<?php
/*
Plugin Name: Add Headers
Plugin URI: https://bitbucket.org/gnotaras/wordpress-add-headers
Description: Adds the ETag, Last-Modified, Expires and Cache-Control headers to HTTP responses generated by WordPress to facilitate caching.
Version: 2.0.1
Author: George Notaras
Author URI: http://www.g-loaded.eu/
License: GPLv3
*/

/**
 *  This file is part of the Add-Headers distribution package.
 *
 *  Add-Headers is an extension for the WordPress publishing platform.
 *
 *  Homepage:
 *  - http://wordpress.org/plugins/add-headers/
 *  Documentation:
 *  - http://www.codetrax.org/projects/wp-add-headers/wiki
 *  Development Web Site and Bug Tracker:
 *  - http://www.codetrax.org/projects/wp-add-headers
 *  Main Source Code Repository (Mercurial):
 *  - https://bitbucket.org/gnotaras/wordpress-add-headers
 *  Mirror repository (Git):
 *  - https://github.com/gnotaras/wordpress-add-headers
 *
 *  Licensing Information
 *
 *  Copyright 2013-2015 George Notaras <gnot@g-loaded.eu>, CodeTRAX.org
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

// Prevent direct access to this file.
if ( ! defined( 'ABSPATH' ) ) {
    header( 'HTTP/1.0 403 Forbidden' );
    echo 'This file should not be accessed directly!';
    exit; // Exit if accessed directly
}

// Store plugin directory
//define( 'ADDH_PLUGIN_DIR', plugin_dir_path( __FILE__ ) );
// Store plugin main file path
//define( 'ADDH_PLUGIN_FILE', __FILE__ );


// Helper function that returns an array of supported post types when is_singular()
function addh_get_supported_post_types_singular() {
    $supported_builtin_types = array('post', 'page', 'attachment');
    //$public_custom_types = get_post_types( array('public'=>true, '_builtin'=>false, 'show_ui'=>true) );
    $public_custom_types = get_post_types( array('public'=>true, '_builtin'=>false) );
    $supported_types = array_merge($supported_builtin_types, $public_custom_types);

    // Allow filtering of the supported content types.
    $supported_types = apply_filters( 'addh_supported_post_types_singular', $supported_types );

    return $supported_types;
}


// Helper function that returns an array of supported post types when is_archive()
function addh_get_supported_post_types_archive() {
    $supported_builtin_types = array('post');
    //$public_custom_types = get_post_types( array('public'=>true, '_builtin'=>false, 'show_ui'=>true) );
    $public_custom_types = get_post_types( array('public'=>true, '_builtin'=>false) );
    $supported_types = array_merge($supported_builtin_types, $public_custom_types);

    // Allow filtering of the supported content types.
    $supported_types = apply_filters( 'addh_supported_post_types_archive', $supported_types );

    return $supported_types;
}


// Send headers to client
function addh_send_headers( $headers_arr, $options ) {
    if ( empty($headers_arr) ) {
        return;
    }
    // First check if headers have already been sent and, if yes, generate friendly warning.
    if ( headers_sent() ) {
        $warning_message = 'Add-Headers tried to append its headers to the headers list, but the header list had already been sent to the client by other code. This is not a problem caused by the Add-Headers plugin. Moreover, the Add-Headers plugin can do nothing about this issue. Please investigate which plugin or theme sends the headers earlier than what is expected.';
        trigger_error($warning_message, E_USER_WARNING);
        return;
    }
    // Clean up pre-existing headers
    if ( array_key_exists('remove_pre_existing_headers', $options) && $options['remove_pre_existing_headers'] === true ) {
        $current_headers = headers_list();
        $supported_headers = array('ETag', 'Last-Modified', 'Expires', 'Cache-Control', 'Pragma');
        foreach ($current_headers as $current_header ) {
            foreach ($supported_headers as $supported_header) {
                if ( strpos($current_header, $supported_header) === false ) {
                    header_remove($supported_header);
                }
            }
        }
    }
    // Send the headers
    foreach ( $headers_arr as $header_name => $header_value ) {
        if ( ! empty($header_name) && ! empty($header_value) ) {
            header( sprintf('%s: %s', $header_name, $header_value) );
        }
    }
}


