#!/usr/bin/env python3
"""Read SUMMARY.md as the only chapter ordering source."""

from __future__ import annotations

import argparse
import os
import re
import sys
from pathlib import Path

LINK_RE = re.compile(r"^\s*-\s+\[[^]]+\]\(([^)]+\.md)\)\s*$")


def default_book_dir() -> Path:
    env = os.environ.get("BOOK_DIR")
    if env:
        return Path(env)
    return Path.cwd()


def chapter_paths(book_dir: Path) -> list[Path]:
    summary = book_dir / "SUMMARY.md"
    paths: list[Path] = []
    seen: set[Path] = set()
    for line_number, line in enumerate(summary.read_text(encoding="utf-8").splitlines(), 1):
        match = LINK_RE.match(line)
        if not match:
            continue
        relative = Path(match.group(1))
        if relative.is_absolute() or ".." in relative.parts:
            raise ValueError(f"SUMMARY.md:{line_number}: chapter path must stay inside the book: {relative}")
        path = (book_dir / relative).resolve()
        if path in seen:
            raise ValueError(f"SUMMARY.md:{line_number}: duplicate chapter: {relative}")
        if not path.is_file():
            raise ValueError(f"SUMMARY.md:{line_number}: missing chapter: {relative}")
        seen.add(path)
        paths.append(path)
    if not paths:
        raise ValueError("SUMMARY.md contains no Markdown chapter links")
    return paths


def main() -> int:
    parser = argparse.ArgumentParser()
    parser.add_argument("--book-dir", type=Path, default=default_book_dir())
    parser.add_argument("--null", action="store_true", help="terminate paths with NUL")
    args = parser.parse_args()
    try:
        paths = chapter_paths(args.book_dir.resolve())
    except (OSError, ValueError) as error:
        print(error, file=sys.stderr)
        return 1
    separator = "\0" if args.null else "\n"
    sys.stdout.write(separator.join(str(path) for path in paths) + separator)
    return 0


if __name__ == "__main__":
    raise SystemExit(main())