/wp-includes/link-template.php

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<?php
/**
 * WordPress Link Template Functions
 *
 * @package WordPress
 * @subpackage Template
 */

/**
 * Displays the permalink for the current post.
 *
 * @since 1.2.0
 * @since 4.4.0 Added the `$post` parameter.
 *
 * @param int|WP_Post $post Optional. Post ID or post object. Default is the global `$post`.
 */
function the_permalink( $post = 0 ) {
	/**
	 * Filters the display of the permalink for the current post.
	 *
	 * @since 1.5.0
	 * @since 4.4.0 Added the `$post` parameter.
	 *
	 * @param string      $permalink The permalink for the current post.
	 * @param int|WP_Post $post      Post ID, WP_Post object, or 0. Default 0.
	 */
	echo esc_url( apply_filters( 'the_permalink', get_permalink( $post ), $post ) );
}

/**
 * Retrieves a trailing-slashed string if the site is set for adding trailing slashes.
 *
 * Conditionally adds a trailing slash if the permalink structure has a trailing
 * slash, strips the trailing slash if not. The string is passed through the
 * {@see 'user_trailingslashit'} filter. Will remove trailing slash from string, if
 * site is not set to have them.
 *
 * @since 2.2.0
 *
 * @global WP_Rewrite $wp_rewrite
 *
 * @param string $string      URL with or without a trailing slash.
 * @param string $type_of_url Optional. The type of URL being considered (e.g. single, category, etc)
 *                            for use in the filter. Default empty string.
 * @return string The URL with the trailing slash appended or stripped.
 */
function user_trailingslashit($string, $type_of_url = '') {
	global $wp_rewrite;
	if ( $wp_rewrite->use_trailing_slashes )
		$string = trailingslashit($string);
	else
		$string = untrailingslashit($string);

	/**
	 * Filters the trailing-slashed string, depending on whether the site is set to use trailing slashes.
	 *
	 * @since 2.2.0
	 *
	 * @param string $string      URL with or without a trailing slash.
	 * @param string $type_of_url The type of URL being considered. Accepts 'single', 'single_trackback',
	 *                            'single_feed', 'single_paged', 'commentpaged', 'paged', 'home', 'feed',
	 *                            'category', 'page', 'year', 'month', 'day', 'post_type_archive'.
	 */
	return apply_filters( 'user_trailingslashit', $string, $type_of_url );
}

/**
 * Displays the permalink anchor for the current post.
 *
 * The permalink mode title will use the post title for the 'a' element 'id'
 * attribute. The id mode uses 'post-' with the post ID for the 'id' attribute.
 *
 * @since 0.71
 *
 * @param string $mode Optional. Permalink mode. Accepts 'title' or 'id'. Default 'id'.
 */
function permalink_anchor( $mode = 'id' ) {
	$post = get_post();
	switch ( strtolower( $mode ) ) {
		case 'title':
			$title = sanitize_title( $post->post_title ) . '-' . $post->ID;
			echo '<a id="'.$title.'"></a>';
			break;
		case 'id':
		default:
			echo '<a id="post-' . $post->ID . '"></a>';
			break;
	}
}

/**
 * Retrieves the full permalink for the current post or post ID.
 *
 * This function is an alias for get_permalink().
 *
 * @since 3.9.0
 *
 * @see get_permalink()
 *
 * @param int|WP_Post $post      Optional. Post ID or post object. Default is the global `$post`.
 * @param bool        $leavename Optional. Whether to keep post name or page name. Default false.
 *
 * @return string|false The permalink URL or false if post does not exist.
 */
function get_the_permalink( $post = 0, $leavename = false ) {
	return get_permalink( $post, $leavename );
}

/**
 * Retrieves the full permalink for the current post or post ID.
 *
 * @since 1.0.0
 *
 * @param int|WP_Post $post      Optional. Post ID or post object. Default is the global `$post`.
 * @param bool        $leavename Optional. Whether to keep post name or page name. Default false.
 * @return string|false The permalink URL or false if post does not exist.
 */
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 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 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 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 );
}

/**
 * Retrieves the permalink for a post of a custom post type.
 *
 * @since 3.0.0
 *
 * @global WP_Rewrite $wp_rewrite
 *
 * @param int $id         Optional. Post ID. Default uses the global `$post`.
 * @param bool $leavename Optional, defaults to false. Whether to keep post name. Default false.
 * @param bool $sample    Optional, defaults to false. Is it a sample permalink. Default false.
 * @return string|WP_Error The post permalink.
 */
function get_post_permalink( $id = 0, $leavename = false, $sample = false ) {
	global $wp_rewrite;

	$post = get_post($id);

	if ( is_wp_error( $post ) )
		return $post;

	$post_link = $wp_rewrite->get_extra_permastruct($post->post_type);

	$slug = $post->post_name;

	$draft_or_pending = get_post_status( $id ) && in_array( get_post_status( $id ), array( 'draft', 'pending', 'auto-draft', 'future' ) );

	$post_type = get_post_type_object($post->post_type);

	if ( $post_type->hierarchical ) {
		$slug = get_page_uri( $id );
	}

	if ( !empty($post_link) && ( !$draft_or_pending || $sample ) ) {
		if ( ! $leavename ) {
			$post_link = str_replace("%$post->post_type%", $slug, $post_link);
		}
		$post_link = home_url( user_trailingslashit($post_link) );
	} else {
		if ( $post_type->query_var && ( isset($post->post_status) && !$draft_or_pending ) )
			$post_link = add_query_arg($post_type->query_var, $slug, '');
		else
			$post_link = add_query_arg(array('post_type' => $post->post_type, 'p' => $post->ID), '');
		$post_link = home_url($post_link);
	}

	/**
	 * Filters the permalink for a post of a custom post type.
	 *
	 * @since 3.0.0
	 *
	 * @param string  $post_link The post's permalink.
	 * @param WP_Post $post      The post in question.
	 * @param bool    $leavename Whether to keep the post name.
	 * @param bool    $sample    Is it a sample permalink.
	 */
	return apply_filters( 'post_type_link', $post_link, $post, $leavename, $sample );
}

