#!/usr/bin/env python3
"""
Render README.md from README.md.in by expanding @snippet directives.

Directive format (in README.md.in):
  <!-- @snippet path/to/file.cpp snippet_name -->

Snippet tags (in C++ source files):
  // --8<-- [start:snippet_name]
  ...content...
  // --8<-- [end:snippet_name]

In Python files use # instead of //.
"""

import re
import sys
import textwrap
from pathlib import Path

ROOT = Path(__file__).parent.parent
TEMPLATE = ROOT / "README.md.in"
OUTPUT = ROOT / "README.md"

DIRECTIVE_RE = re.compile(r'<!--\s*@snippet\s+(\S+)\s+(\S+)\s*-->')

LANG_MAP = {'.cpp': 'cpp', '.hpp': 'cpp', '.h': 'cpp', '.py': 'python', '.c': 'c'}


def comment_prefix(path: Path) -> str:
    return '#' if path.suffix == '.py' else '//'


def extract_snippet(file_path: Path, name: str) -> str:
    prefix = comment_prefix(file_path)
    open_tag  = f'{prefix} --8<-- [start:{name}]'
    close_tag = f'{prefix} --8<-- [end:{name}]'

    text = file_path.read_text()
    lines = text.splitlines()

    in_snippet = False
    found = False
    result = []

    for line in lines:
        stripped = line.strip()
        if stripped == open_tag:
            if in_snippet:
                raise ValueError(f"Nested open tag for '{name}' in {file_path}")
            in_snippet = True
            found = True
            continue
        if stripped == close_tag:
            if not in_snippet:
                raise ValueError(f"Unexpected close tag for '{name}' in {file_path}")
            in_snippet = False
            continue
        if in_snippet:
            result.append(line)

    if not found:
        raise ValueError(f"Snippet '{name}' not found in {file_path}")
    if in_snippet:
        raise ValueError(f"Unclosed snippet '{name}' in {file_path}")

    # Strip leading/trailing blank lines
    while result and not result[0].strip():
        result.pop(0)
    while result and not result[-1].strip():
        result.pop()

    # Dedent common leading whitespace
    content = textwrap.dedent('\n'.join(result))
    return content


def lang_for(file_path: Path) -> str:
    return LANG_MAP.get(file_path.suffix, '')


def render(template: str) -> str:
    errors = []

    def replace(m: re.Match) -> str:
        rel_path = m.group(1)
        name = m.group(2)
        file_path = ROOT / rel_path
        if not file_path.exists():
            errors.append(f"File not found: {file_path}")
            return m.group(0)
        try:
            content = extract_snippet(file_path, name)
        except ValueError as e:
            errors.append(str(e))
            return m.group(0)
        lang = lang_for(file_path)
        return f'```{lang}\n{content}\n```'

    result = DIRECTIVE_RE.sub(replace, template)

    if errors:
        for err in errors:
            print(f"ERROR: {err}", file=sys.stderr)
        sys.exit(1)

    return result


def main() -> None:
    if not TEMPLATE.exists():
        print(f"Error: {TEMPLATE} not found", file=sys.stderr)
        sys.exit(1)

    rendered = render(TEMPLATE.read_text())
    OUTPUT.write_text(rendered)
    print(f"Rendered {TEMPLATE.name} → {OUTPUT.name}")


if __name__ == '__main__':
    main()