from __future__ import annotations

import sys
from collections import deque
from collections.abc import Callable, Hashable, Sequence
from dataclasses import asdict, dataclass
from typing import (
    TYPE_CHECKING,
    Any,
    ClassVar,
    Generic,
    Literal,
    NamedTuple,
    TypeVar,
    final,
)
from warnings import warn

from langchain_core.messages import AnyMessage
from langchain_core.runnables import Runnable, RunnableConfig
from langgraph.checkpoint.base import BaseCheckpointSaver, CheckpointMetadata
from typing_extensions import NotRequired, TypeAliasType, TypedDict, Unpack, deprecated
from xxhash import xxh3_128_hexdigest

from langgraph._internal._cache import default_cache_key
from langgraph._internal._constants import INTERRUPT as _INTERRUPT_KEY
from langgraph._internal._fields import get_cached_annotated_keys, get_update_as_tuples
from langgraph._internal._retry import default_retry_on
from langgraph._internal._typing import MISSING, DeprecatedKwargs
from langgraph.warnings import LangGraphDeprecatedSinceV10, LangGraphDeprecatedSinceV11

# Local TypeVars for generic stream TypedDicts.
# We use separate TypeVars here (rather than importing from langgraph.typing)
# because the typing module TypeVars have defaults that cause mypy issues
# when used in standalone type aliases.
StateT = TypeVar("StateT")
OutputT = TypeVar("OutputT")

if TYPE_CHECKING:
    from langgraph.pregel.protocol import PregelProtocol


try:
    from langchain_core.messages.tool import ToolOutputMixin
except ImportError:

    class ToolOutputMixin:  # type: ignore[no-redef]
        pass


__all__ = (
    "All",
    "Checkpointer",
    "StreamMode",
    "StreamWriter",
    "StreamPart",
    "ValuesStreamPart",
    "UpdatesStreamPart",
    "MessagesStreamPart",
    "CustomStreamPart",
    "CheckpointStreamPart",
    "TasksStreamPart",
    "DebugStreamPart",
    "TaskPayload",
    "TaskResultPayload",
    "CheckpointTask",
    "CheckpointPayload",
    "DebugPayload",
    "RetryPolicy",
    "CachePolicy",
    "Interrupt",
    "StateUpdate",
    "PregelTask",
    "PregelExecutableTask",
    "StateSnapshot",
    "Send",
    "Command",
    "Durability",
    "interrupt",
    "Overwrite",
    "GraphOutput",
    "ensure_valid_checkpointer",
)

Durability = Literal["sync", "async", "exit"]
"""Durability mode for the graph execution.

- `'sync'`: Changes are persisted synchronously before the next step starts.
- `'async'`: Changes are persisted asynchronously while the next step executes.
- `'exit'`: Changes are persisted only when the graph exits.
"""

All = Literal["*"]
"""Special value to indicate that graph should interrupt on all nodes."""

Checkpointer = None | bool | BaseCheckpointSaver
"""Type of the checkpointer to use for a subgraph.

- `True` enables persistent checkpointing for this subgraph.
- `False` disables checkpointing, even if the parent graph has a checkpointer.
- `None` inherits checkpointer from the parent graph.
"""


def ensure_valid_checkpointer(checkpointer: Checkpointer) -> Checkpointer:
    if checkpointer not in (None, True, False) and not isinstance(
        checkpointer, BaseCheckpointSaver
    ):
        raise TypeError(
            "Invalid checkpointer provided. Expected an instance of "
            "`BaseCheckpointSaver`, `True`, `False`, or `None`. "
            f"Received {type(checkpointer).__name__!s}. "
            "Pass a proper saver (e.g., InMemorySaver, AsyncPostgresSaver)."
        )
    return checkpointer


StreamMode = Literal[
    "values", "updates", "checkpoints", "tasks", "debug", "messages", "custom"
]
"""How the stream method should emit outputs.

