Receipt or invoice choice with ΑΦΜ and ΔΟΥ for Greek shops

// Receipt or invoice (Απόδειξη / Τιμολόγιο) with ΑΦΜ, ΔΟΥ, company name and activity.
// Block checkout, WooCommerce 9.9 or later.

// ΑΦΜ check-digit validation (shared with the "Validate a Greek VAT number" snippet).
if ( ! function_exists( 'snipfire_is_valid_afm' ) ) {
	function snipfire_is_valid_afm( $afm ) {
		$afm = preg_replace( '/[\s.\-]/', '', (string) $afm );
		$afm = preg_replace( '/^(EL|GR)/i', '', $afm );

		if ( ! preg_match( '/^[0-9]{9}$/', $afm ) || '000000000' === $afm ) {
			return false;
		}

		$sum = 0;
		for ( $i = 0; $i < 8; $i++ ) {
			$sum += (int) $afm[ $i ] * ( 2 ** ( 8 - $i ) );
		}

		return ( $sum % 11 ) % 10 === (int) $afm[8];
	}
}

add_action(
	'woocommerce_init',
	function () {
		if ( ! function_exists( 'woocommerce_register_additional_checkout_field' ) ) {
			return;
		}

		// Rule: the customer chose "Τιμολόγιο".
		$is_invoice = array(
			'type'       => 'object',
			'properties' => array(
				'checkout' => array(
					'type'       => 'object',
					'properties' => array(
						'additional_fields' => array(
							'type'       => 'object',
							'properties' => array(
								'snipfire/document-type' => array( 'const' => 'invoice' ),
							),
							'required'   => array( 'snipfire/document-type' ),
						),
					),
					'required'   => array( 'additional_fields' ),
				),
			),
		);

		// Rule: anything else (nothing chosen yet, or "Απόδειξη").
		$not_invoice = array(
			'type'       => 'object',
			'properties' => array(
				'checkout' => array(
					'properties' => array(
						'additional_fields' => array(
							'properties' => array(
								'snipfire/document-type' => array( 'not' => array( 'const' => 'invoice' ) ),
							),
						),
					),
				),
			),
		);

		woocommerce_register_additional_checkout_field(
			array(
				'id'       => 'snipfire/document-type',
				'label'    => 'Παραστατικό',
				'location' => 'order',
				'type'     => 'select',
				'required' => true,
				'options'  => array(
					array(
						'value' => 'receipt',
						'label' => 'Απόδειξη',
					),
					array(
						'value' => 'invoice',
						'label' => 'Τιμολόγιο',
					),
				),
			)
		);

		woocommerce_register_additional_checkout_field(
			array(
				'id'                => 'snipfire/vat-number',
				'label'             => 'ΑΦΜ',
				'location'          => 'order',
				'required'          => $is_invoice,
				'hidden'            => $not_invoice,
				'attributes'        => array(
					'autocomplete' => 'off',
					'maxLength'    => 11,
				),
				'sanitize_callback' => function ( $value ) {
					// Store 9 digits only: remove spaces, dots, dashes and an EL/GR prefix.
					$value = strtoupper( preg_replace( '/[\s.\-]/', '', sanitize_text_field( (string) $value ) ) );
					return preg_replace( '/^(EL|GR)/', '', $value );
				},
				'validate_callback' => function ( $value ) {
					if ( '' !== (string) $value && ! snipfire_is_valid_afm( $value ) ) {
						return new WP_Error( 'snipfire_invalid_afm', 'Ο ΑΦΜ δεν είναι έγκυρος. Ελέγξτε ότι έχει 9 ψηφία.' );
					}
				},
			)
		);

		woocommerce_register_additional_checkout_field(
			array(
				'id'       => 'snipfire/tax-office',
				'label'    => 'ΔΟΥ',
				'location' => 'order',
				'required' => $is_invoice,
				'hidden'   => $not_invoice,
			)
		);

		woocommerce_register_additional_checkout_field(
			array(
				'id'         => 'snipfire/company-name',
				'label'      => 'Επωνυμία εταιρείας',
				'location'   => 'order',
				'required'   => $is_invoice,
				'hidden'     => $not_invoice,
				'attributes' => array( 'autocomplete' => 'organization' ),
			)
		);

		woocommerce_register_additional_checkout_field(
			array(
				'id'       => 'snipfire/business-activity',
				'label'    => 'Δραστηριότητα',
				'location' => 'order',
				'required' => $is_invoice,
				'hidden'   => $not_invoice,
			)
		);
	}
);

// If "Απόδειξη" was chosen, don't keep invoice details typed before switching.
add_action(
	'woocommerce_store_api_checkout_update_order_from_request',
	function ( $order ) {
		$prefix = '_wc_other/'; // WooCommerce's meta key prefix for order fields.

		if ( 'invoice' === $order->get_meta( $prefix . 'snipfire/document-type' ) ) {
			return;
		}

		foreach ( array( 'vat-number', 'tax-office', 'company-name', 'business-activity' ) as $field ) {
			$order->delete_meta_data( $prefix . 'snipfire/' . $field );
		}
	}
);