/**
 * Retrieves the permalink for the current page or page ID.
 *
 * Respects page_on_front. Use this one.
 *
 * @since 1.5.0
 *
 * @param int|WP_Post $post      Optional. Post ID or object. Default uses the global `$post`.
 * @param bool        $leavename Optional. Whether to keep the page name. Default false.
 * @param bool        $sample    Optional. Whether it should be treated as a sample permalink.
 *                               Default false.
 * @return string The page permalink.
 */
function get_page_link( $post = false, $leavename = false, $sample = false ) {
	$post = get_post( $post );

	if ( 'page' == get_option( 'show_on_front' ) && $post->ID == get_option( 'page_on_front' ) )
		$link = home_url('/');
	else
		$link = _get_page_link( $post, $leavename, $sample );

	/**
	 * Filters the permalink for a page.
	 *
	 * @since 1.5.0
	 *
	 * @param string $link    The page's permalink.
	 * @param int    $post_id The ID of the page.
	 * @param bool   $sample  Is it a sample permalink.
	 */
	return apply_filters( 'page_link', $link, $post->ID, $sample );
}

/**
 * Retrieves the page permalink.
 *
 * Ignores page_on_front. Internal use only.
 *
 * @since 2.1.0
 * @access private
 *
 * @global WP_Rewrite $wp_rewrite
 *
 * @param int|WP_Post $post      Optional. Post ID or object. Default uses the global `$post`.
 * @param bool        $leavename Optional. Whether to keep the page name. Default false.
 * @param bool        $sample    Optional. Whether it should be treated as a sample permalink.
 *                               Default false.
 * @return string The page permalink.
 */
function _get_page_link( $post = false, $leavename = false, $sample = false ) {
	global $wp_rewrite;

	$post = get_post( $post );

	$draft_or_pending = in_array( $post->post_status, array( 'draft', 'pending', 'auto-draft' ) );

	$link = $wp_rewrite->get_page_permastruct();

	if ( !empty($link) && ( ( isset($post->post_status) && !$draft_or_pending ) || $sample ) ) {
		if ( ! $leavename ) {
			$link = str_replace('%pagename%', get_page_uri( $post ), $link);
		}

		$link = home_url($link);
		$link = user_trailingslashit($link, 'page');
	} else {
		$link = home_url( '?page_id=' . $post->ID );
	}

	/**
	 * Filters the permalink for a non-page_on_front page.
	 *
	 * @since 2.1.0
	 *
	 * @param string $link    The page's permalink.
	 * @param int    $post_id The ID of the page.
	 */
	return apply_filters( '_get_page_link', $link, $post->ID );
}

/**
 * Retrieves the permalink for an attachment.
 *
 * This can be used in the WordPress Loop or outside of it.
 *
 * @since 2.0.0
 *
 * @global WP_Rewrite $wp_rewrite
 *
 * @param int|object $post      Optional. Post ID or object. Default uses the global `$post`.
 * @param bool       $leavename Optional. Whether to keep the page name. Default false.
 * @return string The attachment permalink.
 */
function get_attachment_link( $post = null, $leavename = false ) {
	global $wp_rewrite;

	$link = false;

	$post = get_post( $post );
	$parent = ( $post->post_parent > 0 && $post->post_parent != $post->ID ) ? get_post( $post->post_parent ) : false;
	if ( $parent && ! in_array( $parent->post_type, get_post_types() ) ) {
		$parent = false;
	}

	if ( $wp_rewrite->using_permalinks() && $parent ) {
		if ( 'page' == $parent->post_type )
			$parentlink = _get_page_link( $post->post_parent ); // Ignores page_on_front
		else
			$parentlink = get_permalink( $post->post_parent );

		if ( is_numeric($post->post_name) || false !== strpos(get_option('permalink_structure'), '%category%') )
			$name = 'attachment/' . $post->post_name; // <permalink>/<int>/ is paged so we use the explicit attachment marker
		else
			$name = $post->post_name;

		if ( strpos($parentlink, '?') === false )
			$link = user_trailingslashit( trailingslashit($parentlink) . '%postname%' );

		if ( ! $leavename )
			$link = str_replace( '%postname%', $name, $link );
	} elseif ( $wp_rewrite->using_permalinks() && ! $leavename ) {
		$link = home_url( user_trailingslashit( $post->post_name ) );
	}

	if ( ! $link )
		$link = home_url( '/?attachment_id=' . $post->ID );

	/**
	 * Filters the permalink for an attachment.
	 *
	 * @since 2.0.0
	 *
	 * @param string $link    The attachment's permalink.
	 * @param int    $post_id Attachment ID.
	 */
	return apply_filters( 'attachment_link', $link, $post->ID );
}

/**
 * Retrieves the permalink for the year archives.
 *
 * @since 1.5.0
 *
 * @global WP_Rewrite $wp_rewrite
 *
 * @param int|bool $year False for current year or year for permalink.
 * @return string The permalink for the specified year archive.
 */
function get_year_link( $year ) {
	global $wp_rewrite;
	if ( !$year )
		$year = gmdate('Y', current_time('timestamp'));
	$yearlink = $wp_rewrite->get_year_permastruct();
	if ( !empty($yearlink) ) {
		$yearlink = str_replace('%year%', $year, $yearlink);
		$yearlink = home_url( user_trailingslashit( $yearlink, 'year' ) );
	} else {
		$yearlink = home_url( '?m=' . $year );
	}

	/**
	 * Filters the year archive permalink.
	 *
	 * @since 1.5.0
	 *
	 * @param string $yearlink Permalink for the year archive.
	 * @param int    $year     Year for the archive.
	 */
	return apply_filters( 'year_link', $yearlink, $year );
}

/**
 * Retrieves the permalink for the month archives with year.
 *
 * @since 1.0.0
 *
 * @global WP_Rewrite $wp_rewrite
 *
 * @param bool|int $year  False for current year. Integer of year.
 * @param bool|int $month False for current month. Integer of month.
 * @return string The permalink for the specified month and year archive.
 */
function get_month_link($year, $month) {
	global $wp_rewrite;
	if ( !$year )
		$year = gmdate('Y', current_time('timestamp'));
	if ( !$month )
		$month = gmdate('m', current_time('timestamp'));
	$monthlink = $wp_rewrite->get_month_permastruct();
	if ( !empty($monthlink) ) {
		$monthlink = str_replace('%year%', $year, $monthlink);
		$monthlink = str_replace('%monthnum%', zeroise(intval($month), 2), $monthlink);
		$monthlink = home_url( user_trailingslashit( $monthlink, 'month' ) );
	} else {
		$monthlink = home_url( '?m=' . $year . zeroise( $month, 2 ) );
	}