- `"values"`: Emit all values in the state after each step, including interrupts.
    When used with functional API, values are emitted once at the end of the workflow.
- `"updates"`: Emit only the node or task names and updates returned by the nodes or tasks after each step.
    If multiple updates are made in the same step (e.g. multiple nodes are run) then those updates are emitted separately.
- `"custom"`: Emit custom data using from inside nodes or tasks using `StreamWriter`.
- `"messages"`: Emit LLM messages token-by-token together with metadata for any LLM invocations inside nodes or tasks.
- `"checkpoints"`: Emit an event when a checkpoint is created, in the same format as returned by `get_state()`.
- `"tasks"`: Emit events when tasks start and finish, including their results and errors.
- `"debug"`: Emit `"checkpoints"` and `"tasks"` events for debugging purposes.
"""

StreamWriter = Callable[[Any], None]
"""`Callable` that accepts a single argument and writes it to the output stream.
Always injected into nodes if requested as a keyword argument, but it's a no-op
when not using `stream_mode="custom"`."""


class TaskPayload(TypedDict):
    """Payload for a task start event."""

    id: str
    """Unique identifier for this task."""
    name: str
    """Name of the node being executed."""
    input: Any
    """Input data passed to the task."""
    triggers: list[str]
    """List of triggers that caused this task to be executed (e.g. channel writes)."""


class TaskResultPayload(TypedDict):
    """Payload for a task result event."""

    id: str
    """Unique identifier for this task."""
    name: str
    """Name of the node that was executed."""
    error: str | None
    """Error message if the task failed, otherwise `None`."""
    interrupts: list[dict]
    """List of interrupts that occurred during task execution."""
    result: dict[str, Any]
    """Mapping of channel names to the values written by this task."""


class CheckpointTask(TypedDict):
    """A task entry within a `CheckpointPayload`.

    The keys present depend on the task's state:

    - **Error:** `id`, `name`, `error`, `state`
    - **Has result:** `id`, `name`, `result`, `interrupts`, `state`
    - **Pending:** `id`, `name`, `interrupts`, `state`
    """

    id: str
    """Unique identifier for this task."""
    name: str
    """Name of the node being executed."""
    error: NotRequired[str]
    """Error message, present only if the task failed."""
    result: NotRequired[Any]
    """Result of the task, present only if the task completed successfully."""
    interrupts: NotRequired[list[dict]]
    """List of interrupts, present when the task has been interrupted or completed."""
    state: StateSnapshot | RunnableConfig | None
    """Snapshot of the subgraph state, or a `RunnableConfig` pointing to it. `None` if not a subgraph."""


class CheckpointPayload(TypedDict, Generic[StateT]):
    """Payload for a checkpoint event."""

    config: RunnableConfig | None
    """Configuration for this checkpoint, including the `thread_id` and `checkpoint_id`."""
    metadata: CheckpointMetadata
    """Metadata associated with this checkpoint (e.g. step number, source, writes)."""
    values: StateT
    """Current state values at the time of this checkpoint."""
    next: list[str]
    """Names of the nodes scheduled to execute next."""
    parent_config: RunnableConfig | None
    """Configuration of the parent checkpoint, or `None` if this is the first checkpoint."""
    tasks: list[CheckpointTask]
    """List of tasks associated with this checkpoint."""


class _DebugCheckpointPayload(TypedDict, Generic[StateT]):
    step: int
    """The step number in the graph execution."""
    timestamp: str
    """ISO 8601 timestamp of when this event occurred."""
    type: Literal["checkpoint"]
    """Event type discriminator, always `"checkpoint"`."""
    payload: CheckpointPayload[StateT]
    """The checkpoint payload."""


class _DebugTaskPayload(TypedDict):
    step: int
    """The step number in the graph execution."""
    timestamp: str
    """ISO 8601 timestamp of when this event occurred."""
    type: Literal["task"]
    """Event type discriminator, always `"task"`."""
    payload: TaskPayload
    """The task start payload."""


class _DebugTaskResultPayload(TypedDict):
    step: int
    """The step number in the graph execution."""
    timestamp: str
    """ISO 8601 timestamp of when this event occurred."""
    type: Literal["task_result"]
    """Event type discriminator, always `"task_result"`."""
    payload: TaskResultPayload
    """The task result payload."""


