_update_with_default = "UPDATE %(table)s SET %(column)s = %(default)s WHERE %(column)s IS NULL"

    sql_create_check = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s CHECK (%(check)s)"
    sql_delete_check = "ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"

    sql_create_unique = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s UNIQUE (%(columns)s)"
    sql_delete_unique = "ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"

    sql_create_fk = (
        "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s FOREIGN KEY (%(column)s) "
        "REFERENCES %(to_table)s (%(to_column)s) DEFERRABLE INITIALLY DEFERRED"
    )
    sql_create_inline_fk = None
    sql_delete_fk = "ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"

    sql_create_index = "CREATE INDEX %(name)s ON %(table)s (%(columns)s)%(extra)s"
    sql_delete_index = "DROP INDEX %(name)s"

    sql_create_pk = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s PRIMARY KEY (%(columns)s)"
    sql_delete_pk = "ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"

    def __init__(self, connection, collect_sql=False):
        self.connection = connection
        self.collect_sql = collect_sql
        if self.collect_sql:
            self.collected_sql = []

    # State-managing methods

    def __enter__(self):
        self.deferred_sql = []
        if self.connection.features.can_rollback_ddl:
            self.atomic = atomic(self.connection.alias)
            self.atomic.__enter__()
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        if exc_type is None:
            for sql in self.deferred_sql:
                self.execute(sql)
        if self.connection.features.can_rollback_ddl:
            self.atomic.__exit__(exc_type, exc_value, traceback)

    # Core utility functions

    def execute(self, sql, params=[]):
        """
        Executes the given SQL statement, with optional parameters.
        """
        # Log the command we're running, then run it
        logger.debug("%s; (params %r)" % (sql, params))
        if self.collect_sql:
            ending = "" if sql.endswith(";") else ";"
            if params is not None:
                self.collected_sql.append((sql % tuple(map(self.quote_value, params))) + ending)
            else:
                self.collected_sql.append(sql + ending)
        else:
            with self.connection.cursor() as cursor:
                cursor.execute(sql, params)

    def quote_name(self, name):
        return self.connection.ops.quote_name(name)

    @classmethod
    def _digest(cls, *args):
        """
        Generates 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()[:8]

    # Field <-> database mapping functions

    def column_sql(self, model, field, include_default=False):
        """
        Takes a field and returns its column definition.
        The field must already have had set_attributes_from_name called.
        """
        # Get the column's type and use that as the basis of the SQL
        db_params = field.db_parameters(connection=self.connection)
        sql = db_params['type']
        params = []
        # Check for fields that aren't actually columns (e.g. M2M)
        if sql is None:
            return None, None
        # Work out nullability
        null = field.null
        # If we were told to include a default value, do so
        include_default = include_default and not self.skip_default(field)
        if include_default:
            default_value = self.effective_default(field)
            if default_value is not None:
                if self.connection.features.requires_literal_defaults:
                    # Some databases can't take defaults as a parameter (oracle)
                    # If this is the case, the individual schema backend should
                    # implement prepare_default
                    sql += " DEFAULT %s" % self.prepare_default(default_value)
                else:
                    sql += " DEFAULT %s"
                    params += [default_value]
        # Oracle treats the empty string ('') as null, so coerce the null
        # option whenever '' is a possible value.
        if (field.empty_strings_allowed and not field.primary_key and
                self.connection.features.interprets_empty_strings_as_nulls):
            null = True
        if null and not self.connection.features.implied_column_null:
            sql += " NULL"
        elif not null:
            sql += " NOT NULL"
        # Primary key/unique outputs
        if field.primary_key:
            sql += " PRIMARY KEY"
        elif field.unique:
            sql += " UNIQUE"
        # Optionally add the tablespace if it's an implicitly indexed column
        tablespace = field.db_tablespace or model._meta.db_tablespace
        if tablespace and self.connection.features.supports_tablespaces and field.unique:
            sql += " %s" % self.connection.ops.tablespace_sql(tablespace, inline=True)
        # Return the sql
        return sql, params

    def skip_default(self, field):
        """
        Some backends don't accept default values for certain columns types
        (i.e. MySQL longtext and longblob).
        """
        return False

    def prepare_default(self, value):
        """
        Only used for backends which have requires_literal_defaults feature
        """
        raise NotImplementedError(
            'subclasses of BaseDatabaseSchemaEditor for backends which have '
            'requires_literal_defaults must provide a prepare_default() method'
        )

    def effective_default(self, field):
        """
        Returns a field's effective database default value
        """
        if field.has_default():
            default = field.get_default()
        elif not field.null and field.blank and field.empty_strings_allowed:
            if field.get_internal_type() == "BinaryField":
                default = six.binary_type()
            else:
                default = six.text_type()
        else:
            default = None
        # If it's a callable, call it
        if six.callable(default):
            default = default()
        # Run it through the field's get_db_prep_save method so we can send it
        # to the database.
        default = field.get_db_prep_save(default, self.connection)
        return default

    def quote_value(self, value):
        """
        Returns a quoted version of the value so it's safe to use in an SQL
        string. This is not safe against injection from user code; it is
        intended only for use in making SQL scripts or preparing default values
        for particularly tricky backends (defaults are not user-defined, though,
        so this is safe).
        """
        raise NotImplementedError()

