abstract class Inventory_Presser_Addon {
		/**
		 * initialize_updater
		 *
		 * @param  string $class_name    The class name in the add-on plugin's main file that contains the VERSION and LICENSE_ITEM constants
		 * @param  string $plugin_path   The file path to the plugin for which we are checking for updates
		 * @param  string $license_key   The license key provided by inventorypresser.com upon purchase
		 * @param  string $include_betas Set to true if you wish customers to receive update notifications of beta releases
		 * @return void
		 */
		public static function initialize_updater( $class_name, $plugin_path, $license_key, $include_betas = false ) {
			// check for existence of updater class
			if ( ! class_exists( 'Inventory_Presser_Addon_Updater' ) ) {
				return;
			}

			// Does the plugin class have the VERSION and LICENSE_ITEM constants?
			if ( empty( $class_name::VERSION ) || empty( $class_name::LICENSE_ITEM ) ) {
				return;
			}

			$updater = new Inventory_Presser_Addon_Updater(
				'https://inventorypresser.com',
				$plugin_path,
				array(
					'version' => $class_name::VERSION, // current version number
					'license' => $license_key,
					'item_id' => $class_name::LICENSE_ITEM, // id of this download in inventorypresser.com
					'author'  => 'Corey Salzano',
					'url'     => home_url(), // the site requesting the update, this site
					'beta'    => $include_betas,
				)
			);
		}

		/**
		 * add_license_key_box
		 *
		 * @param  mixed $option_name
		 * @param  mixed $settings_section
		 * @param  mixed $sanitize_values_hook_name
		 * @param  mixed $license_key_key_name
		 * @return void
		 */
		public static function add_license_key_box( $option_name, $settings_section, $sanitize_values_hook_name, $license_key_key_name = 'license_key' ) {
			add_action(
				'admin_init',
				function () use ( $option_name, $settings_section, $license_key_key_name ) {

					// is $settings_section an existing section?
					global $wp_settings_sections;
					if ( empty( $wp_settings_sections[ INVP::option_page() ][ $settings_section ] ) ) {
						// no, abort.
						return;
					}

					$option        = get_option( $option_name );
					$current_value = empty( $option[ $license_key_key_name ] ) ? '' : $option[ $license_key_key_name ];

					add_settings_field(
						$license_key_key_name, // id.
						__( 'License key', 'inventory-presser' ), // title.
						function () use ( $option_name, $current_value, $license_key_key_name ) {
							?><p><input type="text" name="<?php echo esc_attr( $option_name ); ?>[<?php echo esc_attr( $license_key_key_name ); ?>]" class="regular-text code" id="<?php echo esc_attr( $option_name ); ?>[<?php echo esc_attr( $license_key_key_name ); ?>]" value="<?php echo esc_attr( $current_value ); ?>" /></p>
								<p class="description"><?php printf( '%s <a href="https://inventorypresser.com/">https://inventorypresser.com/</a> %s', esc_html__( 'Obtain a key at', 'inventory-presser' ), esc_html__( 'to receive plugin updates.', 'inventory-presser' ) ); ?></p>
							<?php
						}, // callback
						INVP::option_page(), // page.
						$settings_section // section.
					);
				},
				20
			);

			$hook_parameters_count = 'license_key' === $license_key_key_name ? 3 : 4;
			add_filter( $sanitize_values_hook_name, array( __CLASS__, 'sanitize_and_activate_license_key' ), 10, $hook_parameters_count );
		}

		/**
		 * make_sure_license_is_activated
		 *
		 * @param  mixed $license_item
		 * @param  mixed $license_key
		 * @return void
		 */
		public static function make_sure_license_is_activated( $license_item, $license_key ) {
			$license = new Inventory_Presser_Addon_License(
				$license_key,
				$license_item
			);
			if ( ! $license->is_active() ) {
				$license->activate();
			}
		}

		/**
		 * sanitize_and_activate_license_key
		 *
		 * @param  mixed $sanitized
		 * @param  mixed $input
		 * @param  mixed $license_item
		 * @param  mixed $license_key_key_name
		 * @return void
		 */
		public static function sanitize_and_activate_license_key( $sanitized, $input, $license_item, $license_key_key_name = 'license_key' ) {
			if ( isset( $input[ $license_key_key_name ] ) ) {
				$sanitized[ $license_key_key_name ] = sanitize_text_field( $input[ $license_key_key_name ] );
				self::make_sure_license_is_activated( $license_item, $sanitized[ $license_key_key_name ] );
			}
			return $sanitized;
		}
	}