	/**
	 * Filters the month archive permalink.
	 *
	 * @since 1.5.0
	 *
	 * @param string $monthlink Permalink for the month archive.
	 * @param int    $year      Year for the archive.
	 * @param int    $month     The month for the archive.
	 */
	return apply_filters( 'month_link', $monthlink, $year, $month );
}

/**
 * Retrieves the permalink for the day archives with year and month.
 *
 * @since 1.0.0
 *
 * @global WP_Rewrite $wp_rewrite
 *
 * @param bool|int $year  False for current year. Integer of year.
 * @param bool|int $month False for current month. Integer of month.
 * @param bool|int $day   False for current day. Integer of day.
 * @return string The permalink for the specified day, month, and year archive.
 */
function get_day_link($year, $month, $day) {
	global $wp_rewrite;
	if ( !$year )
		$year = gmdate('Y', current_time('timestamp'));
	if ( !$month )
		$month = gmdate('m', current_time('timestamp'));
	if ( !$day )
		$day = gmdate('j', current_time('timestamp'));

	$daylink = $wp_rewrite->get_day_permastruct();
	if ( !empty($daylink) ) {
		$daylink = str_replace('%year%', $year, $daylink);
		$daylink = str_replace('%monthnum%', zeroise(intval($month), 2), $daylink);
		$daylink = str_replace('%day%', zeroise(intval($day), 2), $daylink);
		$daylink = home_url( user_trailingslashit( $daylink, 'day' ) );
	} else {
		$daylink = home_url( '?m=' . $year . zeroise( $month, 2 ) . zeroise( $day, 2 ) );
	}

	/**
	 * Filters the day archive permalink.
	 *
	 * @since 1.5.0
	 *
	 * @param string $daylink Permalink for the day archive.
	 * @param int    $year    Year for the archive.
	 * @param int    $month   Month for the archive.
	 * @param int    $day     The day for the archive.
	 */
	return apply_filters( 'day_link', $daylink, $year, $month, $day );
}

/**
 * Displays the permalink for the feed type.
 *
 * @since 3.0.0
 *
 * @param string $anchor The link's anchor text.
 * @param string $feed   Optional. Feed type. Default empty.
 */
function the_feed_link( $anchor, $feed = '' ) {
	$link = '<a href="' . esc_url( get_feed_link( $feed ) ) . '">' . $anchor . '</a>';

	/**
	 * Filters the feed link anchor tag.
	 *
	 * @since 3.0.0
	 *
	 * @param string $link The complete anchor tag for a feed link.
	 * @param string $feed The feed type, or an empty string for the
	 *                     default feed type.
	 */
	echo apply_filters( 'the_feed_link', $link, $feed );
}

/**
 * Retrieves the permalink for the feed type.
 *
 * @since 1.5.0
 *
 * @global WP_Rewrite $wp_rewrite
 *
 * @param string $feed Optional. Feed type. Default empty.
 * @return string The feed permalink.
 */
function get_feed_link( $feed = '' ) {
	global $wp_rewrite;

	$permalink = $wp_rewrite->get_feed_permastruct();
	if ( '' != $permalink ) {
		if ( false !== strpos($feed, 'comments_') ) {
			$feed = str_replace('comments_', '', $feed);
			$permalink = $wp_rewrite->get_comment_feed_permastruct();
		}

		if ( get_default_feed() == $feed )
			$feed = '';

		$permalink = str_replace('%feed%', $feed, $permalink);
		$permalink = preg_replace('#/+#', '/', "/$permalink");
		$output =  home_url( user_trailingslashit($permalink, 'feed') );
	} else {
		if ( empty($feed) )
			$feed = get_default_feed();

		if ( false !== strpos($feed, 'comments_') )
			$feed = str_replace('comments_', 'comments-', $feed);

		$output = home_url("?feed={$feed}");
	}

	/**
	 * Filters the feed type permalink.
	 *
	 * @since 1.5.0
	 *
	 * @param string $output The feed permalink.
	 * @param string $feed   Feed type.
	 */
	return apply_filters( 'feed_link', $output, $feed );
}

/**
 * Retrieves the permalink for the post comments feed.
 *
 * @since 2.2.0
 *
 * @param int    $post_id Optional. Post ID. Default is the ID of the global `$post`.
 * @param string $feed    Optional. Feed type. Default empty.
 * @return string The permalink for the comments feed for the given post.
 */
function get_post_comments_feed_link( $post_id = 0, $feed = '' ) {
	$post_id = absint( $post_id );

	if ( ! $post_id )
		$post_id = get_the_ID();

	if ( empty( $feed ) )
		$feed = get_default_feed();

	$post = get_post( $post_id );
	$unattached = 'attachment' === $post->post_type && 0 === (int) $post->post_parent;

	if ( '' != get_option('permalink_structure') ) {
		if ( 'page' == get_option('show_on_front') && $post_id == get_option('page_on_front') )
			$url = _get_page_link( $post_id );
		else
			$url = get_permalink($post_id);

		if ( $unattached ) {
			$url =  home_url( '/feed/' );
			if ( $feed !== get_default_feed() ) {
				$url .= "$feed/";
			}
			$url = add_query_arg( 'attachment_id', $post_id, $url );
		} else {
			$url = trailingslashit($url) . 'feed';
			if ( $feed != get_default_feed() )
				$url .= "/$feed";
			$url = user_trailingslashit($url, 'single_feed');
		}
	} else {
		if ( $unattached ) {
			$url = add_query_arg( array( 'feed' => $feed, 'attachment_id' => $post_id ), home_url( '/' ) );
		} elseif ( 'page' == $post->post_type ) {
			$url = add_query_arg( array( 'feed' => $feed, 'page_id' => $post_id ), home_url( '/' ) );
		} else {
			$url = add_query_arg( array( 'feed' => $feed, 'p' => $post_id ), home_url( '/' ) );
		}
	}

	/**
	 * Filters the post comments feed permalink.
	 *
	 * @since 1.5.1
	 *
	 * @param string $url Post comments feed permalink.
	 */
	return apply_filters( 'post_comments_feed_link', $url );
}