// ETag
// The generated ETag is unique for every post page. In order to generate it
// ``$wp->query_vars`` are used among other properties so as to generate a
// unique ETag even for archive on which the latest post is the same.
function addh_generate_etag_header( $post, $mtime, $options ) {
    global $wp;
    if ( $options['add_etag_header'] === true ) {
        $to_hash = array( $mtime, $post->post_date_gmt, $post->guid, $post->ID, serialize( $wp->query_vars ) );
        $header_etag_value = sha1( serialize( $to_hash ) );
        // Generate a weak or strong ETag
        if ( $options['generate_weak_etag'] === true ) {
            return sprintf( 'W/"%s"', $header_etag_value );
        } else {
            return sprintf( '"%s"', $header_etag_value );
        }
    }
}


// Last-Modified
function addh_generate_last_modified_header( $post, $mtime, $options ) {
    if ( $options['add_last_modified_header'] === true ) {
        $header_last_modified_value = str_replace( '+0000', 'GMT', gmdate('r', $mtime) );
        return $header_last_modified_value;
    }
}


// Expires (Calculated from client access time, aka current time)
function addh_generate_expires_header( $post, $mtime, $options ) {
    if ( $options['add_expires_header'] === true ) {
        // See also:  $current_time_gmt = (int) gmdate('U');
        $header_expires_value = str_replace( '+0000', 'GMT', gmdate('r', time() + $options['cache_max_age_seconds'] ) );
        return $header_expires_value;
    }
}


// Cache-Control
function addh_generate_cache_control_header( $post, $mtime, $options ) {
    if ( $options['add_cache_control_header'] === true ) {
        if ( intval($options['cache_max_age_seconds']) > 0 ) {
            $default_cache_control_template = 'public, max-age=%s';
            $cache_control_template = apply_filters( 'addh_cache_control_header_format', $default_cache_control_template );
            $header_cache_control_value = sprintf( $cache_control_template, $options['cache_max_age_seconds'] );
            return $header_cache_control_value;
        } else {
            return 'no-cache, must-revalidate, max-age=0';
        }
    }
}


// Pragma
// This header is set to either `no-cache` or `cache` for HTTP 1.0 compatibility.
// The same checks take place as for the Cache-Control header.
// The addition of this header is controlled by the `add_cache_control_header` option.
// No separate option should be required for this header.
function addh_generate_pragma_header( $post, $mtime, $options ) {
    if ( $options['add_cache_control_header'] === true ) {
        if ( intval($options['cache_max_age_seconds']) > 0 ) {
            return 'public';
        } else {
            return 'no-cache';
        }
    }
}


/**
 * Generates headers in batch
 */
function addh_batch_generate_headers( $post, $mtime, $options ) {

    $headers_arr = array();

    // ETag
    $headers_arr['ETag'] = addh_generate_etag_header( $post, $mtime, $options );
    // Last-Modified
    $headers_arr['Last-Modified'] = addh_generate_last_modified_header( $post, $mtime, $options );
    // Expires (Calculated from client access time, aka current time)
    $headers_arr['Expires'] = addh_generate_expires_header( $post, $mtime, $options );
    // Cache-Control
    $headers_arr['Cache-Control'] = addh_generate_cache_control_header( $post, $mtime, $options );
    // Pragma
    $headers_arr['Pragma'] = addh_generate_pragma_header( $post, $mtime, $options );
    // Allow filtering of the generated headers
    $headers_arr = apply_filters( 'addh_headers', $headers_arr );

    // Send headers
    addh_send_headers( $headers_arr, $options );
}


/**
 * Sets headers on post object pages (posts, pages, attachments, custom
 * post types).
 *
 * In order to calculate the modified time, two time sources are used:
 *   1) the post object's modified time.
 *   2) the modified time of the most recent comment that is attached to the post object.
 * The most "recent" timestamp of the two is returned.
 */
function addh_set_headers_for_object( $options ) {