DebugPayload = TypeAliasType(
    "DebugPayload",
    _DebugCheckpointPayload[StateT] | _DebugTaskPayload | _DebugTaskResultPayload,
    type_params=(StateT,),
)
"""Wrapper payload for debug events. Discriminate on `type`."""


class ValuesStreamPart(TypedDict, Generic[OutputT]):
    """Stream part emitted for `stream_mode="values"`.

    `data` contains the full state after each step, as returned by `read_channels()`.
    """

    type: Literal["values"]
    ns: tuple[str, ...]
    data: OutputT
    interrupts: tuple[Interrupt, ...]


class UpdatesStreamPart(TypedDict):
    """Stream part emitted for `stream_mode="updates"`.

    `data` maps node names to their outputs. May also contain
    `__interrupt__` (tuple of `Interrupt` dicts) and `__metadata__` keys.
    """

    type: Literal["updates"]
    ns: tuple[str, ...]
    data: dict[str, Any]


class MessagesStreamPart(TypedDict):
    """Stream part emitted for `stream_mode="messages"`.

    `data` is a 2-tuple of `(message, metadata)` where `message` is a
    `BaseMessage` (e.g. `AIMessageChunk`) and `metadata` is a dict containing
    keys like `langgraph_step`, `langgraph_node`, `langgraph_triggers`, etc.
    """

    type: Literal["messages"]
    ns: tuple[str, ...]
    data: tuple[AnyMessage, dict[str, Any]]


class CustomStreamPart(TypedDict):
    """Stream part emitted for `stream_mode="custom"`.

    `data` is whatever value was passed to `StreamWriter` inside a node.
    """

    type: Literal["custom"]
    ns: tuple[str, ...]
    data: Any


class CheckpointStreamPart(TypedDict, Generic[StateT]):
    """Stream part emitted for `stream_mode="checkpoints"`."""

    type: Literal["checkpoints"]
    ns: tuple[str, ...]
    data: CheckpointPayload[StateT]


class TasksStreamPart(TypedDict):
    """Stream part emitted for `stream_mode="tasks"`.

    For task start events, `data` is a `TaskPayload` with `id`, `name`,
    `input`, and `triggers` keys.

    For task result events, `data` is a `TaskResultPayload` with `id`,
    `name`, `error`, `interrupts`, and `result` keys.
    """

    type: Literal["tasks"]
    ns: tuple[str, ...]
    data: TaskPayload | TaskResultPayload


class DebugStreamPart(TypedDict, Generic[StateT]):
    """Stream part emitted for `stream_mode="debug"`."""

    type: Literal["debug"]
    ns: tuple[str, ...]
    data: DebugPayload[StateT]


StreamPart = TypeAliasType(
    "StreamPart",
    ValuesStreamPart[OutputT]
    | UpdatesStreamPart
    | MessagesStreamPart
    | CustomStreamPart
    | CheckpointStreamPart[StateT]
    | TasksStreamPart
    | DebugStreamPart[StateT],
    type_params=(StateT, OutputT),
)
"""A discriminated union of all v2 stream part types.

Use `part["type"]` to narrow the type:

```python
async for part in graph.astream(input, version="v2"):
    if part["type"] == "values":
        part["data"]  # OutputT — full state (pydantic/dataclass/dict)
    elif part["type"] == "messages":
        part["data"]  # tuple[BaseMessage, dict] — (message, metadata)
    elif part["type"] == "custom":
        part["data"]  # Any — user-defined
```
"""


@dataclass(frozen=True)
class GraphOutput(Generic[OutputT]):
    """Typed container returned by `invoke()` / `ainvoke()` with `version="v2"`.

