"""Per-instance sync state.

Tracks the snapshot id this pack was last synced to and when. Lives at
``<pack-folder>/.cloud-sync/state.json`` (mode 600).

Purpose: divergence detection. On ``pull``, if the remote latest id
differs from ``last_pulled_snapshot_id`` AND any in-scope local file
has mtime > ``last_pulled_at``, the local and remote diverged from a
common ancestor — surface the conflict dialog.
"""

from __future__ import annotations

import json
from dataclasses import dataclass
from datetime import datetime, timezone
from pathlib import Path


SCHEMA_VERSION = 2


@dataclass(frozen=True)
class State:
    instance_id: str
    last_pulled_snapshot_id: str
    last_pulled_at: datetime
    host_tag: str = "instance-sync"


def state_path(pack_folder: Path) -> Path:
    return pack_folder / ".cloud-sync" / "state.json"


def read(pack_folder: Path) -> State | None:
    """Return parsed state or None if file missing / unreadable / wrong schema."""
    p = state_path(pack_folder)
    if not p.exists():
        return None
    try:
        data = json.loads(p.read_text(encoding="utf-8"))
    except (OSError, json.JSONDecodeError):
        return None
    if data.get("schema") != SCHEMA_VERSION:
        return None
    try:
        return State(
            instance_id=data["instance_id"],
            last_pulled_snapshot_id=data["last_pulled_snapshot_id"],
            last_pulled_at=_parse_iso(data["last_pulled_at"]),
            host_tag=data.get("host_tag", "instance-sync"),
        )
    except (KeyError, ValueError):
        return None


def write(pack_folder: Path, state: State) -> None:
    """Persist state. Creates parent dir + sets mode 600."""
    p = state_path(pack_folder)
    p.parent.mkdir(parents=True, exist_ok=True)
    payload = {
        "schema": SCHEMA_VERSION,
        "instance_id": state.instance_id,
        "last_pulled_snapshot_id": state.last_pulled_snapshot_id,
        "last_pulled_at": state.last_pulled_at.astimezone(timezone.utc)
        .isoformat()
        .replace("+00:00", "Z"),
        "host_tag": state.host_tag,
    }
    p.write_text(json.dumps(payload, indent=2) + "\n", encoding="utf-8")
    p.chmod(0o600)


def clear(pack_folder: Path) -> None:
    """Remove state.json if present. Used when remote has zero snapshots."""
    p = state_path(pack_folder)
    p.unlink(missing_ok=True)


def _parse_iso(s: str) -> datetime:
    """Parse ISO-8601 with trailing Z or +HH:MM, return tz-aware UTC."""
    if s.endswith("Z"):
        s = s[:-1] + "+00:00"
    dt = datetime.fromisoformat(s)
    if dt.tzinfo is None:
        dt = dt.replace(tzinfo=timezone.utc)
    return dt.astimezone(timezone.utc)