import dataclasses
import datetime
from collections.abc import Iterator, Sequence

HIDDEN_LABEL_CATEGORY = '__'
DEFAULT_ESTIMATED_DURATION = datetime.timedelta(hours=1)


@dataclasses.dataclass(frozen=True, order=True)
class Label:
    category: str
    label: str

    def __post_init__(self):
        assert self.category is not None
        assert ':' not in self.category
        assert self.label is not None


@dataclasses.dataclass(frozen=True, order=True)
class ActivitySample:
    labels: Sequence[Label]
    start_at: datetime.datetime | None
    end_at: datetime.datetime | None


@dataclasses.dataclass(frozen=True, order=True)
class RealizedActivitySample(ActivitySample):
    start_at: datetime.datetime
    end_at: datetime.datetime


def heuristically_realize_samples(
    samples: list[ActivitySample],
) -> Iterator[RealizedActivitySample]:
    """Secret sauce.

    Guarentees that:
    * No samples overlap.
    """

    previous_sample_end = None
    for sample in samples:
        end_at = sample.end_at

        if previous_sample_end is None:
            if end_at.tzinfo:
                previous_sample_end = datetime.datetime.fromtimestamp(0, datetime.UTC)
            else:
                previous_sample_end = datetime.datetime.fromtimestamp(0)

        assert previous_sample_end <= end_at, 'Iterating in incorrect order'

        # TODO: Allow end_at is None

        start_at = sample.start_at
        if start_at is None:
            estimated_duration: datetime.timedelta = DEFAULT_ESTIMATED_DURATION
            start_at = max(previous_sample_end, end_at - estimated_duration)
            del estimated_duration

        yield RealizedActivitySample(
            labels=sample.labels,
            end_at=end_at,
            start_at=start_at,
        )

        previous_sample_end = sample.end_at
        del sample