/* ============================================================================
   TRANSLATOR
   ----------------------------------------------------------------------------
   ~2 KB of dependency-free lookup: a dotted key path into the JSON catalogue
   plus {placeholder} interpolation. No i18next, no ICU MessageFormat - the
   catalogues hold UI chrome only, page prose comes from Strapi.

   English is the reference catalogue: `Messages` is derived from en.json, and
   ar.json is typechecked against it, so a key added on one side and forgotten
   on the other is a compile error rather than a blank label in production.
   ========================================================================= */

import en from "./messages/en.json";
import ar from "./messages/ar.json";

import { DEFAULT_LOCALE, type Locale } from "./config";

/** The shape of every catalogue. Derived from the English source of truth. */
export type Messages = typeof en;

/** Anything that may be interpolated into a message. */
export type TranslationParams = Readonly<
  Record<string, string | number | undefined>
>;

/**
 * Every valid dotted key path in the catalogue, e.g. "nav.sections.about".
 * Keys that address a nested object rather than a string are not assignable,
 * so `t("nav")` does not typecheck.
 */
export type MessageKey<T> = T extends string
  ? never
  : {
      [K in Extract<keyof T, string>]: T[K] extends string
        ? K
        : `${K}.${MessageKey<T[K]>}`;
    }[Extract<keyof T, string>];

export type TranslationKey = MessageKey<Messages>;

/** A bound translator. Stable per locale - see `useTranslation()`. */
export type Translator = (
  key: TranslationKey,
  params?: TranslationParams,
) => string;

/* The `Messages` annotation is the parity check: ar.json must supply every
   key en.json declares, with a string in every leaf position. */
const arabicMessages: Messages = ar;

const CATALOGUES: Readonly<Record<Locale, Messages>> = {
  en,
  ar: arabicMessages,
};

/** The whole catalogue for a locale - handy for spreading into a component. */
export function getMessages(locale: Locale): Messages {
  return CATALOGUES[locale] ?? CATALOGUES[DEFAULT_LOCALE];
}

/** Walk a dotted path. Returns undefined for a missing or non-string leaf. */
function lookup(catalogue: Messages, key: string): string | undefined {
  let current: unknown = catalogue;

  for (const segment of key.split(".")) {
    if (typeof current !== "object" || current === null) return undefined;
    current = (current as Record<string, unknown>)[segment];
  }

  return typeof current === "string" ? current : undefined;
}

const PLACEHOLDER = /\{(\w+)\}/g;

/**
 * Replace every `{name}` with `params.name`. An unknown or undefined
 * placeholder is left in place rather than replaced with "undefined", so a
 * missing value is visible in review instead of silently shipping.
 */
export function interpolate(
  template: string,
  params?: TranslationParams,
): string {
  if (!params) return template;

  return template.replace(PLACEHOLDER, (match, name: string) => {
    const value = params[name];
    return value === undefined ? match : String(value);
  });
}

/**
 * Translate one key. Falls back to the English string when a locale is
 * missing it, and finally to the key itself so a page never renders "".
 */
export function translate(
  locale: Locale,
  key: TranslationKey,
  params?: TranslationParams,
): string {
  const template =
    lookup(getMessages(locale), key) ??
    lookup(CATALOGUES[DEFAULT_LOCALE], key);

  if (template === undefined) {
    if (import.meta.env?.DEV) {
      console.warn(`[i18n] missing translation for "${key}" (${locale})`);
    }
    return key;
  }

  return interpolate(template, params);
}

/** A translator bound to one locale. Cheap - safe to call in a `useMemo`. */
export function createTranslator(locale: Locale): Translator {
  return (key, params) => translate(locale, key, params);
}