    Attributes:
        value: The final output of the graph (dict, Pydantic model, dataclass, etc.).
        interrupts: Any interrupts that occurred during execution.
    """

    value: OutputT
    interrupts: tuple[Interrupt, ...] = ()

    def __getitem__(self, key: str) -> Any:
        """Backward compat: `result['__interrupt__']` and dict-key access."""
        warn(
            "Accessing GraphOutput via `result[key]` is deprecated. "
            "Use `result.value` to access the output value directly, "
            "or `result.interrupts` for interrupts.",
            LangGraphDeprecatedSinceV11,
            stacklevel=2,
        )
        if key == _INTERRUPT_KEY:
            return self.interrupts
        if isinstance(self.value, dict):
            return self.value[key]
        try:
            return getattr(self.value, key)
        except AttributeError:
            raise KeyError(key)

    def __contains__(self, key: object) -> bool:
        warn(
            "Accessing GraphOutput via `key in result` is deprecated. "
            "Use `result.value` to access the output value directly, "
            "or `result.interrupts` for interrupts.",
            LangGraphDeprecatedSinceV11,
            stacklevel=2,
        )
        if key == _INTERRUPT_KEY:
            return bool(self.interrupts)
        if isinstance(self.value, dict):
            return key in self.value
        return isinstance(key, str) and hasattr(self.value, key)


_DC_KWARGS = {"kw_only": True, "slots": True, "frozen": True}


class RetryPolicy(NamedTuple):
    """Configuration for retrying nodes.

    !!! version-added "Added in version 0.2.24"
    """

    initial_interval: float = 0.5
    """Amount of time that must elapse before the first retry occurs. In seconds."""
    backoff_factor: float = 2.0
    """Multiplier by which the interval increases after each retry."""
    max_interval: float = 128.0
    """Maximum amount of time that may elapse between retries. In seconds."""
    max_attempts: int = 3
    """Maximum number of attempts to make before giving up, including the first."""
    jitter: bool = True
    """Whether to add random jitter to the interval between retries."""
    retry_on: (
        type[Exception] | Sequence[type[Exception]] | Callable[[Exception], bool]
    ) = default_retry_on
    """List of exception classes that should trigger a retry, or a callable that returns `True` for exceptions that should trigger a retry."""


KeyFuncT = TypeVar("KeyFuncT", bound=Callable[..., str | bytes])


@dataclass(**_DC_KWARGS)
class CachePolicy(Generic[KeyFuncT]):
    """Configuration for caching nodes."""

    key_func: KeyFuncT = default_cache_key  # type: ignore[assignment]
    """Function to generate a cache key from the node's input.
    Defaults to hashing the input with pickle."""

    ttl: int | None = None
    """Time to live for the cache entry in seconds. If `None`, the entry never expires."""


_DEFAULT_INTERRUPT_ID = "placeholder-id"


@final
@dataclass(init=False, slots=True)
class Interrupt:
    """Information about an interrupt that occurred in a node.

    !!! version-added "Added in version 0.2.24"

    !!! version-changed "Changed in version v0.4.0"
        * `interrupt_id` was introduced as a property

    !!! version-changed "Changed in version v0.6.0"

        The following attributes have been removed:

        * `ns`
        * `when`
        * `resumable`
        * `interrupt_id`, deprecated in favor of `id`
    """

    value: Any
    """The value associated with the interrupt."""

    id: str
    """The ID of the interrupt. Can be used to resume the interrupt directly."""

    def __init__(
        self,
        value: Any,
        id: str = _DEFAULT_INTERRUPT_ID,
        **deprecated_kwargs: Unpack[DeprecatedKwargs],
    ) -> None:
        self.value = value

        if (
            (ns := deprecated_kwargs.get("ns", MISSING)) is not MISSING
            and (id == _DEFAULT_INTERRUPT_ID)
            and (isinstance(ns, Sequence))
        ):
            self.id = xxh3_128_hexdigest("|".join(ns).encode())
        else:
            self.id = id