/**
 * Displays the comment feed link for a post.
 *
 * Prints out the comment feed link for a post. Link text is placed in the
 * anchor. If no link text is specified, default text is used. If no post ID is
 * specified, the current post is used.
 *
 * @since 2.5.0
 *
 * @param string $link_text Optional. Descriptive link text. Default 'Comments Feed'.
 * @param int    $post_id   Optional. Post ID. Default is the ID of the global `$post`.
 * @param string $feed      Optional. Feed format. Default empty.
 */
function post_comments_feed_link( $link_text = '', $post_id = '', $feed = '' ) {
	$url = get_post_comments_feed_link( $post_id, $feed );
	if ( empty( $link_text ) ) {
		$link_text = __('Comments Feed');
	}

	$link = '<a href="' . esc_url( $url ) . '">' . $link_text . '</a>';
	/**
	 * Filters the post comment feed link anchor tag.
	 *
	 * @since 2.8.0
	 *
	 * @param string $link    The complete anchor tag for the comment feed link.
	 * @param int    $post_id Post ID.
	 * @param string $feed    The feed type, or an empty string for the default feed type.
	 */
	echo apply_filters( 'post_comments_feed_link_html', $link, $post_id, $feed );
}

/**
 * Retrieves the feed link for a given author.
 *
 * Returns a link to the feed for all posts by a given author. A specific feed
 * can be requested or left blank to get the default feed.
 *
 * @since 2.5.0
 *
 * @param int    $author_id Author ID.
 * @param string $feed      Optional. Feed type. Default empty.
 * @return string Link to the feed for the author specified by $author_id.
 */
function get_author_feed_link( $author_id, $feed = '' ) {
	$author_id = (int) $author_id;
	$permalink_structure = get_option('permalink_structure');

	if ( empty($feed) )
		$feed = get_default_feed();

	if ( '' == $permalink_structure ) {
		$link = home_url("?feed=$feed&amp;author=" . $author_id);
	} else {
		$link = get_author_posts_url($author_id);
		if ( $feed == get_default_feed() )
			$feed_link = 'feed';
		else
			$feed_link = "feed/$feed";

		$link = trailingslashit($link) . user_trailingslashit($feed_link, 'feed');
	}

	/**
	 * Filters the feed link for a given author.
	 *
	 * @since 1.5.1
	 *
	 * @param string $link The author feed link.
	 * @param string $feed Feed type.
	 */
	$link = apply_filters( 'author_feed_link', $link, $feed );

	return $link;
}

/**
 * Retrieves the feed link for a category.
 *
 * Returns a link to the feed for all posts in a given category. A specific feed
 * can be requested or left blank to get the default feed.
 *
 * @since 2.5.0
 *
 * @param int    $cat_id Category ID.
 * @param string $feed   Optional. Feed type. Default empty.
 * @return string Link to the feed for the category specified by $cat_id.
 */
function get_category_feed_link( $cat_id, $feed = '' ) {
	return get_term_feed_link( $cat_id, 'category', $feed );
}

/**
 * Retrieves the feed link for a term.
 *
 * Returns a link to the feed for all posts in a given term. A specific feed
 * can be requested or left blank to get the default feed.
 *
 * @since 3.0.0
 *
 * @param int    $term_id  Term ID.
 * @param string $taxonomy Optional. Taxonomy of `$term_id`. Default 'category'.
 * @param string $feed     Optional. Feed type. Default empty.
 * @return string|false Link to the feed for the term specified by $term_id and $taxonomy.
 */
function get_term_feed_link( $term_id, $taxonomy = 'category', $feed = '' ) {
	$term_id = ( int ) $term_id;

	$term = get_term( $term_id, $taxonomy  );

	if ( empty( $term ) || is_wp_error( $term ) )
		return false;

	if ( empty( $feed ) )
		$feed = get_default_feed();

	$permalink_structure = get_option( 'permalink_structure' );

	if ( '' == $permalink_structure ) {
		if ( 'category' == $taxonomy ) {
			$link = home_url("?feed=$feed&amp;cat=$term_id");
		}
		elseif ( 'post_tag' == $taxonomy ) {
			$link = home_url("?feed=$feed&amp;tag=$term->slug");
		} else {
			$t = get_taxonomy( $taxonomy );
			$link = home_url("?feed=$feed&amp;$t->query_var=$term->slug");
		}
	} else {
		$link = get_term_link( $term_id, $term->taxonomy );
		if ( $feed == get_default_feed() )
			$feed_link = 'feed';
		else
			$feed_link = "feed/$feed";

		$link = trailingslashit( $link ) . user_trailingslashit( $feed_link, 'feed' );
	}

	if ( 'category' == $taxonomy ) {
		/**
		 * Filters the category feed link.
		 *
		 * @since 1.5.1
		 *
		 * @param string $link The category feed link.
		 * @param string $feed Feed type.
		 */
		$link = apply_filters( 'category_feed_link', $link, $feed );
	} elseif ( 'post_tag' == $taxonomy ) {
		/**
		 * Filters the post tag feed link.
		 *
		 * @since 2.3.0
		 *
		 * @param string $link The tag feed link.
		 * @param string $feed Feed type.
		 */
		$link = apply_filters( 'tag_feed_link', $link, $feed );
	} else {
		/**
		 * Filters the feed link for a taxonomy other than 'category' or 'post_tag'.
		 *
		 * @since 3.0.0
		 *
		 * @param string $link The taxonomy feed link.
		 * @param string $feed Feed type.
		 * @param string $feed The taxonomy name.
		 */
		$link = apply_filters( 'taxonomy_feed_link', $link, $feed, $taxonomy );
	}

	return $link;
}

/**
 * Retrieves the permalink for a tag feed.
 *
 * @since 2.3.0
 *
 * @param int    $tag_id Tag ID.
 * @param string $feed   Optional. Feed type. Default empty.
 * @return string The feed permalink for the given tag.
 */
function get_tag_feed_link( $tag_id, $feed = '' ) {
	return get_term_feed_link( $tag_id, 'post_tag', $feed );
}

/**
 * Retrieves the edit link for a tag.
 *
 * @since 2.7.0
 *
 * @param int    $tag_id   Tag ID.
 * @param string $taxonomy Optional. Taxonomy slug. Default 'post_tag'.
 * @return string The edit tag link URL for the given tag.
 */
function get_edit_tag_link( $tag_id, $taxonomy = 'post_tag' ) {
	/**
	 * Filters the edit link for a tag (or term in another taxonomy).
	 *
	 * @since 2.7.0
	 *
	 * @param string $link The term edit link.
	 */
	return apply_filters( 'get_edit_tag_link', get_edit_term_link( $tag_id, $taxonomy ) );
}

