from typing import Callable, Optional, TypeVar, cast

from reactivex import Observable, abc, compose
from reactivex import operators as ops
from reactivex.internal.exceptions import SequenceContainsNoElementsError
from reactivex.typing import Predicate

_T = TypeVar("_T")


def single_or_default_async_(
    has_default: bool = False, default_value: Optional[_T] = None
) -> Callable[[Observable[_T]], Observable[_T]]:
    def single_or_default_async(source: Observable[_T]) -> Observable[_T]:
        def subscribe(
            observer: abc.ObserverBase[_T],
            scheduler: Optional[abc.SchedulerBase] = None,
        ):
            value = cast(_T, default_value)
            seen_value = False

            def on_next(x: _T):
                nonlocal value, seen_value

                if seen_value:
                    observer.on_error(
                        Exception("Sequence contains more than one element")
                    )
                else:
                    value = x
                    seen_value = True

            def on_completed():
                if not seen_value and not has_default:
                    observer.on_error(SequenceContainsNoElementsError())
                else:
                    observer.on_next(value)
                    observer.on_completed()

            return source.subscribe(
                on_next, observer.on_error, on_completed, scheduler=scheduler
            )

        return Observable(subscribe)

    return single_or_default_async


def single_or_default_(
    predicate: Optional[Predicate[_T]] = None, default_value: _T = None
) -> Callable[[Observable[_T]], Observable[_T]]:
    """Returns the only element of an observable sequence that matches
    the predicate, or a default value if no such element exists this
    method reports an exception if there is more than one element in the
    observable sequence.

    Examples:
        >>> res = single_or_default()
        >>> res = single_or_default(lambda x: x == 42)
        >>> res = single_or_default(lambda x: x == 42, 0)
        >>> res = single_or_default(None, 0)

    Args:
        predicate -- [Optional] A predicate function to evaluate for
            elements in the source sequence.
        default_value -- [Optional] The default value if the index is
            outside the bounds of the source sequence.

    Returns:
        An observable Sequence containing the single element in the
    observable sequence that satisfies the condition in the predicate,
    or a default value if no such element exists.
    """

    if predicate:
        return compose(
            ops.filter(predicate), ops.single_or_default(None, default_value)
        )
    else:
        return single_or_default_async_(True, default_value)


__all__ = ["single_or_default_", "single_or_default_async_"]