class FieldInstanceSettings

class FieldInstanceSettings extends ProcessPluginBase {
    
    /**
     * {@inheritdoc}
     *
     * Set the field instance defaults.
     */
    public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
        [
            $widget_type,
            $widget_settings,
            $field_settings,
        ] = $value;
        $settings = [];
        switch ($widget_type) {
            case 'number':
                $settings['min'] = $field_settings['min'];
                $settings['max'] = $field_settings['max'];
                $settings['prefix'] = $field_settings['prefix'];
                $settings['suffix'] = $field_settings['suffix'];
                break;
            case 'link':
                // $settings['url'] = $widget_settings['default_value'][0]['url'];
                // D6 has optional, required, value and none. D8 only has disabled (0)
                // optional (1) and required (2).
                $map = [
                    'disabled' => 0,
                    'optional' => 1,
                    'required' => 2,
                ];
                $settings['title'] = $map[$field_settings['title']];
                break;
            case 'filefield_widget':
                $settings['file_extensions'] = $widget_settings['file_extensions'];
                $settings['file_directory'] = $widget_settings['file_path'];
                $settings['description_field'] = $field_settings['description_field'];
                $settings['max_filesize'] = $this->convertSizeUnit($widget_settings['max_filesize_per_file']);
                break;
            case 'imagefield_widget':
                $settings['file_extensions'] = $widget_settings['file_extensions'];
                $settings['file_directory'] = $widget_settings['file_path'];
                $settings['max_filesize'] = $this->convertSizeUnit($widget_settings['max_filesize_per_file'] ?? '');
                $settings['alt_field'] = $widget_settings['alt'];
                $settings['alt_field_required'] = $widget_settings['custom_alt'];
                $settings['title_field'] = $widget_settings['title'];
                $settings['title_field_required'] = $widget_settings['custom_title'];
                // With nothing entered for min or max resolution in Drupal 6, zero is
                // stored. For Drupal 8 this should be an empty string.
                $settings['max_resolution'] = !empty($widget_settings['max_resolution']) ? $widget_settings['max_resolution'] : '';
                $settings['min_resolution'] = !empty($widget_settings['min_resolution']) ? $widget_settings['min_resolution'] : '';
                break;
        }
        return $settings;
    }
    
    /**
     * Convert file size strings into their D8 format.
     *
     * D6 stores file size using a "K" for kilobytes and "M" for megabytes where
     * as D8 uses "KB" and "MB" respectively.
     *
     * @param string $size_string
     *   The size string, eg 10M
     *
     * @return string
     *   The D8 version of the size string.
     */
    protected function convertSizeUnit($size_string) {
        $size_unit = substr($size_string, strlen($size_string) - 1);
        if ($size_unit == "M" || $size_unit == "K") {
            return $size_string . "B";
        }
        return $size_string;
    }

}