from __future__ import annotations

import json
from datetime import UTC, datetime
from pathlib import Path


class Checkpoint:
    """Per-account cursor persistence.

    File shape: `{"cursor": "...", "updated_at": "2026-04-17T12:00:00+00:00"}`
    One file per (provider, account_id) at `<dir>/<provider>-<account_id>.json`.
    """

    def __init__(self, directory: Path, *, provider: str, account_id: str) -> None:
        self._path = directory / f"{provider}-{account_id}.json"

    def load(self) -> str | None:
        if not self._path.exists():
            return None
        raw = json.loads(self._path.read_text())
        cursor = raw.get("cursor")
        if not isinstance(cursor, str):
            return None
        return cursor

    def save(self, cursor: str) -> None:
        self._path.parent.mkdir(parents=True, exist_ok=True)
        payload = {"cursor": cursor, "updated_at": datetime.now(UTC).isoformat()}
        self._path.write_text(json.dumps(payload))