/**
 * The languages Wellformed speaks, and the one setting that chooses between
 * them.
 *
 * A locale here is a property of the *workspace*, like its currency: it
 * decides the words in the chrome, the comma inside a number, the order
 * inside a date — the same for everyone who opens it. It briefly lived on the
 * account instead, on the theory that words belong to the reader, but a
 * workspace is full of proper names — tables, columns, views — written in one
 * language, and chrome in another language around them reads like two
 * products taped together. One set of books, one language, one way of writing
 * what is in them.
 *
 * Outside a workspace — the login page, the marketing page, mail to someone
 * locked out — there is nothing to ask, and the browser's language decides.
 */
export const SUPPORTED_LOCALES = ['en', 'nl'] as const;

export type Locale = (typeof SUPPORTED_LOCALES)[number];

/** What a reader gets when neither they nor their browser expressed a wish. */
export const DEFAULT_LOCALE: Locale = 'en';

/**
 * Each language named in itself, because the picker is read by exactly the
 * person who may not understand the language the rest of the page is in.
 */
export const LOCALE_LABELS: Record<Locale, string> = {
  en: 'English',
  nl: 'Nederlands',
};

/**
 * The BCP 47 tag `Intl` formats under for each UI language.
 *
 * Broader than the language alone: 'en' could format as en-GB or en-US and
 * they disagree about dates. Picking one region per language keeps every
 * installation rendering identically instead of drifting with the server's or
 * the browser's own idea of English.
 */
export const FORMAT_LOCALES: Record<Locale, string> = {
  en: 'en-US',
  nl: 'nl-NL',
};

export function isLocale(value: unknown): value is Locale {
  return typeof value === 'string' && (SUPPORTED_LOCALES as readonly string[]).includes(value);
}

/**
 * Pick the best supported locale from an ordered preference list, e.g.
 * `navigator.languages` or a parsed Accept-Language header.
 *
 * Only the primary subtag is compared: 'nl-BE' is somebody who reads Dutch,
 * and telling them "sorry, only nl-NL here" by answering in English would be
 * the wrong kind of precision.
 */
export function resolveLocale(preferences: readonly string[]): Locale {
  for (const pref of preferences) {
    const primary = pref.trim().toLowerCase().split('-')[0];
    if (isLocale(primary)) return primary;
  }
  return DEFAULT_LOCALE;
}