Refactor deleted post tracking to scale better

Previously, there was an array item for each post deleted. Since the purpose of tracking deleted posts is to know which daily sitemaps to regenerate, we only need to keep one item per day. On sites with a large number of posts, the previous behavior could result in an incredibly large option being saved with excessive number of db writes.

msm-sitemap.php

@@ -573,20 +573,29 @@ public static function create_post_type() {
 
 	/**
 	 * Record deleted posts in an option to be used by get_last_modified_posts.
+	 *
+	 * This function saves one entry per date to avoid excessive db calls or option
+	 * size when deleting a large number of posts.
 	 */
 	public static function record_deleted_post( $post_id ) {
 		$deleted_posts = get_option( 'msm_sitemap_deleted_posts', array() );
-		$post = get_post( $post_id );
+		$post          = get_post( $post_id );
 		if ( ! $post ) {
 			return;
 		}
 
-		if ( in_array( $post->post_type, self::get_supported_post_types() ) ) {
-			$deleted_posts[] = array(
-				'ID' => (string) $post->ID,
-				'post_date' => $post->post_date,
+		if ( in_array( $post->post_type, self::get_supported_post_types(), true ) ) {
+			// We want to standardize the post date to a specific date. Let's discard the time.
+			$date = date( 'Y-m-d', strtotime( $post->post_date ) );
+
+			// Let's check to see if the date is already on deck for regeneration.
+			if ( ! in_array( $date, array_column( $deleted_posts, 'post_date' ), true ) ) {
+				$deleted_posts[] = array(
+					'ID'        => 0, // Array item is a placeholder to match modified post structure.
+					'post_date' => $date,
 				);
-			update_option( 'msm_sitemap_deleted_posts', $deleted_posts, false );
+				update_option( 'msm_sitemap_deleted_posts', $deleted_posts, false );
+			}
 		}
 	}