/**
 * Displays or retrieves the edit link for a tag with formatting.
 *
 * @since 2.7.0
 *
 * @param string  $link   Optional. Anchor text. Default empty.
 * @param string  $before Optional. Display before edit link. Default empty.
 * @param string  $after  Optional. Display after edit link. Default empty.
 * @param WP_Term $tag    Optional. Term object. If null, the queried object will be inspected.
 *                        Default null.
 */
function edit_tag_link( $link = '', $before = '', $after = '', $tag = null ) {
	$link = edit_term_link( $link, '', '', $tag, false );

	/**
	 * Filters the anchor tag for the edit link for a tag (or term in another taxonomy).
	 *
	 * @since 2.7.0
	 *
	 * @param string $link The anchor tag for the edit link.
	 */
	echo $before . apply_filters( 'edit_tag_link', $link ) . $after;
}

/**
 * Retrieves the URL for editing a given term.
 *
 * @since 3.1.0
 * @since 4.5.0 The `$taxonomy` argument was made optional.
 *
 * @param int    $term_id     Term ID.
 * @param string $taxonomy    Optional. Taxonomy. Defaults to the taxonomy of the term identified
 *                            by `$term_id`.
 * @param string $object_type Optional. The object type. Used to highlight the proper post type
 *                            menu on the linked page. Defaults to the first object_type associated
 *                            with the taxonomy.
 * @return string|null The edit term link URL for the given term, or null on failure.
 */
function get_edit_term_link( $term_id, $taxonomy = '', $object_type = '' ) {
	$term = get_term( $term_id, $taxonomy );
	if ( ! $term || is_wp_error( $term ) ) {
		return;
	}

	$tax = get_taxonomy( $term->taxonomy );
	if ( ! $tax || ! current_user_can( 'edit_term', $term->term_id ) ) {
		return;
	}

	$args = array(
		'taxonomy' => $taxonomy,
		'tag_ID'   => $term->term_id,
	);

	if ( $object_type ) {
		$args['post_type'] = $object_type;
	} elseif ( ! empty( $tax->object_type ) ) {
		$args['post_type'] = reset( $tax->object_type );
	}

	if ( $tax->show_ui ) {
		$location = add_query_arg( $args, admin_url( 'term.php' ) );
	} else {
		$location = '';
	}

	/**
	 * Filters the edit link for a term.
	 *
	 * @since 3.1.0
	 *
	 * @param string $location    The edit link.
	 * @param int    $term_id     Term ID.
	 * @param string $taxonomy    Taxonomy name.
	 * @param string $object_type The object type (eg. the post type).
	 */
	return apply_filters( 'get_edit_term_link', $location, $term_id, $taxonomy, $object_type );
}

/**
 * Displays or retrieves the edit term link with formatting.
 *
 * @since 3.1.0
 *
 * @param string $link   Optional. Anchor text. Default empty.
 * @param string $before Optional. Display before edit link. Default empty.
 * @param string $after  Optional. Display after edit link. Default empty.
 * @param object $term   Optional. Term object. If null, the queried object will be inspected. Default null.
 * @param bool   $echo   Optional. Whether or not to echo the return. Default true.
 * @return string|void HTML content.
 */
function edit_term_link( $link = '', $before = '', $after = '', $term = null, $echo = true ) {
	if ( is_null( $term ) )
		$term = get_queried_object();

	if ( ! $term )
		return;

	$tax = get_taxonomy( $term->taxonomy );
	if ( ! current_user_can( 'edit_term', $term->term_id ) ) {
		return;
	}

	if ( empty( $link ) )
		$link = __('Edit This');

	$link = '<a href="' . get_edit_term_link( $term->term_id, $term->taxonomy ) . '">' . $link . '</a>';

	/**
	 * Filters the anchor tag for the edit link of a term.
	 *
	 * @since 3.1.0
	 *
	 * @param string $link    The anchor tag for the edit link.
	 * @param int    $term_id Term ID.
	 */
	$link = $before . apply_filters( 'edit_term_link', $link, $term->term_id ) . $after;

	if ( $echo )
		echo $link;
	else
		return $link;
}

/**
 * Retrieves the permalink for a search.
 *
 * @since  3.0.0
 *
 * @global WP_Rewrite $wp_rewrite
 *
 * @param string $query Optional. The query string to use. If empty the current query is used. Default empty.
 * @return string The search permalink.
 */
function get_search_link( $query = '' ) {
	global $wp_rewrite;

	if ( empty($query) )
		$search = get_search_query( false );
	else
		$search = stripslashes($query);

	$permastruct = $wp_rewrite->get_search_permastruct();

	if ( empty( $permastruct ) ) {
		$link = home_url('?s=' . urlencode($search) );
	} else {
		$search = urlencode($search);
		$search = str_replace('%2F', '/', $search); // %2F(/) is not valid within a URL, send it un-encoded.
		$link = str_replace( '%search%', $search, $permastruct );
		$link = home_url( user_trailingslashit( $link, 'search' ) );
	}

	/**
	 * Filters the search permalink.
	 *
	 * @since 3.0.0
	 *
	 * @param string $link   Search permalink.
	 * @param string $search The URL-encoded search term.
	 */
	return apply_filters( 'search_link', $link, $search );
}

/**
 * Retrieves the permalink for the search results feed.
 *
 * @since 2.5.0
 *
 * @global WP_Rewrite $wp_rewrite
 *
 * @param string $search_query Optional. Search query. Default empty.
 * @param string $feed         Optional. Feed type. Default empty.
 * @return string The search results feed permalink.
 */
function get_search_feed_link($search_query = '', $feed = '') {
	global $wp_rewrite;
	$link = get_search_link($search_query);

	if ( empty($feed) )
		$feed = get_default_feed();

	$permastruct = $wp_rewrite->get_search_permastruct();

	if ( empty($permastruct) ) {
		$link = add_query_arg('feed', $feed, $link);
	} else {
		$link = trailingslashit($link);
		$link .= "feed/$feed/";
	}

