WooCommerce discount: buy one get one 50% off

Update (related to your comments)

This version will work globally on all product variations in the cart for this defined variable product:

add_action('woocommerce_cart_calculate_fees', 'add_custom_discount_2nd_at_50', 10, 1 );function add_custom_discount_2nd_at_50( $wc_cart ){    if ( is_admin() && ! defined( 'DOING_AJAX' ) ) return;    $discount = 0;    $items_prices = array();    // Set HERE your targeted variable product ID    $targeted_product_id = 40;    foreach ( $wc_cart->get_cart() as $key => $cart_item ) {        if( $cart_item['product_id'] == $targeted_product_id ){            $qty = intval( $cart_item['quantity'] );            for( $i = 0; $i < $qty; $i++ )                $items_prices[] = floatval( $cart_item['data']->get_price());        }    }    $count_items_prices = count($items_prices);    if( $count_items_prices > 1 ) foreach( $items_prices as $key => $price )        if( $key % 2 == 1 ) $discount -= number_format($price / 2, 2 );    if( $discount != 0 ){        // Displaying a custom notice (optional)        wc_clear_notices();        wc_add_notice( __("You get 50% of discount on the 2nd item"), 'notice');        // The discount        $wc_cart->add_fee( 'Discount 2nd at 50%', $discount, true  );        # Note: Last argument in add_fee() method is related to applying the tax or not to the discount (true or false)    }}

Code goes in function.php file of your active child theme (or theme) or also in any plugin file.

This code is tested on Woocommerce 3+ and works.

Original answer:

There is many ways to do add a custom discount of 50% on the 2nd item for a specific variable product ID. Below I am using add_fee() method with a negative value (so it adds a discount).
Optionally it will display a custom notice:

add_action('woocommerce_cart_calculate_fees', 'add_custom_discount_2nd_at_50', 10, 1 );function add_custom_discount_2nd_at_50( $wc_cart ){    if ( is_admin() && ! defined( 'DOING_AJAX' ) ) return;    $discount = 0;    // Set HERE your targeted variable product ID    $targeted_product_id = 40;    foreach ( $wc_cart->get_cart() as $key => $cart_item ) {        if( $cart_item['product_id'] == $targeted_product_id ){            $price = $cart_item['data']->get_price();            $quantity = intval( $cart_item['quantity'] );            for( $i = 1, $j = 0; $i <= $quantity; $i++ ){                if( $i % 2 == 0 &&  $quantity > 1 ) $j++;            }            if( $quantity > 1 ) number_format($discount -= $price * $j / 2, 2 );        }    }    if( $discount != 0 ){        // Displaying a custom notice (optional)        wc_clear_notices();        wc_add_notice( __("You get 50% of discount on the 2nd item"), 'notice');                $wc_cart->add_fee( 'Discount 2nd at 50%', $discount, true  );        # Note: Last argument in add_fee() method is related to applying the tax or not to the discount (true or false)    }}

Code goes in function.php file of your active child theme (or theme) or also in any plugin file.

This code is tested on Woocommerce 3+ and works.

Related: WooCommerce discount: buy one get one 50% off with a notice