"""CSRF protection (session-scoped token, no external dependency)."""

import hmac
import logging
import secrets

from flask import abort, request, session

logger = logging.getLogger(__name__)


def get_csrf_token():
    """Return the per-session CSRF token, generating one on first use."""
    token = session.get("_csrf_token")
    if not token:
        token = secrets.token_hex(32)
        session["_csrf_token"] = token
    return token


def validate_csrf():
    """Abort 403 if the form CSRF token doesn't match the session token."""
    expected = session.get("_csrf_token")
    provided = request.form.get("csrf_token", "")
    if not expected or not provided or not hmac.compare_digest(expected, provided):
        logger.warning(
            "CSRF validation failed for %s from %s",
            request.path,
            request.remote_addr,
        )
        abort(403)


def init_app(app):
    """Expose csrf_token() to Jinja templates as a global."""
    app.jinja_env.globals["csrf_token"] = get_csrf_token