class MakeUniqueBase

abstract class MakeUniqueBase extends ProcessPluginBase {
    
    /**
     * Creates a unique value based on the source value.
     *
     * @param string $value
     *   The input string.
     * @param \Drupal\migrate\MigrateExecutableInterface $migrate_executable
     *   The migration in which this process is being executed.
     * @param \Drupal\migrate\Row $row
     *   The row from the source to process.
     * @param string $destination_property
     *   The destination property currently worked on. This is only used together
     *   with the $row above.
     *
     * @return string
     *   The unique version of the input value.
     *
     * @throws \Drupal\migrate\MigrateException
     */
    public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
        $i = 1;
        $postfix = $this->configuration['postfix'] ?? '';
        $start = $this->configuration['start'] ?? 0;
        if (!is_int($start)) {
            throw new MigrateException('The start position configuration key should be an integer. Omit this key to capture from the beginning of the string.');
        }
        $length = $this->configuration['length'] ?? NULL;
        if (!is_null($length) && !is_int($length)) {
            throw new MigrateException('The character length configuration key should be an integer. Omit this key to capture the entire string.');
        }
        // Use optional start or length to return a portion of the unique value.
        $value = mb_substr($value, $start, $length);
        $new_value = $value;
        while ($this->exists($new_value)) {
            $new_value = $value . $postfix . $i++;
        }
        return $new_value;
    }
    
    /**
     * This is a query checking the existence of some value.
     *
     * @param mixed $value
     *   The value to check.
     *
     * @return bool
     *   TRUE if the value exists.
     */
    protected abstract function exists($value);

}