    // Get current queried object.
    $post = get_queried_object();
    // Valid post types: post, page, attachment, public custom post types
    if ( ! is_object($post) || ! isset($post->post_type) || ! in_array( get_post_type($post), addh_get_supported_post_types_singular() ) ) {
        return;
    }

    // Check for password protected posts
    if ( post_password_required() ) {
        return;
    }

    // Retrieve stored time of post object
    $post_mtime = $post->post_modified_gmt;
    $post_mtime_unix = strtotime( $post_mtime );

    // Initially set the $mtime to the post mtime timestamp
    $mtime = $post_mtime_unix;

    // If there are comments attached to this post object, find the mtime of
    // the most recent comment.
    if ( intval($post->comment_count) > 0 ) {

        // Retrieve the mtime of the most recent comment
        $comments = get_comments( array(
            'status' => 'approve',
            'orderby' => 'comment_date_gmt',
            'number' => '1',
            'post_id' => $post->ID
        ) );
        if ( ! empty($comments) ) {
            $comment = $comments[0];
            $comment_mtime = $comment->comment_date_gmt;
            $comment_mtime_unix = strtotime( $comment_mtime );
            // Compare the two mtimes and keep the most recent (higher) one.
            if ( $comment_mtime_unix > $post_mtime_unix ) {
                $mtime = $comment_mtime_unix;
            }
        }
    }

    // Check for old content (only if a threshold is set)
    if ( absint($options['cache_old_content_threshold_seconds']) > 0 ) {
        // Find the time that determines whether a resource should be treated as 'old content'.
        // Basically this is the current GM time minus the 'old content threshold in seconds' as set in the option.
        $threshold_time_unix = absint(gmdate('U')) - absint($options['cache_old_content_threshold_seconds']);
        // Any content which has been last modified before that time should be treated as old
        // and the 'cache_old_content_max_age_seconds' option should be used as the caching
        // timeout, instead of 'cache_max_age_seconds'.
        // Here we check whether the mtime (last modified time) is older than the threshold time.
        if ( $mtime < $threshold_time_unix ) {
            // This content is old. Set the caching timeout to the caching timeout of old content.
            $options['cache_max_age_seconds'] = $options['cache_old_content_max_age_seconds'];
        }
    }

    addh_batch_generate_headers( $post, $mtime, $options );
}


/**
 * Sets headers on archives
 */
function addh_set_headers_for_archive( $options ) {

    // WordPress archives list the posts that belong to the archive from
    // newest to oldest. We set the HTTP headers for the archive page based
    // on the first post of the archive (newest).
    // There is no need to check for pagination, since every page of the archive
    // has different posts.
    //global $post; // Using this is possibly a mistake. This should be term/author/date object of the archive.

    // Get our post object from the list of posts.
    global $posts;
    if ( empty($posts) ) {
        return;
    }
    $post = $posts[0];

    // The post object we use for the HTTP headers is the latest post.
    // Here it is possible to filter this post object and use what you want.
    $post = apply_filters( 'addh_archive_post', $post );

    // Valid post types: post
    if ( ! is_object($post) || ! isset($post->post_type) || ! in_array( get_post_type($post), addh_get_supported_post_types_archive() ) ) {
        return;
    }

    // Retrieve stored time of post object
    $post_mtime = $post->post_modified_gmt;
    $mtime = strtotime( $post_mtime );