	/**
	 * Filters the search feed link.
	 *
	 * @since 2.5.0
	 *
	 * @param string $link Search feed link.
	 * @param string $feed Feed type.
	 * @param string $type The search type. One of 'posts' or 'comments'.
	 */
	return apply_filters( 'search_feed_link', $link, $feed, 'posts' );
}

/**
 * Retrieves the permalink for the search results comments feed.
 *
 * @since 2.5.0
 *
 * @global WP_Rewrite $wp_rewrite
 *
 * @param string $search_query Optional. Search query. Default empty.
 * @param string $feed         Optional. Feed type. Default empty.
 * @return string The comments feed search results permalink.
 */
function get_search_comments_feed_link($search_query = '', $feed = '') {
	global $wp_rewrite;

	if ( empty($feed) )
		$feed = get_default_feed();

	$link = get_search_feed_link($search_query, $feed);

	$permastruct = $wp_rewrite->get_search_permastruct();

	if ( empty($permastruct) )
		$link = add_query_arg('feed', 'comments-' . $feed, $link);
	else
		$link = add_query_arg('withcomments', 1, $link);

	/** This filter is documented in wp-includes/link-template.php */
	return apply_filters( 'search_feed_link', $link, $feed, 'comments' );
}

/**
 * Retrieves the permalink for a post type archive.
 *
 * @since 3.1.0
 * @since 4.5.0 Support for posts was added.
 *
 * @global WP_Rewrite $wp_rewrite
 *
 * @param string $post_type Post type.
 * @return string|false The post type archive permalink.
 */
function get_post_type_archive_link( $post_type ) {
	global $wp_rewrite;
	if ( ! $post_type_obj = get_post_type_object( $post_type ) )
		return false;

	if ( 'post' === $post_type ) {
		$show_on_front = get_option( 'show_on_front' );
		$page_for_posts  = get_option( 'page_for_posts' );

		if ( 'page' == $show_on_front && $page_for_posts ) {
			$link = get_permalink( $page_for_posts );
		} else {
			$link = get_home_url();
		}
		/** This filter is documented in wp-includes/link-template.php */
		return apply_filters( 'post_type_archive_link', $link, $post_type );
	}

	if ( ! $post_type_obj->has_archive )
		return false;

	if ( get_option( 'permalink_structure' ) && is_array( $post_type_obj->rewrite ) ) {
		$struct = ( true === $post_type_obj->has_archive ) ? $post_type_obj->rewrite['slug'] : $post_type_obj->has_archive;
		if ( $post_type_obj->rewrite['with_front'] )
			$struct = $wp_rewrite->front . $struct;
		else
			$struct = $wp_rewrite->root . $struct;
		$link = home_url( user_trailingslashit( $struct, 'post_type_archive' ) );
	} else {
		$link = home_url( '?post_type=' . $post_type );
	}

	/**
	 * Filters the post type archive permalink.
	 *
	 * @since 3.1.0
	 *
	 * @param string $link      The post type archive permalink.
	 * @param string $post_type Post type name.
	 */
	return apply_filters( 'post_type_archive_link', $link, $post_type );
}

/**
 * Retrieves the permalink for a post type archive feed.
 *
 * @since 3.1.0
 *
 * @param string $post_type Post type
 * @param string $feed      Optional. Feed type. Default empty.
 * @return string|false The post type feed permalink.
 */
function get_post_type_archive_feed_link( $post_type, $feed = '' ) {
	$default_feed = get_default_feed();
	if ( empty( $feed ) )
		$feed = $default_feed;

	if ( ! $link = get_post_type_archive_link( $post_type ) )
		return false;

	$post_type_obj = get_post_type_object( $post_type );
	if ( get_option( 'permalink_structure' ) && is_array( $post_type_obj->rewrite ) && $post_type_obj->rewrite['feeds'] ) {
		$link = trailingslashit( $link );
		$link .= 'feed/';
		if ( $feed != $default_feed )
			$link .= "$feed/";
	} else {
		$link = add_query_arg( 'feed', $feed, $link );
	}

	/**
	 * Filters the post type archive feed link.
	 *
	 * @since 3.1.0
	 *
	 * @param string $link The post type archive feed link.
	 * @param string $feed Feed type.
	 */
	return apply_filters( 'post_type_archive_feed_link', $link, $feed );
}

/**
 * Retrieves the URL used for the post preview.
 *
 * Allows additional query args to be appended.
 *
 * @since 4.4.0
 *
 * @param int|WP_Post $post         Optional. Post ID or `WP_Post` object. Defaults to global `$post`.
 * @param array       $query_args   Optional. Array of additional query args to be appended to the link.
 *                                  Default empty array.
 * @param string      $preview_link Optional. Base preview link to be used if it should differ from the
 *                                  post permalink. Default empty.
 * @return string|null URL used for the post preview, or null if the post does not exist.
 */
function get_preview_post_link( $post = null, $query_args = array(), $preview_link = '' ) {
	$post = get_post( $post );
	if ( ! $post ) {
		return;
	}

	$post_type_object = get_post_type_object( $post->post_type );
	if ( is_post_type_viewable( $post_type_object ) ) {
		if ( ! $preview_link ) {
			$preview_link = set_url_scheme( get_permalink( $post ) );
		}

		$query_args['preview'] = 'true';
		$preview_link = add_query_arg( $query_args, $preview_link );
	}

	/**
	 * Filters the URL used for a post preview.
	 *
	 * @since 2.0.5
	 * @since 4.0.0 Added the `$post` parameter.
	 *
	 * @param string  $preview_link URL used for the post preview.
	 * @param WP_Post $post         Post object.
	 */
	return apply_filters( 'preview_post_link', $preview_link, $post );
}

/**
 * Retrieves the edit post link for post.
 *
 * Can be used within the WordPress loop or outside of it. Can be used with
 * pages, posts, attachments, and revisions.
 *
 * @since 2.3.0
 *
 * @param int    $id      Optional. Post ID. Default is the ID of the global `$post`.
 * @param string $context Optional. How to output the '&' character. Default '&amp;'.
 * @return string|null The edit post link for the given post. null if the post type is invalid or does
 *                     not allow an editing UI.
 */
function get_edit_post_link( $id = 0, $context = 'display' ) {
	if ( ! $post = get_post( $id ) )
		return;

	if ( 'revision' === $post->post_type )
		$action = '';
	elseif ( 'display' == $context )
		$action = '&amp;action=edit';
	else
		$action = '&action=edit';

