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

<?php


function computed_field_field_index_compute($entity_type_manager, $entity, $fields, $delta){
  // drush_print($entity->field_index->value);
  if (!empty($entity->field_index->value)) {  // the node is not new
	  return $entity->field_index->value;
	}


  /* FROM D7

  // dsm('-- computed --');
	// dsm($entity_field, '$entity_field');
	// dsm($entity_type, '$entity_type');
	// dsm($entity, '$entity');
	// dsm($field, '$field');
	// dsm($instance, '$instance');
	// dsm($items, '$items');

	// dsm($entity_field[0]['value'], 'entity_field value');

	if (!empty($entity_field[0]['value'])) {  // the node is not new

	  return $entity_field[0]['value'];
	}
	else {  // the node is new
	  // get all same entities in same family
		$query = new EntityFieldQuery();
		$query
			->entityCondition('entity_type', $entity_type)
			->entityCondition('bundle', $entity->type)
			->fieldCondition('field_famille', 'value', $entity->field_famille['und'][0]['value']);
		$result = $query->execute();
		if(is_array($result[$entity_type])){
			$entities = entity_load($entity_type, array_keys($result[$entity_type]));

			// get identifiants fo these entities
			$ids = array();
			foreach ($entities as $id => $e) {
				$identifiant = field_view_field($entity_type, $e, 'field_identifiant');
				$ids[] = $identifiant[0]['#markup'] ? intval($identifiant[0]['#markup']) : 0;
			}
			sort($ids);

			$value = strval(array_pop($ids) + 1);
		}else{
			$value = '1';
		}

		// parse the value to a string as 0023 or 4458 or 0001
		$value_parts = str_split($value);
		while (count($value_parts) < 4) {
			array_unshift($value_parts, "0");
		}

			// record the result
			$entity_field[0]['value'] = implode('', $value_parts);
	}
  */
}


// 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;
  }

  /* FROM D7

	// dsm('-- computed --');
	// 	dsm($entity_field, '$entity_field');
	// 	dsm($entity_type, '$entity_type');
	// 	dsm($entity, '$entity');
	// 	dsm($field, '$field');
	// 	dsm($instance, '$instance');
	// 	dsm($items, '$items');
	if (!empty($entity_field[0]['value'])) {  // the node is not new
	  return $entity_field[0]['value'];
	}
	else {  // the node is new
		$entity_field[0]['value'] = $entity->field_famille['und'][0]['value'].'-'.$entity->field_identifiant['und'][0]['value'];
	}
 */
}

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