* exist. It loads automatically, cannot be deactivated from the admin (which is
 * deliberate: it cannot be switched off by accident), and to remove it you
 * delete the file.
 *
 * ── WHY THIS EXISTS ──────────────────────────────────────────────────────────
 * Polylang (free) keeps a post's language in a hidden `language` taxonomy that
 * is not registered with show_in_rest. So /wp/v2/posts exposes no `lang` and no
 * `translations`, and ?lang=xx does not filter the collection. A REST client can
 * therefore create a post that carries NO language — and Polylang's frontend
 * tax_query then excludes it from every listing while it stays readable at ?p=N.
 * Published into invisibility, silently.
 *
 * This bridge closes that hole by exposing Polylang's own public API:
 *
 *   /wp/v2/posts       "lang":         "ar"                    read + write
 *                      "translations": { "ar": 12, "en": 13 }   read + write
 *   /wp/v2/categories  same two fields                          read + write
 *   /helix/v1/status   public probe, so a client can confirm the bridge is live
 *
 * Everything is capability-checked and validated. If Polylang is absent the
 * fields are simply not registered — nothing fatals.
 */

if ( ! defined( 'ABSPATH' ) ) {
      exit;
}

/** Polylang's post API present? */
function helix_pll_posts_available() {
      return function_exists( 'pll_set_post_language' )
              && function_exists( 'pll_get_post_language' )
              && function_exists( 'pll_get_post_translations' )
              && function_exists( 'pll_save_post_translations' )
              && function_exists( 'pll_languages_list' );
}

/** Polylang's term API present? */
function helix_pll_terms_available() {
      return function_exists( 'pll_set_term_language' )
              && function_exists( 'pll_get_term_language' )
              && function_exists( 'pll_get_term_translations' )
              && function_exists( 'pll_save_term_translations' )
              && function_exists( 'pll_languages_list' );
}

/** The configured language slugs, or an empty array if Polylang is absent. */
function helix_pll_slugs() {
      if ( ! function_exists( 'pll_languages_list' ) ) {
              return array();
      }
      $slugs = pll_languages_list( array( 'fields' => 'slug' ) );
      return is_array( $slugs ) ? $slugs : array();
}

/**
 * Validate a { language_slug => object_id } map. Returns a clean array of
 * slug => (int) id, or a WP_Error.
 *
 * $get_lang( $id )  reads an object's CURRENT language slug.
 * $can_edit( $id )  decides whether this user may link that object.
 */
function helix_pll_clean_group( $value, $get_lang, $can_edit ) {
      if ( ! is_array( $value ) || empty( $value ) ) {
              return new WP_Error(
                      'helix_bad_group',
                      'translations must be a non-empty object of language slug => id.',
                      array( 'status' => 400 )
              );
      }

      $slugs = helix_pll_slugs();
      $clean = array();

      foreach ( $value as $slug => $id ) {
              $id = (int) $id;

              if ( ! in_array( $slug, $slugs, true ) ) {
                      return new WP_Error(
                              'helix_bad_language',
                              sprintf( 'Unknown language slug "%s".', $slug ),
                              array( 'status' => 400 )
                      );
              }
              if ( $id <= 0 ) {
                      return new WP_Error(
                              'helix_bad_id',
                              sprintf( 'Invalid id for language "%s".', $slug ),
                              array( 'status' => 400 )
                      );
              }
              if ( ! call_user_func( $can_edit, $id ) ) {
                      return new WP_Error(
                              'helix_forbidden',
                              sprintf( 'Not allowed to link object %d.', $id ),
                              array( 'status' => 403 )
                      );
              }
              // Each object must ALREADY carry the language it is being linked as.
              // pll_save_*_translations misbehaves quietly otherwise — which is exactly
              // the silent breakage this bridge exists to prevent, so it is an error
              // rather than something to paper over.
              if ( call_user_func( $get_lang, $id ) !== $slug ) {
                      return new WP_Error(
                              'helix_language_mismatch',
                              sprintf( 'Object %d is not in language "%s" — set its language before linking.', $id, $s
                              array( 'status' => 400 )
                      );
              }

              $clean[ $slug ] = $id;
      }

      return $clean;
}

// ── posts ────────────────────────────────────────────────────────────────────

function helix_pll_get_post_lang( $arr ) {
      $lang = pll_get_post_language( (int) $arr['id'], 'slug' );
      return $lang ? $lang : null;
}

function helix_pll_update_post_lang( $value, $post ) {
      if ( null === $value || '' === $value ) {
              return;
      }
      if ( ! is_string( $value ) ) {
              return new WP_Error( 'helix_bad_language', 'lang must be a language slug string.', array( 'status' => 40
      }
      if ( ! current_user_can( 'edit_post', $post->ID ) ) {
              return new WP_Error( 'helix_forbidden', 'Not allowed to set the language of this post.', array( 'status'
      }
      if ( ! in_array( $value, helix_pll_slugs(), true ) ) {
              return new WP_Error(
                      'helix_bad_language',
                      sprintf( 'Unknown language slug "%s".', $value ),
                      array( 'status' => 400 )
              );
      }
      pll_set_post_language( $post->ID, $value );
}

function helix_pll_get_post_translations( $arr ) {
      $t = pll_get_post_translations( (int) $arr['id'] );
      return is_array( $t ) ? array_map( 'intval', $t ) : array();
}

function helix_pll_update_post_translations( $value, $post ) {
      if ( null === $value || '' === $value ) {
              return;
      }
      if ( ! current_user_can( 'edit_post', $post->ID ) ) {
              return new WP_Error( 'helix_forbidden', 'Not allowed to link translations for this post.', array( 'statu
      }