    @classmethod
    def from_ns(cls, value: Any, ns: str) -> Interrupt:
        return cls(value=value, id=xxh3_128_hexdigest(ns.encode()))

    @property
    @deprecated("`interrupt_id` is deprecated. Use `id` instead.", category=None)
    def interrupt_id(self) -> str:
        warn(
            "`interrupt_id` is deprecated. Use `id` instead.",
            LangGraphDeprecatedSinceV10,
            stacklevel=2,
        )
        return self.id


class StateUpdate(NamedTuple):
    values: dict[str, Any] | None
    as_node: str | None = None
    task_id: str | None = None


class PregelTask(NamedTuple):
    """A Pregel task."""

    id: str
    name: str
    path: tuple[str | int | tuple, ...]
    error: Exception | None = None
    interrupts: tuple[Interrupt, ...] = ()
    state: None | RunnableConfig | StateSnapshot = None
    result: Any | None = None


if sys.version_info > (3, 11):
    _T_DC_KWARGS = {"weakref_slot": True, "slots": True, "frozen": True}
else:
    _T_DC_KWARGS = {"frozen": True}


class CacheKey(NamedTuple):
    """Cache key for a task."""

    ns: tuple[str, ...]
    """Namespace for the cache entry."""
    key: str
    """Key for the cache entry."""
    ttl: int | None
    """Time to live for the cache entry in seconds."""


@dataclass(**_T_DC_KWARGS)
class PregelExecutableTask:
    name: str
    input: Any
    proc: Runnable
    writes: deque[tuple[str, Any]]
    config: RunnableConfig
    triggers: Sequence[str]
    retry_policy: Sequence[RetryPolicy]
    cache_key: CacheKey | None
    id: str
    path: tuple[str | int | tuple, ...]
    writers: Sequence[Runnable] = ()
    subgraphs: Sequence[PregelProtocol] = ()


class StateSnapshot(NamedTuple):
    """Snapshot of the state of the graph at the beginning of a step."""

    values: dict[str, Any] | Any
    """Current values of channels."""
    next: tuple[str, ...]
    """The name of the node to execute in each task for this step."""
    config: RunnableConfig
    """Config used to fetch this snapshot."""
    metadata: CheckpointMetadata | None
    """Metadata associated with this snapshot."""
    created_at: str | None
    """Timestamp of snapshot creation."""
    parent_config: RunnableConfig | None
    """Config used to fetch the parent snapshot, if any."""
    tasks: tuple[PregelTask, ...]
    """Tasks to execute in this step. If already attempted, may contain an error."""
    interrupts: tuple[Interrupt, ...]
    """Interrupts that occurred in this step that are pending resolution."""


class Send:
    """A message or packet to send to a specific node in the graph.

    The `Send` class is used within a `StateGraph`'s conditional edges to
    dynamically invoke a node with a custom state at the next step.

    Importantly, the sent state can differ from the core graph's state,
    allowing for flexible and dynamic workflow management.

    One such example is a "map-reduce" workflow where your graph invokes
    the same node multiple times in parallel with different states,
    before aggregating the results back into the main graph's state.

    Attributes:
        node (str): The name of the target node to send the message to.
        arg (Any): The state or message to send to the target node.

    !!! example

        ```python
        from typing import Annotated
        from langgraph.types import Send
        from langgraph.graph import END, START
        from langgraph.graph import StateGraph
        import operator

        class OverallState(TypedDict):
            subjects: list[str]
            jokes: Annotated[list[str], operator.add]

        def continue_to_jokes(state: OverallState):
            return [Send("generate_joke", {"subject": s}) for s in state["subjects"]]

        builder = StateGraph(OverallState)
        builder.add_node("generate_joke", lambda state: {"jokes": [f"Joke about {state['subject']}"]})
        builder.add_conditional_edges(START, continue_to_jokes)
        builder.add_edge("generate_joke", END)
        graph = builder.compile()

        # Invoking with two subjects results in a generated joke for each
        graph.invoke({"subjects": ["cats", "dogs"]})
        # {'subjects': ['cats', 'dogs'], 'jokes': ['Joke about cats', 'Joke about dogs']}
        ```
    """

    __slots__ = ("node", "arg")

    node: str
    arg: Any

    def __init__(self, /, node: str, arg: Any) -> None:
        """
        Initialize a new instance of the `Send` class.

