Variable Surcharges based on spaces/hrs and booking form field

<?php
/**
 * This class will handle variable surcharges based on event duration and extra charges based on form field.
 *
 * The first charge is based on number of spaces booked and hours of the event, this will automatically be applied to all events, modify $variant and $variant name to change the surcharge price and name.
 *
 * The second charge will come into effect if you create a custom booking form field with the id 'extra_charge' (or the name of $extra_field_id if you modify it) and will add a fixed amount to the booking.
 * If the field is a dropdown or anything that has a numeric value, that will be multipled by the $extra_cost field. If a checkbox, then $extra_cost will be applied.
 *
 * To install, download the contents of this snippet into a file with .php extension, and save it to your /wp-content/mu-plugins folder (create if necessary).
 *
 */
class EM_Variable_Surcharges {

    // This is a variable fee based on number of spaces x hr x variant
    public static $variant = 10;
    public static $variant_name = "Fuel Surcharge";
    // Uncomment the line below and the lines further down starting with 'desc' to add a description to the extra charge
    //public static $variant_description = "Price based on €10 per hour.";

    // This is for the extra fee based on form field
    public static $extra_field_id = 'extra_charge';
    public static $extra_cost = 10;
    public static $extra_name = 'Extra Charge';
    // Uncomment the line below and the lines further down starting with 'desc' to add a description to the extra charge
    //public static $extra_description = 'This is an extra charge description.';

    public static function init() {
        add_action('em_multiple_booking', array( static::class, 'em_mb_add_price_adjustments'), 10, 2);
        add_filter('em_booking_get_post', array( static::class, 'em_add_price_adjustments'), 100, 2);
        add_filter('em_bookings_get_intent_default', array( static::class, 'em_bookings_get_intent_default'), 10);
        add_action( 'em_booking_form_confirm_footer', array( static::class, 'js_intent'), 10 );
    }

    /**
     * @param bool $result
     * @param EM_Booking $EM_Booking
     *
     * @return mixed
     */
    public static function em_add_price_adjustments( $result, $EM_Booking ){
        //Only apply surcharge if booking has passed validation and therefore can be saved to database
        //You could also do further checks here if you want to give discounts to specific events or bookings
        if( $result ){
            //Ensure we have arrays assigned to booking meta, if not create them to avoid PHP warnings
            if( empty($EM_Booking->booking_meta['surcharges']) ) $EM_Booking->booking_meta['surcharges'] = array();

            // calculate hrs of event, then add surcharge based on that
            $EM_Event = $EM_Booking->get_event();
            $duration_seconds = $EM_Event->end()->getTimestamp() - $EM_Event->start()->getTimestamp();
            $duration_hours = $duration_seconds / 3600;
            $spaces = $EM_Booking->get_spaces();

            //This one adds a fixed $25 surcharge and is applied before taxes
            $EM_Booking->booking_meta['surcharges'][] = array(
                'name' => static::$variant_name,
                //'desc' => static::$variant_description,
                'type' => '#', //numerical discount i.e. $10.00 off
                'amount' => $spaces * ($duration_hours * static::$variant),
                'tax' => 'pre' //discount applied BEFORE taxes have been added, and IS taxable
            );

            // Extra Fee = Field Id from Events > Forms Editor
            if ( !empty($EM_Booking->booking_meta['booking'][static::$extra_field_id]) ) {

                if ( is_numeric($EM_Booking->booking_meta['booking'][static::$extra_field_id]) ) {
                    $qty = $EM_Booking->booking_meta['booking'][static::$extra_field_id];
                    $amount = $qty * static::$extra_cost;
                } else {
                    $amount = static::$extra_cost;
                }

                $EM_Booking->booking_meta['surcharges'][] = array(
                    'name' => static::$extra_name,
                    //'desc' => 'Some type of surcharge description',
                    'type' => '#', //numerical discount i.e. $10.00 off
                    'amount' => $amount,
                    'tax' => 'pre' //discount applied BEFORE taxes have been added, and IS taxable
                );
            };
        }
        $EM_Booking->calculate_price();
        return $result;
    }

    public static function em_bookings_get_intent_default( $EM_Booking ){
        if( $EM_Booking ) {
            static::em_add_price_adjustments( true, $EM_Booking );
            return $EM_Booking;
        }
        return $EM_Booking;
    }

    /**
     * Example function for handling discounts and surcharges within Multiple Bookings. Adjustments are made (and can currently only be made) to the multiple booking object, not individual bookings.
     * @param EM_Multiple_Booking Multiple Booking object, containing individual bookings within.
     * @param mixed $booking_data - Supplied information to create EM_Multiple_Booking object.
     */
    public static function em_mb_add_price_adjustments( $EM_Multiple_Booking, $booking_data ){
        //$bookings_data must be empty here, because otherwise it means this booking object already existed at one point.
        //If booking already existed, we'd either apply adjustments multiple times before checkout, or apply to an existing booking when modified which is not correct either.
        if( !$booking_data ){
            static::em_add_price_adjustments(false, $EM_Multiple_Booking, true);
        }
    }

    /**
     * Outputs JS that will update the booking summary when the donation field is changed.
     * @return void
     */
    public static function js_intent() {
        ?>
        <script type="text/javascript">
            document.addEventListener("em_booking_form_init", function( e ) {
                let booking_form = e.target;
                booking_form.addEventListener("change", function( e ) {
                    if( e.target.matches('[name="<?php echo esc_js(static::$extra_field_id); ?>"]') ){
                        booking_form.dispatchEvent( new CustomEvent('em_booking_form_updated') );
                    }
                });
            });
        </script>
        <?php
    }
}
EM_Variable_Surcharges::init();