Inventory_Presser_Taxonomy_Overlapper::maintain_taxonomy_terms_during_meta_updates( int $meta_id, int $object_id, string $meta_key, string|object $meta_value )

	function maintain_taxonomy_terms_during_meta_updates( $meta_id, $object_id, $meta_key, $meta_value )
	{
		if( '_edit_lock' == strtolower( $meta_key ) )
		{
			return;
		}
		
		//These are the unprefixed meta keys that have overlapping taxonomies
		$overlapping_keys = $this->overlapping_meta_keys();
		//unprefix the meta key
		$unprefixed = apply_filters( 'invp_unprefix_meta_key', $meta_key );

		//does $meta_key have a corresponding taxonomy?
		if( ! in_array( $unprefixed, array_keys( $overlapping_keys ) ) )
		{
			//No
			return;
		}

		$taxonomy = $overlapping_keys[$unprefixed];

		/**
		 * If we are in the Availability taxonomy, the end of this method
		 * appends terms instead of replacing. That means if the $meta_value is
		 * For Sale or Sold, we need to remove the opposite term.
		 */
		if( 'availability' == strtolower( $unprefixed ) 
			&& ! empty( $meta_value ) 
			&& in_array( INVP::sluggify( $meta_value ), array( 'for-sale', 'sold' ) ) )
		{
			$for_sale_and_sold_term_ids = get_terms( array(
				'taxonomy' => $taxonomy,
				'fields'   => 'ids',
				'slug'     => 'sold' == INVP::sluggify( $meta_value ) ? 'for-sale' : 'sold',
			) );
			$this->hooks_remove();
			wp_remove_object_terms( $object_id, $for_sale_and_sold_term_ids, $taxonomy );
			$this->hooks_add();
		}

		//if $meta_value is empty, then remove a term & exit
		//will this only happen for wholesale?
		if( empty( $meta_value ) )
		{
			//remove a term actually
			$terms = array();
			if( 'availability' == strtolower( $taxonomy ) )
			{
				$terms = wp_get_object_terms( $object_id, $taxonomy );
				for( $t=0; $t<sizeof($terms); $t++ )
				{
					if( $unprefixed == $terms[$t]->slug )
					{
						/**
						 * Both $unprefixed and $terms[$t]->slug are 'wholesale'
						 * or $unprefixed and $taxonomy are both 'availability'
						 */
						//trash this one
						unset( $terms[$t] );
						break;
					}
				}
			}

			$this->hooks_remove();
			wp_set_object_terms( $object_id, array_column( $terms, 'term_id' ), $taxonomy );
			$this->hooks_add();
			return;
		}

		/**
		 * Wholesale is a term in the Availability taxonomy rather than a real
		 * boolean as the meta field suggests & is registered.
		 */
		if( 'wholesale' == strtolower( $unprefixed ) )
		{
			$meta_value = 'Wholesale';
		}

		//is there already a term for this $meta_value in the taxonomy?
		$term = get_term_by( 'slug', INVP::sluggify( $meta_value ), $taxonomy );
		if( ! $term )
		{
			//it's not a slug, what about a name?
			$term = get_term_by( 'name', $meta_value, $taxonomy );
			if( ! $term )
			{
				//No, create a term
				$term_id_array = wp_insert_term( $meta_value, $taxonomy, array(
					'description' => $meta_value,
					'slug'        => INVP::sluggify( $meta_value ),
				) );
				if( ! empty( $term_id_array['term_id'] ) )
				{
					$term = get_term( $term_id_array['term_id'], $taxonomy );
				}
			}
		}

		/** 
		 * Assign the new term for this $object_id. The Availability taxonomy
		 * holds For Sale/Sold and Wholesale, so append in that taxonomy.
		 */
		$this->hooks_remove();
		wp_set_object_terms( $object_id, $term->term_id, $taxonomy, ( 'availability' == strtolower( $taxonomy ) ) );
		$this->hooks_add();
	}