	$post_type_object = get_post_type_object( $post->post_type );
	if ( !$post_type_object )
		return;

	if ( !current_user_can( 'edit_post', $post->ID ) )
		return;

	if ( $post_type_object->_edit_link ) {
		$link = admin_url( sprintf( $post_type_object->_edit_link . $action, $post->ID ) );
	} else {
		$link = '';
	}

	/**
	 * Filters the post edit link.
	 *
	 * @since 2.3.0
	 *
	 * @param string $link    The edit link.
	 * @param int    $post_id Post ID.
	 * @param string $context The link context. If set to 'display' then ampersands
	 *                        are encoded.
	 */
	return apply_filters( 'get_edit_post_link', $link, $post->ID, $context );
}

/**
 * Displays the edit post link for post.
 *
 * @since 1.0.0
 * @since 4.4.0 The `$class` argument was added.
 *
 * @param string $text   Optional. Anchor text. If null, default is 'Edit This'. Default null.
 * @param string $before Optional. Display before edit link. Default empty.
 * @param string $after  Optional. Display after edit link. Default empty.
 * @param int    $id     Optional. Post ID. Default is the ID of the global `$post`.
 * @param string $class  Optional. Add custom class to link. Default 'post-edit-link'.
 */
function edit_post_link( $text = null, $before = '', $after = '', $id = 0, $class = 'post-edit-link' ) {
	if ( ! $post = get_post( $id ) ) {
		return;
	}

	if ( ! $url = get_edit_post_link( $post->ID ) ) {
		return;
	}

	if ( null === $text ) {
		$text = __( 'Edit This' );
	}

	$link = '<a class="' . esc_attr( $class ) . '" href="' . esc_url( $url ) . '">' . $text . '</a>';

	/**
	 * Filters the post edit link anchor tag.
	 *
	 * @since 2.3.0
	 *
	 * @param string $link    Anchor tag for the edit link.
	 * @param int    $post_id Post ID.
	 * @param string $text    Anchor text.
	 */
	echo $before . apply_filters( 'edit_post_link', $link, $post->ID, $text ) . $after;
}

/**
 * Retrieves the delete posts link for post.
 *
 * Can be used within the WordPress loop or outside of it, with any post type.
 *
 * @since 2.9.0
 *
 * @param int    $id           Optional. Post ID. Default is the ID of the global `$post`.
 * @param string $deprecated   Not used.
 * @param bool   $force_delete Optional. Whether to bypass trash and force deletion. Default false.
 * @return string|void The delete post link URL for the given post.
 */
function get_delete_post_link( $id = 0, $deprecated = '', $force_delete = false ) {
	if ( ! empty( $deprecated ) )
		_deprecated_argument( __FUNCTION__, '3.0.0' );

	if ( !$post = get_post( $id ) )
		return;

	$post_type_object = get_post_type_object( $post->post_type );
	if ( !$post_type_object )
		return;

	if ( !current_user_can( 'delete_post', $post->ID ) )
		return;

	$action = ( $force_delete || !EMPTY_TRASH_DAYS ) ? 'delete' : 'trash';

	$delete_link = add_query_arg( 'action', $action, admin_url( sprintf( $post_type_object->_edit_link, $post->ID ) ) );

	/**
	 * Filters the post delete link.
	 *
	 * @since 2.9.0
	 *
	 * @param string $link         The delete link.
	 * @param int    $post_id      Post ID.
	 * @param bool   $force_delete Whether to bypass the trash and force deletion. Default false.
	 */
	return apply_filters( 'get_delete_post_link', wp_nonce_url( $delete_link, "$action-post_{$post->ID}" ), $post->ID, $force_delete );
}

/**
 * Retrieves the edit comment link.
 *
 * @since 2.3.0
 *
 * @param int|WP_Comment $comment_id Optional. Comment ID or WP_Comment object.
 * @return string|void The edit comment link URL for the given comment.
 */
function get_edit_comment_link( $comment_id = 0 ) {
	$comment = get_comment( $comment_id );

	if ( !current_user_can( 'edit_comment', $comment->comment_ID ) )
		return;

	$location = admin_url('comment.php?action=editcomment&amp;c=') . $comment->comment_ID;

	/**
	 * Filters the comment edit link.
	 *
	 * @since 2.3.0
	 *
	 * @param string $location The edit link.
	 */
	return apply_filters( 'get_edit_comment_link', $location );
}

/**
 * Displays the edit comment link with formatting.
 *
 * @since 1.0.0
 *
 * @param string $text   Optional. Anchor text. If null, default is 'Edit This'. Default null.
 * @param string $before Optional. Display before edit link. Default empty.
 * @param string $after  Optional. Display after edit link. Default empty.
 */
function edit_comment_link( $text = null, $before = '', $after = '' ) {
	$comment = get_comment();

	if ( ! current_user_can( 'edit_comment', $comment->comment_ID ) ) {
		return;
	}

	if ( null === $text ) {
		$text = __( 'Edit This' );
	}

	$link = '<a class="comment-edit-link" href="' . esc_url( get_edit_comment_link( $comment ) ) . '">' . $text . '</a>';

	/**
	 * Filters the comment edit link anchor tag.
	 *
	 * @since 2.3.0
	 *
	 * @param string $link       Anchor tag for the edit link.
	 * @param int    $comment_id Comment ID.
	 * @param string $text       Anchor text.
	 */
	echo $before . apply_filters( 'edit_comment_link', $link, $comment->comment_ID, $text ) . $after;
}

/**
 * Displays the edit bookmark link.
 *
 * @since 2.7.0
 *
 * @param int|stdClass $link Optional. Bookmark ID. Default is the id of the current bookmark.
 * @return string|void The edit bookmark link URL.
 */
function get_edit_bookmark_link( $link = 0 ) {
	$link = get_bookmark( $link );

	if ( !current_user_can('manage_links') )
		return;

	$location = admin_url('link.php?action=edit&amp;link_id=') . $link->link_id;

	/**
	 * Filters the bookmark edit link.
	 *
	 * @since 2.7.0
	 *
	 * @param string $location The edit link.
	 * @param int    $link_id  Bookmark ID.
	 */
	return apply_filters( 'get_edit_bookmark_link', $location, $link->link_id );
}

