Auto fill content field with random values I have field_x with values "A" or "B". 


They must be assigned randomly by the system between A or B at every node added ( must not be changed on "node edit" by the node author).
The total numbers of nodes will be "30" and the assignment must be symmetrical ( after all 30 nodes will be added: 15 nodes must have "A" value and 15 nodes "B" value)


I've tried an solution with RULES but I didn't find an random function to use. Anyone has an better solution ? Thank you!

More rational explanation of the problem:

Insurance company "X" has 5 workers who upload client files (nodes), every worker must upload 30 files per month to the system. Every file has an field named "file_inspector" that has the name of the inspector who will check the insurance problem from that respective file. The field "file_inspector" can have "inspector1" or "inspector2" values that will not be completed by the worker but will be assigned by Drupal randomly based on the fact that at the end of the month total number of nodes will be 30 for every worker and "inspector1" and "inspector2" must be assigned equally 15 / 15. 
The assignment must happen in real time (when the node is added) not at the end of the month. (Hope this makes more sense now)
This should be done in a custom module by creating a custom field type

Good starting points would be either one of serial or unique_field modules and changing the logic of how the number is generated to match the logic described in your post