Events Manager Pro – Calculate Coupon "2 for 1"

<?php
/**
 * Adjusts the EM Coupon Discount based on number of booked tickets.
 *
 * If you have an EM Coupon with "2 for 1" and someone books 5 tickets, they should only get the discount for 4 tickets.
 * Events Manager Pro can only calculate the discount over the complete booking. This snippets will alter that.
 * Requires: An existing EM Coupon with 50% discount.
 *
 * @param  object $EM_Event   The current Events Manager Event object.
 * @param  object $EM_Booking The current Events Manager Booking object.
 * @return object
 */
function stonehenge_change_coupon_value( $EM_Event, $EM_Booking ) {
    // Change '2for1' to your the actual coupon code in your database.
    if( isset( $EM_Booking->booking_meta['coupon'] ) && '2for1' === $EM_Booking->booking_meta['coupon']['coupon_code'] ) {

        // Only change the 50% discount if the number of tickets is an odd number.
        $odd = $EM_Booking->spaces % 2 == 0 ? false : true;

        if( $odd ) {
            // Get raw booking price.
            $EM_Tickets = $EM_Booking->get_tickets_bookings();
            foreach( $EM_Tickets as $EM_Ticket ) {
                $total = $EM_Tickets->get_price( false );   // Raw value: total without discount and tax.
            }
            $single_ticket_price = ($raw_total / $spaces);

            // Calculate the allowed amount of discount.
            $discount_spaces    = ($spaces - 1);    // Make even -> exclude 1 ticket that does not get the discount.
            $discountable       = ($single_ticket_price * $discount_spaces);
            $discount_value     = ($discountable / 2); // 50% discount.

            // Send new discount to booking.
            $EM_Booking->booking_meta['coupon']['coupon_type'] = '#';   // Fixed amount.
            $EM_Booking->booking_meta['coupon']['coupon_discount'] = $discount_value;
            $EM_Booking->booking_meta['coupon']['coupon_tax'] = 'pre'; // We calculated excluding taxes.
        }
    }
    return $EM_Booking;
}
add_action( 'em_booking_add', 'stonehenge_change_coupon_value', 11, 2 );