/**
 * Displays the edit bookmark link anchor content.
 *
 * @since 2.7.0
 *
 * @param string $link     Optional. Anchor text. Default empty.
 * @param string $before   Optional. Display before edit link. Default empty.
 * @param string $after    Optional. Display after edit link. Default empty.
 * @param int    $bookmark Optional. Bookmark ID. Default is the current bookmark.
 */
function edit_bookmark_link( $link = '', $before = '', $after = '', $bookmark = null ) {
	$bookmark = get_bookmark($bookmark);

	if ( !current_user_can('manage_links') )
		return;

	if ( empty($link) )
		$link = __('Edit This');

	$link = '<a href="' . esc_url( get_edit_bookmark_link( $bookmark ) ) . '">' . $link . '</a>';

	/**
	 * Filters the bookmark edit link anchor tag.
	 *
	 * @since 2.7.0
	 *
	 * @param string $link    Anchor tag for the edit link.
	 * @param int    $link_id Bookmark ID.
	 */
	echo $before . apply_filters( 'edit_bookmark_link', $link, $bookmark->link_id ) . $after;
}

/**
 * Retrieves the edit user link.
 *
 * @since 3.5.0
 *
 * @param int $user_id Optional. User ID. Defaults to the current user.
 * @return string URL to edit user page or empty string.
 */
function get_edit_user_link( $user_id = null ) {
	if ( ! $user_id )
		$user_id = get_current_user_id();

	if ( empty( $user_id ) || ! current_user_can( 'edit_user', $user_id ) )
		return '';

	$user = get_userdata( $user_id );

	if ( ! $user )
		return '';

	if ( get_current_user_id() == $user->ID )
		$link = get_edit_profile_url( $user->ID );
	else
		$link = add_query_arg( 'user_id', $user->ID, self_admin_url( 'user-edit.php' ) );

	/**
	 * Filters the user edit link.
	 *
	 * @since 3.5.0
	 *
	 * @param string $link    The edit link.
	 * @param int    $user_id User ID.
	 */
	return apply_filters( 'get_edit_user_link', $link, $user->ID );
}

// Navigation links

/**
 * Retrieves the previous post that is adjacent to the current post.
 *
 * @since 1.5.0
 *
 * @param bool         $in_same_term   Optional. Whether post should be in a same taxonomy term. Default false.
 * @param array|string $excluded_terms Optional. Array or comma-separated list of excluded term IDs. Default empty.
 * @param string       $taxonomy       Optional. Taxonomy, if $in_same_term is true. Default 'category'.
 * @return null|string|WP_Post Post object if successful. Null if global $post is not set. Empty string if no
 *                             corresponding post exists.
 */
function get_previous_post( $in_same_term = false, $excluded_terms = '', $taxonomy = 'category' ) {
	return get_adjacent_post( $in_same_term, $excluded_terms, true, $taxonomy );
}

/**
 * Retrieves the next post that is adjacent to the current post.
 *
 * @since 1.5.0
 *
 * @param bool         $in_same_term   Optional. Whether post should be in a same taxonomy term. Default false.
 * @param array|string $excluded_terms Optional. Array or comma-separated list of excluded term IDs. Default empty.
 * @param string       $taxonomy       Optional. Taxonomy, if $in_same_term is true. Default 'category'.
 * @return null|string|WP_Post Post object if successful. Null if global $post is not set. Empty string if no
 *                             corresponding post exists.
 */
function get_next_post( $in_same_term = false, $excluded_terms = '', $taxonomy = 'category' ) {
	return get_adjacent_post( $in_same_term, $excluded_terms, false, $taxonomy );
}

/**
 * Retrieves the adjacent post.
 *
 * Can either be next or previous post.
 *
 * @since 2.5.0
 *
 * @global wpdb $wpdb WordPress database abstraction object.
 *
 * @param bool         $in_same_term   Optional. Whether post should be in a same taxonomy term. Default false.
 * @param array|string $excluded_terms Optional. Array or comma-separated list of excluded term IDs. Default empty.
 * @param bool         $previous       Optional. Whether to retrieve previous post. Default true
 * @param string       $taxonomy       Optional. Taxonomy, if $in_same_term is true. Default 'category'.
 * @return null|string|WP_Post Post object if successful. Null if global $post is not set. Empty string if no
 *                             corresponding post exists.
 */
function get_adjacent_post( $in_same_term = false, $excluded_terms = '', $previous = true, $taxonomy = 'category' ) {
	global $wpdb;

	if ( ( ! $post = get_post() ) || ! taxonomy_exists( $taxonomy ) )
		return null;

	$current_post_date = $post->post_date;

	$join = '';
	$where = '';
	$adjacent = $previous ? 'previous' : 'next';

	if ( $in_same_term || ! empty( $excluded_terms ) ) {
		if ( ! empty( $excluded_terms ) && ! is_array( $excluded_terms ) ) {
			// back-compat, $excluded_terms used to be $excluded_terms with IDs separated by " and "
			if ( false !== strpos( $excluded_terms, ' and ' ) ) {
				_deprecated_argument( __FUNCTION__, '3.3.0', sprintf( __( 'Use commas instead of %s to separate excluded terms.' ), "'and'" ) );
				$excluded_terms = explode( ' and ', $excluded_terms );
			} else {
				$excluded_terms = explode( ',', $excluded_terms );
			}

			$excluded_terms = array_map( 'intval', $excluded_terms );
		}

		if ( $in_same_term ) {
			$join .= " INNER JOIN $wpdb->term_relationships AS tr ON p.ID = tr.object_id INNER JOIN $wpdb->term_taxonomy tt ON tr.term_taxonomy_id = tt.term_taxonomy_id";
			$where .= $wpdb->prepare( "AND tt.taxonomy = %s", $taxonomy );

			if ( ! is_object_in_taxonomy( $post->post_type, $taxonomy ) )
				return '';
			$term_array = wp_get_object_terms( $post->ID, $taxonomy, array( 'fields' => 'ids' ) );

			// Remove any exclusions from the term array to include.
			$term_array = array_diff( $term_array, (array) $excluded_terms );
			$term_array = array_map( 'intval', $term_array );

			if ( ! $term_array || is_wp_error( $term_array ) )
				return '';

			$where .= " AND tt.term_id IN (" . implode( ',', $term_array ) . ")";
		}

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