    // Check for old content (only if a threshold is set)
    if ( absint($options['cache_old_content_threshold_seconds']) > 0 ) {
        // Find the time that determines whether a resource should be treated as 'old content'.
        // Basically this is the current GM time minus the 'old content threshold in seconds' as set in the option.
        $threshold_time_unix = absint(gmdate('U')) - absint($options['cache_old_content_threshold_seconds']);
        // Any content which has been last modified before that time should be treated as old
        // and the 'cache_old_content_max_age_seconds' option should be used as the caching
        // timeout, instead of 'cache_max_age_seconds'.
        // Here we check whether the mtime (last modified time) is older than the threshold time.
        if ( $mtime < $threshold_time_unix ) {
            // This content is old. Set the caching timeout to the caching timeout of old content.
            $options['cache_max_age_seconds'] = $options['cache_old_content_max_age_seconds'];
        }
    }

    addh_batch_generate_headers( $post, $mtime, $options );
}


/**
 * Main function.
 */
function addh_headers(){

    // Early checks
    //
    // Note: These might not be necessary since conditional tags possibly
    // exclude any processing in the following cases.
    //
    if ( defined( 'DOING_AJAX' ) && DOING_AJAX ) {
        // Check for AJAX call.
        return;
    } elseif( defined('XMLRPC_REQUEST') && XMLRPC_REQUEST ) {
        // Check for XMLRPC request.
        return;
    } elseif( defined('REST_REQUEST') && REST_REQUEST ) {
        // Check for REST request.
        return;
    } elseif ( is_admin() ) {
        // Check if we are in the WP admin interface.
        return;
    }

    // Notes
    //
    // 1. WordPress by default generates the ETag and Last-Modified headers for feeds.
    // If 'add_etag_header' and/or 'add_last_modified_header' has been set to 'false',
    // then an ETag header and/or a Last-Modified header might appear in the response.
    // These are not added by Add-Headers, but by WordPress. To get rid of them,
    // you will have to enable the 'remove_pre_existing_headers' option,
    // which deletes any of the headers (supported by this plugin) before adding its own.

    // Options
    $default_options = array(
        'add_etag_header' => true,
        'generate_weak_etag' => false,
        'add_last_modified_header' => true,
        'add_expires_header' => true,
        'add_cache_control_header' => true,
        'cache_max_age_seconds' => 0,
        'cache_max_age_seconds_for_search_results' => 0,
        'cache_max_age_seconds_for_authenticated_users' => 0,
        'cache_old_content_threshold_seconds' => 0,
        'cache_old_content_max_age_seconds' => 0,
        'remove_pre_existing_headers' => false,
    );
    $options = apply_filters( 'addh_options', $default_options );

    // Adjust `cache_max_age_seconds` for authenticated users.
    if ( is_user_logged_in() ) {
        $options['cache_max_age_seconds'] = $options['cache_max_age_seconds_for_authenticated_users'];
    }

    // Feeds
    // Note: At the time of writing, WordPress 3.8 feeds already have ETag and
    // Last-Modified headers. Here we reprocess them and add our own.
    // One problem with the default feed ETag, it is the same for RSS and Atom
    // and possibly other feed types, which is not the right way to do it.
    if ( is_feed() ) {
        // Process feeds as archives
        addh_set_headers_for_archive( $options );
    }

    // Adds headers to:
    // - Post objects (posts, pages, attachments, custom post types)
    // - Static front page
    elseif ( is_singular() ) {
        addh_set_headers_for_object( $options );
    }
    
    // Adds headers to:
    // - Category, Tag, other Taxonomy Term, custom post type archive, Author and Date-based archives.
    // - Search results
    // - Default front page displaying the latest posts
    // - Static page displaying the latest posts
    elseif ( is_archive() || is_search() || is_home() ) {
        if ( is_search() ) {
            $options['cache_max_age_seconds'] = $options['cache_max_age_seconds_for_search_results'];
        }
        addh_set_headers_for_archive( $options );
    }

}


// Action hooks - order of running
// * https://codex.wordpress.org/Plugin_API/Action_Reference
// Also see action hooks:
// * https://codex.wordpress.org/Plugin_API/Action_Reference/send_headers
// * https://codex.wordpress.org/Plugin_API/Action_Reference/wp
// * https://codex.wordpress.org/Plugin_API/Action_Reference/template_redirect
// * https://codex.wordpress.org/Plugin_API/Action_Reference/posts_selection
//
// http://wordpress.stackexchange.com/a/20193
add_action('template_redirect', 'addh_headers');