"""Local reading-progress storage.

Positions are stored as the same "blockIndex:innerFraction" anchor string the
server uses (books/models.py, EBookProgress.save_progress) — see
diora_tui/blocks.py for how blockIndex is computed to match static/js/app.js
exactly, and anchor_is_further() for the identical "furthest wins" comparison
used server-side (_progress_is_further in books/views.py). A saved position
only ever advances (unless forced), so an older/offline run can't regress a
further-along read position — matching that same rule locally.
"""

from __future__ import annotations

import json
import time
from dataclasses import dataclass
from pathlib import Path

from platformdirs import user_data_dir

from .blocks import anchor_is_further

_DATA_DIR = Path(user_data_dir("diora-tui", "diora"))
_PROGRESS_FILE = _DATA_DIR / "progress.json"


@dataclass
class Position:
    anchor: str  # "blockIndex:innerFraction", e.g. "42:0.500000"
    updated_at: float = 0.0


def _load_all() -> dict[str, dict]:
    if not _PROGRESS_FILE.exists():
        return {}
    try:
        return json.loads(_PROGRESS_FILE.read_text())
    except (json.JSONDecodeError, OSError):
        return {}


def _save_all(data: dict[str, dict]) -> None:
    _DATA_DIR.mkdir(parents=True, exist_ok=True)
    _PROGRESS_FILE.write_text(json.dumps(data, indent=2))


def load(book_id: str) -> Position | None:
    entry = _load_all().get(book_id)
    if not isinstance(entry, dict) or not entry.get("anchor"):
        return None
    return Position(anchor=entry["anchor"], updated_at=entry.get("updated_at", 0.0))


def save(book_id: str, position: Position, *, force: bool = False) -> bool:
    """Returns True if the position was actually written (i.e. it was further
    along, or forced) — callers that push to the server only need to do so
    when this returns True."""
    data = _load_all()
    existing = data.get(book_id)
    if existing is not None and not force:
        old_anchor = existing.get("anchor", "")
        if not anchor_is_further(position.anchor, old_anchor):
            return False
    if not position.updated_at:
        position.updated_at = time.time()
    data[book_id] = {"anchor": position.anchor, "updated_at": position.updated_at}
    _save_all(data)
    return True