_grouping_latest_value_cache.py

# License: MIT
# Copyright © 2025 Frequenz Energy-as-a-Service GmbH

"""The GroupingLatestValueCache caches the latest values in a receiver grouped by key.

It provides a way to look up on demand, the latest value in a stream for any key, as
long as there has been at least one value received for that key.

[GroupingLatestValueCache][frequenz.channels.experimental.GroupingLatestValueCache]
takes a [Receiver][frequenz.channels.Receiver] and a `key` function as arguments and
stores the latest value received by that receiver for each key separately.

The `GroupingLatestValueCache` implements the [`Mapping`][collections.abc.Mapping]
interface, so it can be used like a dictionary.  In addition, it provides a
[has_value][frequenz.channels.experimental.GroupingLatestValueCache.has_value] method to
check if a value has been received for a specific key, and a
[clear][frequenz.channels.experimental.GroupingLatestValueCache.clear] method to clear
the cached value for a specific key.

Example:
```python
from frequenz.channels import Broadcast
from frequenz.channels.experimental import GroupingLatestValueCache

channel = Broadcast[tuple[int, str]](name="lvc_test")

cache = GroupingLatestValueCache(channel.new_receiver(), key=lambda x: x[0])
sender = channel.new_sender()

assert not cache.has_value(6)

await sender.send((6, "twenty-six"))

assert cache.has_value(6)
assert cache.get(6) == (6, "twenty-six")
```
"""


import asyncio
import typing
from collections.abc import ItemsView, Iterator, KeysView, Mapping, ValuesView

from typing_extensions import override

from .._receiver import Receiver

T_co = typing.TypeVar("T_co", covariant=True)
T = typing.TypeVar("T")
HashableT = typing.TypeVar("HashableT", bound=typing.Hashable)


class GroupingLatestValueCache(Mapping[HashableT, T_co]):
    """A cache that stores the latest value in a receiver, grouped by key."""

    def __init__(
        self,
        receiver: Receiver[T_co],
        *,
        key: typing.Callable[[T_co], HashableT],
        unique_id: str | None = None,
    ) -> None:
        """Create a new cache.

        Args:
            receiver: The receiver to cache values from.
            key: An function that takes a value and returns a key to group the values
                by.
            unique_id: A string to help uniquely identify this instance. If not
                provided, a unique identifier will be generated from the object's
                [`id()`][id]. It is used mostly for debugging purposes.
        """
        self._receiver: Receiver[T_co] = receiver
        self._key: typing.Callable[[T_co], HashableT] = key
        self._unique_id: str = hex(id(self)) if unique_id is None else unique_id
        self._latest_value_by_key: dict[HashableT, T_co] = {}
        self._task: asyncio.Task[None] = asyncio.create_task(
            self._run(), name=f"LatestValueCache«{self._unique_id}»"
        )

    @property
    def unique_id(self) -> str:
        """The unique identifier of this instance."""
        return self._unique_id

    @override
    def keys(self) -> KeysView[HashableT]:
        """Return the set of keys for which values have been received.

        If no key function is provided, this will return an empty set.
        """
        return self._latest_value_by_key.keys()

    @override
    def items(self) -> ItemsView[HashableT, T_co]:
        """Return an iterator over the key-value pairs of the latest values received."""
        return self._latest_value_by_key.items()

    @override
    def values(self) -> ValuesView[T_co]:
        """Return an iterator over the latest values received."""
        return self._latest_value_by_key.values()

    @typing.overload
    def get(self, key: HashableT, default: None = None) -> T_co | None:
        """Return the latest value that has been received for a specific key."""

    # MyPy passes this overload as a valid signature, but pylint does not like it.
    @typing.overload
    def get(  # pylint: disable=signature-differs
        self, key: HashableT, default: T
    ) -> T_co | T:
        """Return the latest value that has been received for a specific key."""

    @override
    def get(self, key: HashableT, default: T | None = None) -> T_co | T | None:
        """Return the latest value that has been received.

        Args:
            key: An optional key to retrieve the latest value for that key. If not
                provided, it retrieves the latest value received overall.
            default: The default value to return if no value has been received yet for
                the specified key. If not provided, it defaults to `None`.

        Returns:
            The latest value that has been received.
        """
        return self._latest_value_by_key.get(key, default)

    @override
    def __iter__(self) -> Iterator[HashableT]:
        """Return an iterator over the keys for which values have been received."""
        return iter(self._latest_value_by_key)

    @override
    def __len__(self) -> int:
        """Return the number of keys for which values have been received."""
        return len(self._latest_value_by_key)

    @override
    def __getitem__(self, key: HashableT) -> T_co:
        """Return the latest value that has been received for a specific key.

        Args:
            key: The key to retrieve the latest value for.

        Returns:
            The latest value that has been received for that key.

        Raises:
            KeyError: If no value has been received yet for that key.
        """
        if key not in self._latest_value_by_key:
            raise KeyError(f"No value received for key: {key!r}")
        return self._latest_value_by_key[key]

    @override
    def __contains__(self, key: object, /) -> bool:
        """Check if a value has been received for a specific key.

        Args:
            key: The key to check for.

        Returns:
            `True` if a value has been received for that key, `False` otherwise.
        """
        return key in self._latest_value_by_key

    @override
    def __eq__(self, other: object, /) -> bool:
        """Check if this cache is equal to another object.

        Two caches are considered equal if they have the same keys and values.

        Args:
            other: The object to compare with.

        Returns:
            `True` if the caches are equal, `False` otherwise.
        """
        if not isinstance(other, GroupingLatestValueCache):
            return NotImplemented
        return self._latest_value_by_key == other._latest_value_by_key

    @override
    def __ne__(self, value: object, /) -> bool:
        """Check if this cache is not equal to another object.

        Args:
            value: The object to compare with.

        Returns:
            `True` if the caches are not equal, `False` otherwise.
        """
        if not isinstance(value, GroupingLatestValueCache):
            return NotImplemented
        return self._latest_value_by_key != value._latest_value_by_key

    def clear(self, key: HashableT) -> None:
        """Clear the latest value for a specific key.

        Args:
            key: The key for which to clear the latest value.
        """
        _ = self._latest_value_by_key.pop(key, None)

    async def stop(self) -> None:
        """Stop the cache."""
        if not self._task.done():
            self._task.cancel()
            try:
                await self._task
            except asyncio.CancelledError:
                pass

    def __repr__(self) -> str:
        """Return a string representation of this cache."""
        return (
            f"<GroupingLatestValueCache num_keys={len(self._latest_value_by_key.keys())}, "
            f"receiver={self._receiver!r}, unique_id={self._unique_id!r}>"
        )

    async def _run(self) -> None:
        async for value in self._receiver:
            key = self._key(value)
            self._latest_value_by_key[key] = value