materio-d8/web/modules/custom/materio_id/materio_id.module

<?php


function computed_field_field_index_compute($entity_type_manager, $entity, $fields, $delta){
  // ksm($entity_type_manager);
  // ksm($entity);
  // ksm($fields);
  // ksm($delta);
  $famille = $entity->get('field_famille')->getValue()[0]['value'];

  // $entity->isNew(); ??
  // TODO: what if familly change ?
  if (!empty($entity->field_index->value)) {  // the node is not new
	  return $entity->field_index->value;
	}

  // the node is new
  // get all same entities in same family
  $bundle = $entity->bundle();
  $entity_type_id = $entity->getEntityTypeId();
  $query = \Drupal::entityQuery($entity->getEntityTypeId())
    ->condition('type', $entity->bundle())
    ->condition('field_famille', $famille)
    ->sort('field_index', 'DESC')
    ->accessCheck(FALSE)
    ->range(0,1);

  $nids = $query->execute();

  $index = null;
	if(is_array($nids)){
    $nid = array_pop($nids);
    if ($nid) {
      // $last = entity_load($entity->getEntityTypeId(), $nid); // array_pop($nids)
      $last = \Drupal::entityTypeManager()->getStorage($entity->getEntityTypeId())->load($nid);
      // get identifiants fo these entities
      $index = $last->get('field_index')->getValue()[0]['value'] + 1;
    }
	}
  // if(!$index){
	// 	$index = '1';
	// }
  return $index ? $index : '1';
}


// function computed_field_field_identifiant_display($field, $entity_field_item, $entity_lang = "en", $langcode = "en") {
// 	return $entity_field_item['value'];
// }

function computed_field_field_reference_compute($entity_type_manager, $entity, $fields, $delta) {
  if (!empty($entity->field_reference->value)) {  // the node is not new
    return $entity->field_reference->value;
  }
  // the node is new
  // parse the value to a string as 0023 or 4458 or 0001
	$index_parts = str_split($entity->field_index->value);
	while (count($index_parts) < 4) {
		array_unshift($index_parts, "0");
	}
	return $entity->field_famille->value.implode('', $index_parts);
}

// function computed_field_field_reference_materio_display($field, $entity_field_item, $entity_lang = "en", $langcode = "en") {
// 	return $entity_field_item['value'];
// }

/**
 * Implements hook_form_alter().
 */
function materio_id_form_node_materiau_edit_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  // dsm($form_id);
  // ksm($form);
  $node = $form_state->getFormObject()->getEntity();

  // lock the family filed if already filled
  if(isset($form['field_famille']) && $family = $node->field_famille->value){
    // dsm($family);
    $form['field_famille']['widget']['#disabled'] = true;
  }

  // adding reference to the form title
  $ref = $node->field_reference->value;
  // dsm($ref);
	if($ref != ''){
    $string = $form['#title']->getUntranslatedString() . ' (@ref)';
    // dsm($string);
    $args = $form['#title']->getArguments() + array('@ref'=>$ref);
    // ksm($args);
    $form['#title'] = t($string, $args);
	}
}