      $clean = helix_pll_clean_group(
              $value,
              function ( $id ) {
                      return pll_get_post_language( $id, 'slug' );
              },
              function ( $id ) {
                      return get_post( $id ) && current_user_can( 'edit_post', $id );
              }
      );
      if ( is_wp_error( $clean ) ) {
              return $clean;
      }

      pll_save_post_translations( $clean );
}

// ── categories ───────────────────────────────────────────────────────────────

function helix_pll_get_term_lang( $arr ) {
      $lang = pll_get_term_language( (int) $arr['id'], 'slug' );
      return $lang ? $lang : null;
}

function helix_pll_update_term_lang( $value, $term ) {
      if ( null === $value || '' === $value ) {
              return;
      }
      if ( ! is_string( $value ) ) {
              return new WP_Error( 'helix_bad_language', 'lang must be a language slug string.', array( 'status' => 40
      }
      if ( ! current_user_can( 'manage_categories' ) ) {
              return new WP_Error( 'helix_forbidden', 'Not allowed to set the language of this category.', array( 'sta);
      }
      if ( ! in_array( $value, helix_pll_slugs(), true ) ) {
              return new WP_Error(
                      'helix_bad_language',
                      sprintf( 'Unknown language slug "%s".', $value ),
                      array( 'status' => 400 )
              );
      }
      pll_set_term_language( $term->term_id, $value );
}

function helix_pll_get_term_translations( $arr ) {
      $t = pll_get_term_translations( (int) $arr['id'] );
      return is_array( $t ) ? array_map( 'intval', $t ) : array();
}

function helix_pll_update_term_translations( $value, $term ) {
      if ( null === $value || '' === $value ) {
              return;
      }
      if ( ! current_user_can( 'manage_categories' ) ) {
              return new WP_Error( 'helix_forbidden', 'Not allowed to link category translations.', array( 'status' =>
      }

      $clean = helix_pll_clean_group(
              $value,
              function ( $id ) {
                      return pll_get_term_language( $id, 'slug' );
              },
              function ( $id ) {
                      $term = get_term( $id, 'category' );
                      return $term && ! is_wp_error( $term );
              }
      );
      if ( is_wp_error( $clean ) ) {
              return $clean;
      }

      pll_save_term_translations( $clean );
}

// ── registration ─────────────────────────────────────────────────────────────

add_action( 'rest_api_init', 'helix_pll_rest_init' );

function helix_pll_rest_init() {

      // Public probe. Lets a client confirm the bridge is installed BEFORE it tries
      // to rely on it, so "bridge missing" is a clear message rather than a post
      // that silently lands with no language. Exposes nothing that the existing
      // /pll/v1/languages endpoint does not already expose publicly.
      register_rest_route(
              'helix/v1',
              '/status',
              array(
                      'methods'             => 'GET',
                      'permission_callback' => '__return_true',
                      'callback'            => 'helix_pll_status',
              )
      );

      $schema_lang = array(
              'description' => 'Polylang language slug.',
              'type'        => array( 'string', 'null' ),
              'context'     => array( 'view', 'edit' ),
      );
      $schema_group = array(
              'description' => 'Translation group as { language slug: object id }.',
              'type'        => 'object',
              'context'     => array( 'view', 'edit' ),
      );

      // ORDER MATTERS. "lang" is registered before "translations" so that, within a
      // single request, the language is assigned before the link is attempted —
      // WP_REST_Controller::update_additional_fields_for_object() runs these in
      // registration order, and helix_pll_clean_group() validates the language of
      // every member before linking.
      if ( helix_pll_posts_available() ) {
              register_rest_field(
                      'post',
                      'lang',
                      array(
                              'get_callback'    => 'helix_pll_get_post_lang',
                              'update_callback' => 'helix_pll_update_post_lang',
                              'schema'          => $schema_lang,
                      )
              );
              register_rest_field(
                      'post',
                      'translations',
                      array(
                              'get_callback'    => 'helix_pll_get_post_translations',
                              'update_callback' => 'helix_pll_update_post_translations',
                              'schema'          => $schema_group,
                      )
              );
      }

      if ( helix_pll_terms_available() ) {
              register_rest_field(
                      'category',
                      'lang',
                      array(
                              'get_callback'    => 'helix_pll_get_term_lang',
                              'update_callback' => 'helix_pll_update_term_lang',
                              'schema'          => $schema_lang,
                      )
              );
              register_rest_field(
                      'category',
                      'translations',
                      array(
                              'get_callback'    => 'helix_pll_get_term_translations',
                              'update_callback' => 'helix_pll_update_term_translations',
                              'schema'          => $schema_group,
                      )
              );
      }
}

function helix_pll_status() {
      return rest_ensure_response(
              array(
                      'ok'        => true,
                      'plugin'    => 'helix-polylang-rest',
                      'version'   => '1.0.0',
                      'posts'     => helix_pll_posts_available(),
                      'terms'     => helix_pll_terms_available(),
                      'languages' => helix_pll_slugs(),
                      'default'   => function_exists( 'pll_default_language' ) ? pll_default_language( 'slug' ) : null
              )
      );
}