    # Actions

    def create_model(self, model):
        """
        Takes a model and creates a table for it in the database.
        Will also create any accompanying indexes or unique constraints.
        """
        # Create column SQL, add FK deferreds if needed
        column_sqls = []
        params = []
        for field in model._meta.local_fields:
            # SQL
            definition, extra_params = self.column_sql(model, field)
            if definition is None:
                continue
            # Check constraints can go on the column SQL here
            db_params = field.db_parameters(connection=self.connection)
            if db_params['check']:
                definition += " CHECK (%s)" % db_params['check']
            # Autoincrement SQL (for backends with inline variant)
            col_type_suffix = field.db_type_suffix(connection=self.connection)
            if col_type_suffix:
                definition += " %s" % col_type_suffix
            params.extend(extra_params)
            # FK
            if field.remote_field and field.db_constraint:
                to_table = field.remote_field.model._meta.db_table
                to_column = field.remote_field.model._meta.get_field(field.remote_field.field_name).column
                if self.connection.features.supports_foreign_keys:
                    self.deferred_sql.append(self._create_fk_sql(model, field, "_fk_%(to_table)s_%(to_column)s"))
                elif self.sql_create_inline_fk:
                    definition += " " + self.sql_create_inline_fk % {
                        "to_table": self.quote_name(to_table),
                        "to_column": self.quote_name(to_column),
                    }
            # Add the SQL to our big list
            column_sqls.append("%s %s" % (
                self.quote_name(field.column),
                definition,
            ))
            # Autoincrement SQL (for backends with post table definition variant)
            if field.get_internal_type() == "AutoField":
                autoinc_sql = self.connection.ops.autoinc_sql(model._meta.db_table, field.column)
                if autoinc_sql:
                    self.deferred_sql.extend(autoinc_sql)

        # Add any unique_togethers (always deferred, as some fields might be
        # created afterwards, like geometry fields with some backends)
        for fields in model._meta.unique_together:
            columns = [model._meta.get_field(field).column for field in fields]
            self.deferred_sql.append(self._create_unique_sql(model, columns))
        # Make the table
        sql = self.sql_create_table % {
            "table": self.quote_name(model._meta.db_table),
            "definition": ", ".join(column_sqls)
        }
        if model._meta.db_tablespace:
            tablespace_sql = self.connection.ops.tablespace_sql(model._meta.db_tablespace)
            if tablespace_sql:
                sql += ' ' + tablespace_sql
        # Prevent using [] as params, in the case a literal '%' is used in the definition
        self.execute(sql, params or None)

        # Add any field index and index_together's (deferred as SQLite3 _remake_table needs it)
        self.deferred_sql.extend(self._model_indexes_sql(model))

        # Make M2M tables
        for field in model._meta.local_many_to_many:
            if field.remote_field.through._meta.auto_created:
                self.create_model(field.remote_field.through)

    def delete_model(self, model):
        """
        Deletes a model from the database.
        """
        # Handle auto-created intermediary models
        for field in model._meta.local_many_to_many:
            if field.remote_field.through._meta.auto_created:
                self.delete_model(field.remote_field.through)

        # Delete the table
        self.execute(self.sql_delete_table % {
            "table": self.quote_name(model._meta.db_table),
        })

    def alter_unique_together(self, model, old_unique_together, new_unique_together):
        """
        Deals with a model changing its unique_together.
        Note: The input unique_togethers must be doubly-nested, not the single-
        nested ["foo", "bar"] format.
        """
        olds = set(tuple(fields) for fields in old_unique_together)
        news = set(tuple(fields) for fields in new_unique_together)
        # Deleted uniques
        for fields in olds.difference(news):
            self._delete_composed_index(model, fields, {'unique': True}, self.sql_delete_unique)
        # Created uniques
        for fields in news.difference(olds):
            columns = [model._meta.get_field(field).column for field in fields]
            self.execute(self._create_unique_sql(model, columns))

    def alter_index_together(self, model, old_index_together, new_index_together):
        """
        Deals with a model changing its index_together.
        Note: The input index_togethers must be doubly-nested, not the single-
        nested ["foo", "bar"] format.
        """
        olds = set(tuple(fields) for fields in old_index_together)
        news = set(tuple(fields) for fields in new_index_together)
        # Deleted indexes
        for fields in olds.difference(news):
            self._delete_composed_index(model, fields, {'index': True}, self.sql_delete_index)
        # Created indexes
        for field_names in news.difference(olds):
            fields = [model._meta.get_field(field) for field in field_names]
            self.execute(self._create_index_sql(model, fields, suffix="_idx"))

    def _delete_composed_index(self, model, fields, constraint_kwargs, sql):
        columns = [model._meta.get_field(field).column for field in fields]
        constraint_names = self._constraint_names(model, columns, **constraint_kwargs)
        if len(constraint_names) != 1:
            raise ValueError("Found wrong number (%s) of constraints for %s(%s)" % (
                len(constraint_names),