        Args:
            node: The name of the target node to send the message to.
            arg: The state or message to send to the target node.
        """
        self.node = node
        self.arg = arg

    def __hash__(self) -> int:
        return hash((self.node, self.arg))

    def __repr__(self) -> str:
        return f"Send(node={self.node!r}, arg={self.arg!r})"

    def __eq__(self, value: object) -> bool:
        return (
            isinstance(value, Send)
            and self.node == value.node
            and self.arg == value.arg
        )


N = TypeVar("N", bound=Hashable)


@dataclass(**_DC_KWARGS)
class Command(Generic[N], ToolOutputMixin):
    """One or more commands to update the graph's state and send messages to nodes.

    Args:
        graph: Graph to send the command to. Supported values are:

            - `None`: the current graph
            - `Command.PARENT`: closest parent graph
        update: Update to apply to the graph's state.
        resume: Value to resume execution with. To be used together with [`interrupt()`][langgraph.types.interrupt].
            Can be one of the following:

            - Mapping of interrupt ids to resume values
            - A single value with which to resume the next interrupt
        goto: Can be one of the following:

            - Name of the node to navigate to next (any node that belongs to the specified `graph`)
            - Sequence of node names to navigate to next
            - `Send` object (to execute a node with the input provided)
            - Sequence of `Send` objects
    """

    graph: str | None = None
    update: Any | None = None
    resume: dict[str, Any] | Any | None = None
    goto: Send | Sequence[Send | N] | N = ()

    def __repr__(self) -> str:
        # get all non-None values
        contents = ", ".join(
            f"{key}={value!r}" for key, value in asdict(self).items() if value
        )
        return f"Command({contents})"

    def _update_as_tuples(self) -> Sequence[tuple[str, Any]]:
        if isinstance(self.update, dict):
            return list(self.update.items())
        elif isinstance(self.update, (list, tuple)) and all(
            isinstance(t, tuple) and len(t) == 2 and isinstance(t[0], str)
            for t in self.update
        ):
            return self.update
        elif keys := get_cached_annotated_keys(type(self.update)):
            return get_update_as_tuples(self.update, keys)
        elif self.update is not None:
            return [("__root__", self.update)]
        else:
            return []

    PARENT: ClassVar[Literal["__parent__"]] = "__parent__"


def interrupt(value: Any) -> Any:
    """Interrupt the graph with a resumable exception from within a node.

    The `interrupt` function enables human-in-the-loop workflows by pausing graph
    execution and surfacing a value to the client. This value can communicate context
    or request input required to resume execution.

    In a given node, the first invocation of this function raises a `GraphInterrupt`
    exception, halting execution. The provided `value` is included with the exception
    and sent to the client executing the graph.

    A client resuming the graph must use the [`Command`][langgraph.types.Command]
    primitive to specify a value for the interrupt and continue execution.
    The graph resumes from the start of the node, **re-executing** all logic.

    If a node contains multiple `interrupt` calls, LangGraph matches resume values
    to interrupts based on their order in the node. This list of resume values
    is scoped to the specific task executing the node and is not shared across tasks.

    To use an `interrupt`, you must enable a checkpointer, as the feature relies
    on persisting the graph state.

