get_permalink( int|WP_Post $post, bool $leavename = false )

function get_permalink( $post = 0, $leavename = false ) {
	$rewritecode = array(
		'%year%',
		'%monthnum%',
		'%day%',
		'%hour%',
		'%minute%',
		'%second%',
		$leavename? '' : '%postname%',
		'%post_id%',
		'%category%',
		'%author%',
		$leavename? '' : '%pagename%',
	);

	if ( is_object( $post ) && isset( $post->filter ) && 'sample' == $post->filter ) {
		$sample = true;
	} else {
		$post = get_post( $post );
		$sample = false;
	}

	if ( empty($post->ID) )
		return false;

	if ( $post->post_type == 'page' )
		return get_page_link($post, $leavename, $sample);
	elseif ( $post->post_type == 'attachment' )
		return get_attachment_link( $post, $leavename );
	elseif ( in_array($post->post_type, get_post_types( array('_builtin' => false) ) ) )
		return get_post_permalink($post, $leavename, $sample);

	$permalink = get_option('permalink_structure');

	/**
	 * Filters the permalink structure for a post before token replacement occurs.
	 *
	 * Only applies to posts with post_type of 'post'.
	 *
	 * @since WP-3.0.0
	 *
	 * @param string  $permalink The site's permalink structure.
	 * @param WP_Post $post      The post in question.
	 * @param bool    $leavename Whether to keep the post name.
	 */
	$permalink = apply_filters( 'pre_post_link', $permalink, $post, $leavename );

	if ( '' != $permalink && !in_array( $post->post_status, array( 'draft', 'pending', 'auto-draft', 'future' ) ) ) {
		$unixtime = strtotime($post->post_date);

		$category = '';
		if ( strpos($permalink, '%category%') !== false ) {
			$cats = get_the_category($post->ID);
			if ( $cats ) {
				$cats = wp_list_sort( $cats, array(
					'term_id' => 'ASC',
				) );

				/**
				 * Filters the category that gets used in the %category% permalink token.
				 *
				 * @since WP-3.5.0
				 *
				 * @param WP_Term  $cat  The category to use in the permalink.
				 * @param array    $cats Array of all categories (WP_Term objects) associated with the post.
				 * @param WP_Post  $post The post in question.
				 */
				$category_object = apply_filters( 'post_link_category', $cats[0], $cats, $post );

				$category_object = get_term( $category_object, 'category' );
				$category = $category_object->slug;
				if ( $parent = $category_object->parent )
					$category = get_category_parents($parent, false, '/', true) . $category;
			}
			// show default category in permalinks, without
			// having to assign it explicitly
			if ( empty($category) ) {
				$default_category = get_term( get_option( 'default_category' ), 'category' );
				if ( $default_category && ! is_wp_error( $default_category ) ) {
					$category = $default_category->slug;
				}
			}
		}

		$author = '';
		if ( strpos($permalink, '%author%') !== false ) {
			$authordata = get_userdata($post->post_author);
			$author = $authordata->user_nicename;
		}

		$date = explode(" ",date('Y m d H i s', $unixtime));
		$rewritereplace =
		array(
			$date[0],
			$date[1],
			$date[2],
			$date[3],
			$date[4],
			$date[5],
			$post->post_name,
			$post->ID,
			$category,
			$author,
			$post->post_name,
		);
		$permalink = home_url( str_replace($rewritecode, $rewritereplace, $permalink) );
		$permalink = user_trailingslashit($permalink, 'single');
	} else { // if they're not using the fancy permalink option
		$permalink = home_url('?p=' . $post->ID);
	}

	/**
	 * Filters the permalink for a post.
	 *
	 * Only applies to posts with post_type of 'post'.
	 *
	 * @since WP-1.5.0
	 *
	 * @param string  $permalink The post's permalink.
	 * @param WP_Post $post      The post in question.
	 * @param bool    $leavename Whether to keep the post name.
	 */
	return apply_filters( 'post_link', $permalink, $post, $leavename );
}