Calculate distance between to coordinates of latitude and longitude using the WP REST API and return posts ordered by distance from user's coordinates

geodistance.php

<?php

/**
 * Heavily borrowed from: http://xplus3.net/2010/08/08/filtering-on-a-non-standard-database-field-with-wordpress/
 **/

class CoordinatesTable extends DB {

	protected $db_option = "coordinates_db";

	protected $db_version = 2;

	protected $id_field = 'pid';

	protected $columns = [
		'pid'       => '%d',
		'latitude'  => '%f',
		'longitude' => '%f',
	];

	/**
	 * CoordinatesTable constructor.
	 *
	 * Create table if needed
	 */
	private function __construct() {
		global $wpdb;
		$this->table = $wpdb->prefix . 'coordinates';

		if ( $this->update_required() ) {
			$this->run_updates();
		}
	}

	/**
	 * Create table with indexes for lat/lon
	 */
	protected function create_table() {
		global $wpdb;
		require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
		foreach ( array( 'providers' ) as $table ) {
			$sql = "CREATE TABLE {$this->table} (
            pid bigint(20) unsigned NOT NULL,
            latitude double,
            longitude double,
            UNIQUE KEY pid (pid),
            INDEX latitude (latitude),
            INDEX longitude (longitude)
          ) /*!40100 DEFAULT CHARACTER SET utf8 */;";
			dbDelta( $sql );
		}
	}

	/**
	 * @param $fields
	 *
	 * @return string
	 *
	 * To be hooked into 'post_fields' filter
	 */
	public function query_posts_fields( $fields ) {
		global $wpdb;
		global $wp_query;

		list( $lat, $lng ) = explode(',', $wp_query->query_vars['latlong'] );
		$fields .= sprintf(", 
		( 6371000 
			* acos( cos( radians( %f ) ) 
			* cos( radians( {$this->table}.latitude ) ) 
			* cos( radians( {$this->table}.longitude ) - radians( %f ) ) 
			+ sin( radians( %f ) ) 
			* sin( radians({$this->table}.latitude ) ) ) ) AS distance", $lat, $lng, $lat );

		return $fields;
	}

	/**
	 * @param $join
	 *
	 * @return string
	 *
	 * To be hooked into 'posts_join' filter
	 */
	public function query_posts_join( $join ) {
		global $wpdb;

		$join = " LEFT JOIN {$this->table} ON {$wpdb->posts}.ID = {$this->table}.pid" . $join;

		return $join;
	}

	/**
	 * @param $orderby
	 *
	 * @return string
	 */
	public function query_posts_orderby( $orderby ) {
		$myorder = "distance ASC";

		if ( $orderby ) {
			$myorder .= ',' . $orderby;
		}

		return $myorder; // return the default if we haven't changed it
	}

	/**
	 * @param $groupby
	 *
	 * @return string
	 *
	 * To be hooked into 'posts_groupby' filter.  This groups by the distance and orders closest in meters.  The limit
	 * is necessary for max distance to get properly ordered results.
	 */
	public function query_posts_groupby( $groupby ) {
		global $wpdb;

		if ( null === $max_distance = get_field( Options::GEO_MAX_DISTANCE, 'option' ) ) {
			$max_distance = Options::GEO_MAX_DISTANCE_DEFAULT;
		}
		$groupby = $wpdb->prepare( " {$wpdb->posts}.ID HAVING distance < %d", $max_distance );

		return $groupby;
	}

	/**
	 * Inject our query's sql string
	 */
	public function query_filters() {

		add_filter( 'posts_clauses', function( $clauses ) {

			$clauses['fields'] = $this->query_posts_fields( $clauses['fields'] );
			$clauses['join'] = $this->query_posts_join( $clauses['join'] );
			$clauses['orderby'] = $this->query_posts_orderby( $clauses['orderby'] );
			$clauses['groupby'] = $this->query_posts_groupby( $clauses['groupby'] );

			return $clauses;

		} );

	}

	public function save_coordinates( $post_id, $lat, $long ) {
		$this->add( [
			'pid' => $post_id,
			'latitude' => $lat,
			'longitude' => $long,
		] );
	}

	public function delete_coordinates( $post_id ) {
		$this->remove( [
			'pid' => $post_id,
		] );
	}

	public function get_latitude( $post_id ) {
		return $this->get( 'latitude', [ 'pid' => $post_id, ], 1 );
	}

	public function get_longitude( $post_id ) {
		return $this->get( 'longitude', [ 'pid' => $post_id, ], 1 );
	}
  
}