Source string usage details

The variables available to your code include: @fields. To set the value of the field, set @entity_field. For multi-value computed fields continue with @entity_field_multi. Here's a simple example which sets the computed field's value to the value of the sum of the number fields (@field_a and @field_b) in a node entity:

Alternately, this code can be supplied by your own custom function named: @compute_func(&$entity_field, $entity_type, $entity, $field, $instance, $langcode, $items).