"""Local reading-progress storage.

Mirrors diora's web-reader "furthest wins" semantics (see books/models.py,
EBookProgress.save_progress): a saved position only ever advances, so an
older or offline run can't regress a further-along read position. The
position is kept anchor-shaped (chapter index + fraction within it) so a
future sync layer against diora's API can adopt it without a rewrite.
"""

from __future__ import annotations

import json
from dataclasses import asdict, dataclass
from pathlib import Path

from platformdirs import user_data_dir

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


@dataclass
class Position:
    chapter_index: int
    scroll_fraction: float  # 0..1 within the chapter's paragraph list


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 entry is None:
        return None
    return Position(chapter_index=entry["chapter_index"], scroll_fraction=entry["scroll_fraction"])


def _is_further(new: Position, old: Position) -> bool:
    if new.chapter_index != old.chapter_index:
        return new.chapter_index > old.chapter_index
    return new.scroll_fraction > old.scroll_fraction


def save(book_id: str, position: Position, *, force: bool = False) -> None:
    data = _load_all()
    existing = data.get(book_id)
    if existing is not None and not force:
        old = Position(chapter_index=existing["chapter_index"], scroll_fraction=existing["scroll_fraction"])
        if not _is_further(position, old):
            return
    data[book_id] = asdict(position)
    _save_all(data)