"""Locale detection and management for i18n support.

This module provides locale detection from Accept-Language headers and cookies,
following the same middleware pattern as the flash message system.
"""

from fastapi import Request

from app.infrastructure.i18n.translator import DEFAULT_LOCALE, SUPPORTED_LOCALES

LOCALE_COOKIE_NAME = "locale"
SUPPORTED_LOCALES_SET: frozenset[str] = SUPPORTED_LOCALES


def _parse_accept_language(header: str) -> list[str]:
    """Parse Accept-Language header into ordered list of locale codes.

    Args:
        header: Raw Accept-Language header value.

    Returns:
        List of locale codes in preference order, with region subtags removed.
    """
    if not header:
        return []

    locales: list[str] = []
    for part in header.split(","):
        part = part.strip()
        if not part:
            continue
        locale = part.split(";")[0].strip().split("-")[0]
        if locale:
            locales.append(locale)
    return locales


def _get_best_locale(request: Request) -> str:
    """Detect the best locale for the current request.

    Priority order: cookie → Accept-Language header → default.

    Args:
        request: FastAPI request object.

    Returns:
        Best matching locale code, defaulting to ``en``.
    """
    cookie_locale = request.cookies.get(LOCALE_COOKIE_NAME)
    if cookie_locale and cookie_locale in SUPPORTED_LOCALES_SET:
        return cookie_locale

    accept_language = request.headers.get("accept-language", "")
    for lang in _parse_accept_language(accept_language):
        if lang in SUPPORTED_LOCALES_SET:
            return lang

    return DEFAULT_LOCALE


async def setup_locale_manager(request: Request) -> None:
    """Set the detected locale on request state.

    Called early in the request lifecycle so that route handlers and
    template rendering can access the current locale via
    ``request.state.locale``.

    Args:
        request: FastAPI request object.
    """
    if not hasattr(request.state, "locale"):
        request.state.locale = _get_best_locale(request)