WordPress | Inject a new custom post type posting into the loop every X amount of regular posts

inject-cpt-post-every-X-posts.php

<?php
add_action( 'wp', 'fs_only_do_on_homepage' );
function fs_only_do_on_homepage() {

	if( is_front_page() ) {
	  
	  	/* change the query to inject ads after every X frequency */
		// https://www.gowp.com/blog/inserting-custom-posts-loop/
	  
		add_filter( 'the_posts', 'fs_insert_ads', 10, 2 );
		function fs_insert_ads( $posts, $query ) {

		  	// We don't want our filter to affect post lists in the admin dashboard
			if ( is_admin() ) return $posts;

		  	// We also only want our filter to affect the main query
			if ( ! is_main_query() ) return $posts;
			
		  	// Set a variable for the frequency of the insertion which we'll use in some math later on
			$freq = 2;

		  	/* Retrieve the ads, and calculate the posts_per_page on the fly, 
		  	 * based on the value from the original/main query AND our frequency
			 * this helps ensure that the inserted posts stay in sync regardless of pagination settings
			 */
			$args = array(
				'post_type' 		=> 'fs_ads',
				'posts_per_page' 	=> floor( $query->query_vars['posts_per_page'] / $freq ),
				'paged' 			=> ( $query->query_vars['paged'] ) ? $query->query_vars['paged'] : 1,
			  	'orderby'			=> 'menu_order',
				'order' 			=> 'ASC',
				'meta_key' 			=> 'status',
				'meta_value' 		=> 'enable',
			);

		  	// inserts the ads into the $posts array, using array_slice() ... and do some math to ensure they are inserted correctly
			if ( $fs_ads = get_posts( $args ) ) {
				$insert = -1;
				foreach ( $fs_ads as $fs_ad ) {
					$insert =  $insert + ( $freq + 1 );
					array_splice( $posts, $insert, 0, array( $fs_ad ) );
				}
			}

			return $posts;
		} // end the fs_insert_ads function

	} // end checking is_home
  
} // end the fs_only_do_on_homepage function