import contextlib

from contextlib import contextmanager

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.pool import (
    NullPool,
    QueuePool,
    StaticPool,
)

from sqlalchemy_utils import database_exists, create_database

from ..constants import (
    DB_ENGINE,
    DB_HOST,
    DB_NAME,
    DB_PASSWORD,
    DB_PORT,
    DB_USERNAME,
    SQLITE,
    POSTGRESQL,
)

__session_factory = None
__session = None
__engine = None
__is_test = False


def _get_metadata():
    from .mixins import Base
    return Base.metadata


def setup_db(*, is_test=False, **db_config):
    """Main setup function for our database.

    This will perform the initial DB connection and also create tables as needed.

    """
    global __engine
    global __is_test

    if __engine:
        return

    __is_test = is_test

    connection_string = get_connection_string()
    connection_kwargs = db_config.get('connection_kwargs', {})

    # In a serverless environment use a staic pool which is a single-connection pool per Lambda
    connection_kwargs.update({
        'poolclass': StaticPool,
    })

    session_kwargs = db_config.get('session_kwargs', {})

    __engine = create_engine(connection_string, **connection_kwargs)
    print('Connected to: %s' % (__engine.url, ))

    if not database_exists(__engine.url):    # pragma: no cover
        print('Creating database: %s' % (__engine.url, ))
        create_database(__engine.url)

    create_tables()
    get_session(**session_kwargs)


def get_connection_string(**kwargs):
    """Return a connection string for sqlalchemy::

        dialect+driver://username:password@host:port/database

    """
    global DB_NAME

    if DB_ENGINE not in (SQLITE, POSTGRESQL):
        raise ValueError(
                'Invalid database engine specified: %s. Only sqlite' \
                ' and postgresql are supported' % (DB_ENGINE, ))

    if DB_ENGINE == SQLITE:
        # missing filename creates an in-memory db
        return 'sqlite://%s' % kwargs.get('filename', '')

    if __is_test and not DB_NAME.startswith('test_'):
        DB_NAME = 'test_%s' % (DB_NAME, )

    return 'postgresql://%s:%s@%s:%s/%s' % (
        DB_USERNAME,
        DB_PASSWORD,
        DB_HOST,
        DB_PORT,
        DB_NAME,
    )


def close_db():    # pragma: no cover
    if not __session:
        return

    try:
        __session.commit()
    except:
        __session.rollback()
    finally:
        __session.close()


def commit_session(_raise=False):    # pragma: no cover
    if not __session:
        return

    try:
        __session.commit()
    except Exception as e:
        __session.rollback()
        if _raise:
            raise


def create_tables():
    assert __engine
    meta = _get_metadata()
    meta.create_all(__engine)


def get_session(**kwargs):
    """Main API for connection to the DB via the SQLAlchemy session.

    Clients should use this for any DB interactions as it will connect and setup the database as
    needed. After initialization the global session will be returned so this is safe to call
    multiple times in a single thread.

    """
    setup_db()

    assert __engine
    global __session
    global __session_factory

    if __session is not None:
        return __session

    if __session_factory is None:    # pragma: no cover
        __session_factory = sessionmaker(bind=__engine, **kwargs)

    __session = __session_factory()
    return __session


def session_committer(func):
    """Decorator to comming the DB session.

    Use this from high-level functions such as handler so that the session is always committed or
    closed.

    """

    def wrapper(*args, **kwargs):
        try:
            return func(*args, **kwargs)
        finally:
            commit_session()

    return wrapper


def session_getter(func):
    """Decorator to get a session and inject it as the first argument in a function"""

    def wrapper(*args, **kwargs):
        session = get_session()
        return func(session, *args, **kwargs)

    return wrapper


@contextmanager
def dbtransaction():    # pragma: no cover
    """Use as a context manager to commit a transaction"""
    session = get_session()
    try:
        yield session
        session.commit()
    except:
        session.rollback()
        raise


def _drop_tables(*, force=False):
    if not __is_test and not force:
        return

    assert __engine
    meta = _get_metadata()
    meta.drop_all(__engine)


def _clear_tables(*, force=False):
    if not __is_test and not force:
        return

    assert __engine

    meta = _get_metadata()
    with contextlib.closing(__engine.connect()) as con:
        trans = con.begin()
        for table in reversed(meta.sorted_tables):
            try:
                con.execute(table.delete())
            except:
                pass
        trans.commit()