"""
Useful auxiliary data structures for query construction. Not useful outside
the SQL domain.
"""
from django.db.models.sql.constants import INNER, LOUTER


class EmptyResultSet(Exception):
    pass


class MultiJoin(Exception):
    """
    Used by join construction code to indicate the point at which a
    multi-valued join was attempted (if the caller wants to treat that
    exceptionally).
    """
    def __init__(self, names_pos, path_with_names):
        self.level = names_pos
        # The path travelled, this includes the path to the multijoin.
        self.names_with_path = path_with_names


class Empty(object):
    pass


class Join(object):
    """
    Used by sql.Query and sql.SQLCompiler to generate JOIN clauses into the
    FROM entry. For example, the SQL generated could be
        LEFT OUTER JOIN "sometable" T1 ON ("othertable"."sometable_id" = "sometable"."id")

    This class is primarily used in Query.alias_map. All entries in alias_map
    must be Join compatible by providing the following attributes and methods:
        - table_name (string)
        - table_alias (possible alias for the table, can be None)
        - join_type (can be None for those entries that aren't joined from
          anything)
        - parent_alias (which table is this join's parent, can be None similarly
          to join_type)
        - as_sql()
        - relabeled_clone()
    """
    def __init__(self, table_name, parent_alias, table_alias, join_type,
                 join_field, nullable):
        # Join table
        self.table_name = table_name
        self.parent_alias = parent_alias
        # Note: table_alias is not necessarily known at instantiation time.
        self.table_alias = table_alias
        # LOUTER or INNER
        self.join_type = join_type
        # A list of 2-tuples to use in the ON clause of the JOIN.
        # Each 2-tuple will create one join condition in the ON clause.
        self.join_cols = join_field.get_joining_columns()
        # Along which field (or ForeignObjectRel in the reverse join case)
        self.join_field = join_field
        # Is this join nullabled?
        self.nullable = nullable

    def as_sql(self, compiler, connection):
        """
        Generates the full
           LEFT OUTER JOIN sometable ON sometable.somecol = othertable.othercol, params
        clause for this join.
        """
        join_conditions = []
        params = []
        qn = compiler.quote_name_unless_alias
        qn2 = connection.ops.quote_name

        # Add a join condition for each pair of joining columns.
        for index, (lhs_col, rhs_col) in enumerate(self.join_cols):
            join_conditions.append('%s.%s = %s.%s' % (
                qn(self.parent_alias),
                qn2(lhs_col),
                qn(self.table_alias),
                qn2(rhs_col),
            ))

        # Add a single condition inside parentheses for whatever
        # get_extra_restriction() returns.
        extra_cond = self.join_field.get_extra_restriction(
            compiler.query.where_class, self.table_alias, self.parent_alias)
        if extra_cond:
            extra_sql, extra_params = compiler.compile(extra_cond)
            join_conditions.append('(%s)' % extra_sql)
            params.extend(extra_params)

        if not join_conditions:
            # This might be a rel on the other end of an actual declared field.
            declared_field = getattr(self.join_field, 'field', self.join_field)
            raise ValueError(
                "Join generated an empty ON clause. %s did not yield either "
                "joining columns or extra restrictions." % declared_field.__class__
            )
        on_clause_sql = ' AND '.join(join_conditions)
        alias_str = '' if self.table_alias == self.table_name else (' %s' % self.table_alias)
        sql = '%s %s%s ON (%s)' % (self.join_type, qn(self.table_name), alias_str, on_clause_sql)
        return sql, params

    def relabeled_clone(self, change_map):
        new_parent_alias = change_map.get(self.parent_alias, self.parent_alias)
        new_table_alias = change_map.get(self.table_alias, self.table_alias)
        return self.__class__(
            self.table_name, new_parent_alias, new_table_alias, self.join_type,
            self.join_field, self.nullable)

    def __eq__(self, other):
        if isinstance(other, self.__class__):
            return (
                self.table_name == other.table_name and
                self.parent_alias == other.parent_alias and
                self.join_field == other.join_field
            )
        return False

    def demote(self):
        new = self.relabeled_clone({})
        new.join_type = INNER
        return new

    def promote(self):
        new = self.relabeled_clone({})
        new.join_type = LOUTER
        return new


class BaseTable(object):
    """
    The BaseTable class is used for base table references in FROM clause. For
    example, the SQL "foo" in
        SELECT * FROM "foo" WHERE somecond
    could be generated by this class.
    """
    join_type = None
    parent_alias = None

    def __init__(self, table_name, alias):
        self.table_name = table_name
        self.table_alias = alias

    def as_sql(self, compiler, connection):
        alias_str = '' if self.table_alias == self.table_name else (' %s' % self.table_alias)
        base_sql = compiler.quote_name_unless_alias(self.table_name)
        return base_sql + alias_str, []

    def relabeled_clone(self, change_map):
        return self.__class__(self.table_name, change_map.get(self.table_alias, self.table_alias))