    !!! example

        ```python
        import uuid
        from typing import Optional
        from typing_extensions import TypedDict

        from langgraph.checkpoint.memory import InMemorySaver
        from langgraph.constants import START
        from langgraph.graph import StateGraph
        from langgraph.types import interrupt, Command


        class State(TypedDict):
            \"\"\"The graph state.\"\"\"

            foo: str
            human_value: Optional[str]
            \"\"\"Human value will be updated using an interrupt.\"\"\"


        def node(state: State):
            answer = interrupt(
                # This value will be sent to the client
                # as part of the interrupt information.
                \"what is your age?\"
            )
            print(f\"> Received an input from the interrupt: {answer}\")
            return {\"human_value\": answer}


        builder = StateGraph(State)
        builder.add_node(\"node\", node)
        builder.add_edge(START, \"node\")

        # A checkpointer must be enabled for interrupts to work!
        checkpointer = InMemorySaver()
        graph = builder.compile(checkpointer=checkpointer)

        config = {
            \"configurable\": {
                \"thread_id\": uuid.uuid4(),
            }
        }

        for chunk in graph.stream({\"foo\": \"abc\"}, config):
            print(chunk)

        # > {'__interrupt__': (Interrupt(value='what is your age?', id='45fda8478b2ef754419799e10992af06'),)}

        command = Command(resume=\"some input from a human!!!\")

        for chunk in graph.stream(Command(resume=\"some input from a human!!!\"), config):
            print(chunk)

        # > Received an input from the interrupt: some input from a human!!!
        # > {'node': {'human_value': 'some input from a human!!!'}}
        ```

    Args:
        value: The value to surface to the client when the graph is interrupted.

    Returns:
        Any: On subsequent invocations within the same node (same task to be precise), returns the value provided during the first invocation

    Raises:
        GraphInterrupt: On the first invocation within the node, halts execution and surfaces the provided value to the client.
    """
    from langgraph._internal._constants import (
        CONFIG_KEY_CHECKPOINT_NS,
        CONFIG_KEY_SCRATCHPAD,
        CONFIG_KEY_SEND,
        RESUME,
    )
    from langgraph.config import get_config
    from langgraph.errors import GraphInterrupt

    conf = get_config()["configurable"]
    # track interrupt index
    scratchpad = conf[CONFIG_KEY_SCRATCHPAD]
    idx = scratchpad.interrupt_counter()
    # find previous resume values
    if scratchpad.resume:
        if idx < len(scratchpad.resume):
            conf[CONFIG_KEY_SEND]([(RESUME, scratchpad.resume)])
            return scratchpad.resume[idx]
    # find current resume value
    v = scratchpad.get_null_resume(True)
    if v is not None:
        assert len(scratchpad.resume) == idx, (scratchpad.resume, idx)
        scratchpad.resume.append(v)
        conf[CONFIG_KEY_SEND]([(RESUME, scratchpad.resume)])
        return v
    # no resume value found
    raise GraphInterrupt(
        (
            Interrupt.from_ns(
                value=value,
                ns=conf[CONFIG_KEY_CHECKPOINT_NS],
            ),
        )
    )


@dataclass(slots=True)
class Overwrite:
    """Bypass a reducer and write the wrapped value directly to a `BinaryOperatorAggregate` channel.

    Receiving multiple `Overwrite` values for the same channel in a single super-step
    will raise an `InvalidUpdateError`.

    !!! example

        ```python
        from typing import Annotated
        import operator
        from langgraph.graph import StateGraph
        from langgraph.types import Overwrite

        class State(TypedDict):
            messages: Annotated[list, operator.add]

        def node_a(state: TypedDict):
            # Normal update: uses the reducer (operator.add)
            return {"messages": ["a"]}

        def node_b(state: State):
            # Overwrite: bypasses the reducer and replaces the entire value
            return {"messages": Overwrite(value=["b"])}

        builder = StateGraph(State)
        builder.add_node("node_a", node_a)
        builder.add_node("node_b", node_b)
        builder.set_entry_point("node_a")
        builder.add_edge("node_a", "node_b")
        graph = builder.compile()

        # Without Overwrite in node_b, messages would be ["START", "a", "b"]
        # With Overwrite, messages is just ["b"]
        result = graph.invoke({"messages": ["START"]})
        assert result == {"messages": ["b"]}
        ```
    """

    value: Any
    """The value to write directly to the channel, bypassing any reducer."""
