import re
from typing import Any, Dict

import jinja2

# Create a minimal Jinja2 environment for text templates.
# Use StrictUndefined so missing variables raise exceptions during rendering.
ENV = jinja2.Environment(
    undefined=jinja2.StrictUndefined,
    autoescape=False,
)


def _convert_braced_format_to_jinja(template: str) -> str:
    """
    Convert simple `{var}` style placeholders into Jinja `{{ var }}` so legacy templates keep working.
    This will not touch existing `{{ }}` or `{% %}`.
    """
    # Skip conversion if template already contains jinja markers
    if ("{{" in template) or ("{%" in template):
        return template

    # Replace single-brace simple identifiers like {a_b} or {a.b} -> {{ a_b }} or {{ a.b }}
    pattern = re.compile(r'(?<!\{)\{([A-Za-z0-9_\.]+)\}(?!\})')
    return pattern.sub(r'{{ \1 }}', template)


def safe_render(template_str: str, context: Dict[str, Any]) -> str:
    """
    Safely render a template string using Jinja2 with StrictUndefined.
    - Accepts legacy `{var}` placeholders by converting them to Jinja style.
    - Raises jinja2 exceptions when rendering fails.
    """
    if template_str is None:
        raise ValueError("template_str is None")

    tpl = _convert_braced_format_to_jinja(template_str)
    jtpl = ENV.from_string(tpl)
    # Jinja expects normal Python types in context; if context contains Pydantic models,
    # they should be converted by caller to plain dicts.
    return jtpl.render(**context)