"""On-disk cache for the (FlatBook, BookLayout) pair a book open computes —
extraction + row-layout for very large books (tens of thousands of blocks)
can take several seconds each; reopening the same book at the same terminal
width should be near-instant instead of paying that cost again every time.

Not a correctness-critical cache: any miss (new book, different width, edited
file) just falls back to recomputing from scratch, so a stale/corrupt cache
entry is handled by overwriting it, never by crashing the reader.
"""

from __future__ import annotations

import hashlib
import pickle
from pathlib import Path

from platformdirs import user_cache_dir

from .blocks import FlatBook
from .layout import BookLayout

_CACHE_DIR = Path(user_cache_dir("diora-tui", "diora")) / "layout_cache"


def _cache_key(path: Path, width: int) -> str:
    stat = path.stat()
    raw = f"{path.resolve()}|{stat.st_size}|{stat.st_mtime_ns}|{width}"
    return hashlib.sha256(raw.encode()).hexdigest()[:32]


def load(path: Path, width: int) -> tuple[FlatBook, BookLayout] | None:
    cache_file = _CACHE_DIR / f"{_cache_key(path, width)}.pickle"
    if not cache_file.exists():
        return None
    try:
        with cache_file.open("rb") as f:
            book, book_layout = pickle.load(f)
        if not isinstance(book, FlatBook) or not isinstance(book_layout, BookLayout):
            return None
        return book, book_layout
    except Exception:
        return None


def save(path: Path, width: int, book: FlatBook, book_layout: BookLayout) -> None:
    _CACHE_DIR.mkdir(parents=True, exist_ok=True)
    cache_file = _CACHE_DIR / f"{_cache_key(path, width)}.pickle"
    try:
        with cache_file.open("wb") as f:
            pickle.dump((book, book_layout), f, protocol=pickle.HIGHEST_PROTOCOL)
    except Exception:
        pass  # best-effort — a failed cache write shouldn't break reading