from collections.abc import Mapping
from datetime import datetime, timezone
from typing import Optional

from langgraph.channels.base import BaseChannel
from langgraph.checkpoint.base import Checkpoint
from langgraph.checkpoint.base.id import uuid6
from langgraph.constants import MISSING

LATEST_VERSION = 3


def empty_checkpoint() -> Checkpoint:
    return Checkpoint(
        v=LATEST_VERSION,
        id=str(uuid6(clock_seq=-2)),
        ts=datetime.now(timezone.utc).isoformat(),
        channel_values={},
        channel_versions={},
        versions_seen={},
        pending_sends=[],
    )


def create_checkpoint(
    checkpoint: Checkpoint,
    channels: Optional[Mapping[str, BaseChannel]],
    step: int,
    *,
    id: Optional[str] = None,
) -> Checkpoint:
    """Create a checkpoint for the given channels."""
    ts = datetime.now(timezone.utc).isoformat()
    if channels is None:
        values = checkpoint["channel_values"]
    else:
        values = {}
        for k in channels:
            if k not in checkpoint["channel_versions"]:
                continue
            v = channels[k].checkpoint()
            if v is not MISSING:
                values[k] = v
    return Checkpoint(
        v=LATEST_VERSION,
        ts=ts,
        id=id or str(uuid6(clock_seq=step)),
        channel_values=values,
        channel_versions=checkpoint["channel_versions"],
        versions_seen=checkpoint["versions_seen"],
        pending_sends=checkpoint.get("pending_sends", []),
    )