Generate / Check the Check Digit of a Container

The application allows to Generate or simply Verify the Check Digit (control code) of
the identification number of a Container (Container) as per ISO 6346:1995 specifications.

For example by entering the partial CNTR identifier: ABCU123456
(So ​​4 Letters and 6 numbers) we will have 0 as the answer which
is Check Digit calculated for those values ​​entered.

Now let's give two quick explanations:

1) the 4 initial letters represent:
  - the first three are : The proprietary code is composed of three capital letters of the Latin alphabet to indicate the owner or principal operator of the container.
  - the fourth : represents the identifier of the equipment category consists of one of the following capital letters of the Latin alphabet:
 * U for all containers
 * J for equipment related to the transport of detachable containers
 * Z for trailers and chassis

2) the 6 numbers are the serial number assigned by the owner or operator, which uniquely identifies the container within the owner / operator fleet.

https://en.wikipedia.org/wiki/ISO_6346