#!/usr/bin/env python3
"""Verify every relative link, anchor and image path in a folder of Markdown.

Catches the two things that break most often after a transcription pass:
image paths that point at figures which were never extracted, and TOC anchors
that no longer match a heading because the heading was reworded.

Anchor slugs follow GitHub's rules: lowercase, strip punctuation except
hyphens/underscores, spaces to hyphens.

Usage:
    check-links.py [DIR]        # defaults to ./chapters

Exits non-zero if anything is broken, so it can gate a commit.
"""

from __future__ import annotations

import re
import sys
from pathlib import Path

LINK = re.compile(r"\[([^\]]*)\]\(([^)]+)\)")
HEADING = re.compile(r"^#{1,6} +(.*)$")


def anchor(heading: str) -> str:
    s = heading.strip().lower().replace("`", "")
    s = re.sub(r"[^\w\s-]", "", s, flags=re.UNICODE)
    return re.sub(r"\s+", "-", s.strip())


def main() -> int:
    root = Path(sys.argv[1] if len(sys.argv) > 1 else "chapters")
    if not root.is_dir():
        print(f"not a directory: {root}")
        return 2

    md_files = sorted(root.glob("*.md"))
    anchors: dict[str, set[str]] = {}
    for f in md_files:
        seen: dict[str, int] = {}
        slugs: set[str] = set()
        for line in f.read_text().splitlines():
            m = HEADING.match(line)
            if not m:
                continue
            a = anchor(m.group(1))
            n = seen.get(a, 0)
            seen[a] = n + 1
            # GitHub disambiguates repeated headings by appending -1, -2, ...
            slugs.add(a if n == 0 else f"{a}-{n}")
        anchors[f.name] = slugs

    bad = 0
    linked_images: set[Path] = set()

    for f in md_files:
        for _text, target in LINK.findall(f.read_text()):
            if target.startswith(("http://", "https://", "mailto:", "#")):
                continue
            rel, _, frag = target.partition("#")
            if not rel:
                continue
            resolved = (f.parent / rel).resolve()
            if not resolved.exists():
                print(f"{f.name}: MISSING FILE  {target}")
                bad += 1
                continue
            if resolved.suffix.lower() == ".png":
                linked_images.add(resolved)
            if frag and resolved.suffix == ".md":
                if frag not in anchors.get(resolved.name, set()):
                    print(f"{f.name}: MISSING ANCHOR  #{frag} in {rel}")
                    bad += 1

    for img in sorted(root.rglob("*.png")):
        if img.resolve() not in linked_images:
            print(f"ORPHAN IMAGE: {img.relative_to(root)}")
            bad += 1

    print(f"\n{len(md_files)} file(s) checked, {bad} problem(s)")
    return 1 if bad else 0


if __name__ == "__main__":
    sys.exit(main())