3. Post Translation Functions

/**
 * Get the language of a post.
 *
 * @param  int    $post_id WordPress post ID
 * @return string Language code. Falls back to the default language
 *                if no language meta is set.
 */
$lang = lf_get_post_language(42);
// Returns: 'en'

// In The Loop
$lang = lf_get_post_language(get_the_ID());
$languages = lf_get_languages();
$lang_name = $languages[$lang]['native_name'] ?? $lang;
$flag = $languages[$lang]['flag'] ?? $lang;
$flag_url = plugins_url('langforge/assets/flags/' . $flag . '.svg');

echo '<span class="lang-badge">';
echo '<img src="' . esc_url($flag_url) . '" alt="" width="16" height="12"> ';
echo esc_html($lang_name);
echo '</span>';

// Build a language breakdown for a set of posts
$all_posts = get_posts(['posts_per_page' => -1, 'post_type' => 'post']);
$by_language = [];

foreach ($all_posts as $post) {
    $lang = lf_get_post_language($post->ID);
    if (!isset($by_language[$lang])) {
        $by_language[$lang] = 0;
    }
    $by_language[$lang]++;
}

// $by_language = ['ru' => 45, 'en' => 38, 'de' => 12]

// single.php - Show a translation notice if the post is not in the visitor's preferred language
$post_lang = lf_get_post_language(get_the_ID());
$current_lang = lf_get_current_language();

if ($post_lang !== $current_lang) {
    $translations = lf_get_post_translations(get_the_ID());
    if (isset($translations[$current_lang])) {
        $translated_url = get_permalink($translations[$current_lang]);
        echo '<div class="translation-notice">';
        echo '<a href="' . esc_url($translated_url) . '">';
        _e('This page is available in your language', 'theme');
        echo '</a>';
        echo '</div>';
    }
}

/**
 * Get all translations for a post.
 *
 * Uses an in-memory cache: when you fetch translations for one post
 * in a group, all posts in that group are cached for the rest of
 * the request.
 *
 * @param  int   $post_id WordPress post ID
 * @return array Associative array: language_code => post_id
 */
$translations = lf_get_post_translations(42);
// Returns: ['ru' => 42, 'en' => 99, 'de' => 155]

// single.php
$translations = lf_get_post_translations(get_the_ID());
$languages    = lf_get_languages();
$current      = lf_get_current_language();

if (count($translations) > 1) {
    echo '<div class="also-available">';
    echo '<span>' . esc_html__('Also available in:', 'theme') . ' </span>';
    $links = [];
    foreach ($translations as $lang_code => $trans_id) {
        if ($lang_code === $current) continue;
        $name = $languages[$lang_code]['native_name'] ?? $lang_code;
        $links[] = '<a href="' . esc_url(get_permalink($trans_id)) . '">' . esc_html($name) . '</a>';
    }
    echo implode(', ', $links);
    echo '</div>';
}

// Show which languages are missing translations for this post
$translations = lf_get_post_translations(get_the_ID());
$all_languages = lf_get_languages();
$missing = array_diff_key($all_languages, $translations);

if (!empty($missing)) {
    echo '<div class="missing-translations">';
    echo '<strong>Missing translations:</strong> ';
    $names = [];
    foreach ($missing as $code => $lang) {
        $names[] = esc_html($lang['name']);
    }
    echo implode(', ', $names);
    echo '</div>';
}

// Efficient pattern: preload translations before a loop
$query = new WP_Query(['post_type' => 'post', 'posts_per_page' => 20]);
$post_ids = wp_list_pluck($query->posts, 'ID');

// Warm the cache: first call per translation group triggers 1 query
foreach ($post_ids as $pid) {
    lf_get_post_translations($pid);
}

// Now display posts with translation info - all cache hits
while ($query->have_posts()) {
    $query->the_post();
    $trans = lf_get_post_translations(get_the_ID()); // 0 queries
    $trans_count = count($trans);
    echo '<p>' . get_the_title() . ' (' . $trans_count . ' languages)</p>';
}
wp_reset_postdata();

/**
 * Get a single translation of a post.
 *
 * @param  int    $post_id  WordPress post ID
 * @param  string $language Target language code
 * @return int|null          Translated post ID, or null if not found
 */
$en_id = lf_get_translation(42, 'en');
// Returns: 99

$en_version = lf_get_translation(get_the_ID(), 'en');

if ($en_version) {
    echo '<a href="' . esc_url(get_permalink($en_version)) . '">';
    echo '<img src="' . esc_url(plugins_url('langforge/assets/flags/gb.svg')) . '" alt="English" width="20"> ';
    echo 'Read in English';
    echo '</a>';
} else {
    echo '<span class="no-translation">English version not available</span>';
}

add_action('wp_head', function () {
    if (!is_singular()) return;

    $languages = lf_get_languages();
    $post_id = get_the_ID();

    foreach ($languages as $code => $lang) {
        $trans_id = lf_get_translation($post_id, $code);
        if ($trans_id) {
            $url = get_permalink($trans_id);
            $hreflang = str_replace('_', '-', $lang['locale']);
            echo '<link rel="alternate" hreflang="' . esc_attr($hreflang) . '" href="' . esc_url($url) . '">' . "n";
        }
    }
});

// Show the same post in other languages as "Read in other languages" widget
function theme_cross_language_widget($post_id) {
    $languages = lf_get_languages();
    $current = lf_get_current_language();
    $has_translations = false;

    echo '<div class="widget cross-language-widget">';
    echo '<h3>' . esc_html__('Read in other languages', 'theme') . '</h3>';
    echo '<ul>';

    foreach ($languages as $code => $lang) {
        if ($code === $current) continue;
        $trans_id = lf_get_translation($post_id, $code);
        if ($trans_id) {
            $has_translations = true;
            $title = get_the_title($trans_id);
            echo '<li>';
            echo '<a href="' . esc_url(get_permalink($trans_id)) . '">';
            echo esc_html($lang['native_name']) . ': ' . esc_html($title);
            echo '</a>';
            echo '</li>';
        }
    }

    echo '</ul>';
    echo '</div>';

    return $has_translations;
}