import itertools
from collections.abc import Callable, Iterable, Iterator
from typing import Protocol, Self, TypeVar


class SupportsLessThan(Protocol):
    def __lt__(self, other: Self) -> bool:
        raise NotImplementedError()


T = TypeVar("T")
KeyT = TypeVar("KeyT", bound=SupportsLessThan)
GroupT = TypeVar("GroupT")


def groupby(
    it: Iterable[T],
    key_fn: Callable[[T], KeyT],
    group_fn: Callable[[Iterable[T]], GroupT] = lambda x: x,
) -> Iterator[tuple[KeyT, GroupT]]:
    for k, g in itertools.groupby(sorted(it, key=key_fn), key=key_fn):
        yield k, group_fn(g)