actuel-inactuel/site/collections/categories.php

<?php

function createCategories($textsPage) {
    $categories = array();
    foreach ($textsPage->categories()->split() as $category) {
        $categories[$category] = array(
          'title' => $category,
          'texts' => new Pages()
        );
    }
    return $categories;
}

function fillCategoriesWithTexts($emptyCategories, $texts) {
    $filledCategories = $emptyCategories;
    foreach ($texts as $text) {
        try {
            $textCategory = $text->category()->value();
            $filledCategories[$textCategory]['texts']->add($text);
        } catch (\Throwable $th) {
            throw new Exception(json_encode($th->getFile() . ' : ' . $th->getMessage()));
        }
    }

    // exclude empty categories
    return array_filter($filledCategories, function($category) {
        return count($category['texts']) > 0;
    });
}

return function ($site) {
    $textsPage = $site->find('textes');
    $years = $textsPage->children();
    $texts = $years->children()->index();

    $emptyCategories = createCategories($textsPage);

    $filledCategories = fillCategoriesWithTexts($emptyCategories, $texts);

    return $filledCategories;
};