"""Per-device preferences: theme, language and German form of address (AD-15).

They live in cookies and are never stored on the server. Values are checked against fixed
lists, so a forged cookie can only fall back to the default.
"""

from __future__ import annotations

from dataclasses import dataclass

from django.http import HttpRequest, HttpResponse
from django.utils import translation

THEME_COOKIE = "pm_theme"
LANGUAGE_COOKIE = "pm_lang"
FORM_COOKIE = "pm_form"
COOKIE_MAX_AGE = 365 * 24 * 60 * 60

THEMES = ("light", "dark", "system")
LANGUAGES = ("de", "en")
FORMS = ("sie", "du")

DEFAULT_THEME = "system"
DEFAULT_FORM = "sie"
INFORMAL_GERMAN = "de-du"  # catalogue with only the informal differences; falls back to "de"

_CHOICES = {
    THEME_COOKIE: ("theme", THEMES),
    LANGUAGE_COOKIE: ("language", LANGUAGES),
    FORM_COOKIE: ("form", FORMS),
}


@dataclass(frozen=True)
class Preferences:
    theme: str = DEFAULT_THEME
    language: str = "en"
    form: str = DEFAULT_FORM

    @property
    def language_code(self) -> str:
        """The code the translations are activated with."""
        if self.language == "de" and self.form == "du":
            return INFORMAL_GERMAN
        return self.language


def from_request(request: HttpRequest) -> Preferences:
    theme = request.COOKIES.get(THEME_COOKIE)
    language = request.COOKIES.get(LANGUAGE_COOKIE)
    form = request.COOKIES.get(FORM_COOKIE)
    if language not in LANGUAGES:
        # Default from Accept-Language: German or English, English otherwise.
        language = translation.get_language_from_request(request, check_path=False)
        language = language if language in LANGUAGES else "en"
    return Preferences(
        theme=theme if theme in THEMES else DEFAULT_THEME,
        language=language,
        form=form if form in FORMS else DEFAULT_FORM,
    )


def chosen(values) -> dict[str, str]:
    """The cookie name and value for every valid preference in `values` (a POST or a dict)."""
    result = {}
    for cookie, (field, allowed) in _CHOICES.items():
        value = values.get(field)
        if value in allowed:
            result[cookie] = value
    return result


def set_cookies(response: HttpResponse, cookies: dict[str, str], *, secure: bool) -> None:
    for name, value in cookies.items():
        response.set_cookie(
            name,
            value,
            max_age=COOKIE_MAX_AGE,
            secure=secure,
            httponly=False,  # shell.js applies a change without a reload
            samesite="Lax",
        )