def build(opt):
    """Main driver."""
    files = _find_files(opt)
    markdown, others = _separate_files(files)
    _handle_markdown(opt, markdown)
    _handle_others(opt, others)

def construct_parser(parser):
    """Parse command-line arguments."""
    parser.add_argument("--dst", type=Path, default="docs", help="output directory")
    parser.add_argument("--src", type=Path, default=".", help="source directory")
    parser.add_argument(
        "--templates", type=Path, default="templates", help="templates directory"
    )

def _do_bibliography_links(opt, dest, doc):
    """Handle 'b:' bibliography links."""
    for node in doc.select("a[href]"):
        if not node["href"].startswith("b:"):
            continue
        assert node["href"].count(":") == 1
        key = node["href"].split(":")[1]
        node.string = key
        node["href"] = _make_root_prefix(opt, dest) + f"bibliography/#{key}"

def _do_glossary_links(opt, dest, doc):
    """Handle 'g:' glossary links."""
    for node in doc.select("a[href]"):
        if not node["href"].startswith("g:"):
            continue
        assert node["href"].count(":") == 1
        key = node["href"].split(":")[1]
        node["href"] = _make_root_prefix(opt, dest) + f"glossary/#{key}"

def _do_root_links(opt, dest, doc):
    """Handle '@/' links."""
    prefix = _make_root_prefix(opt, dest)
    targets = (
        ("a[href]", "href"),
        ("img[src]", "src"),
        ("link[href]", "href"),
        ("script[src]", "src"),
    )
    for selector, attr in targets:
        for node in doc.select(selector):
            if "@/" in node[attr]:
                node[attr] = node[attr].replace("@/", prefix)

def _do_title(opt, dest, doc):
    """Make sure title element is filled in."""
    if doc.title is None:
        _warn(f"{dest} does not have <title> element")
    try:
        doc.title.string = doc.h1.get_text()
    except Exception:
        _warn(f"{dest} lacks H1 heading")

def _find_files(opt):
    """Collect all interesting files."""
    return [
        path for path in Path(opt.src).glob("**/*.*") if _is_interesting_file(opt, path)
    ]

def _handle_markdown(opt, files):
    """Handle Markdown files."""
    env = Environment(loader=FileSystemLoader(opt.templates))
    for source in files:
        dest = _make_output_path(opt, source)
        html = _render_markdown(opt, env, source, dest)
        dest.write_text(html)

def _handle_others(opt, files):
    """Handle copy-only files."""
    for source in files:
        dest = _make_output_path(opt, source)
        content = source.read_bytes()
        dest.write_bytes(content)

def _is_interesting_file(opt, path):
    """Is this file worth copying over?"""
    if not path.is_file():
        return False
    if str(path).startswith("."):
        return False
    if str(path.parent.name).startswith("."):
        return False
    if path.is_relative_to(opt.dst):
        return False
    if path.is_relative_to(opt.templates):
        return False
    return True

def _make_output_path(opt, source):
    """Build output path."""
    if source.suffix == ".md":
        temp = source.with_suffix("").with_suffix(".html")
    else:
        temp = source
    result = opt.dst / temp.relative_to(opt.src)
    result.parent.mkdir(parents=True, exist_ok=True)
    return result

def _make_root_prefix(opt, path):
    """Create prefix to root for path."""
    relative = path.relative_to(opt.dst)
    depth = len(relative.parents) - 1
    assert depth >= 0
    return "./" if (depth == 0) else "../" * depth

def _render_markdown(opt, env, source, dest):
    """Convert Markdown to HTML."""
    content = source.read_text()
    template = env.get_template("page.html")
    raw_html = markdown(content, extensions=MARKDOWN_EXTENSIONS)
    rendered_html = template.render(content=raw_html)

    doc = BeautifulSoup(rendered_html, "html.parser")
    for func in [_do_bibliography_links, _do_glossary_links, _do_root_links, _do_title]:
        func(opt, dest, doc)

    return str(doc)

def _separate_files(files):
    """Divide files into categories."""
    markdown = []
    others = []
    for path in files:
        if path.suffix == ".md":
            markdown.append(path)
        else:
            others.append(path)
    return markdown, others

def _warn(msg):
    """Print warning."""
    print(msg, file=sys.stderr)