<?php
/**
 * Speculative loading functions.
 *
 * @package WordPress
 * @subpackage Speculative Loading
 * @since 6.8.0
 */

/**
 * Returns the speculation rules configuration.
 *
 * @since 6.8.0
 *
 * @return array<string, string>|null Associative array with 'mode' and 'eagerness' keys, or null if speculative
 *                                    loading is disabled.
 */
function wp_get_speculation_rules_configuration(): ?array {
    // By default, speculative loading is only enabled for sites with pretty permalinks when no user is logged in.
    if ( ! is_user_logged_in() && get_option( 'permalink_structure' ) ) {
        $config = array(
            'mode'      => 'auto',
            'eagerness' => 'auto',
        );
    } else {
        $config = null;
    }

    /**
     * Filters the way that speculation rules are configured.
     *
     * The Speculation Rules API is a web API that allows to automatically prefetch or prerender certain URLs on the
     * page, which can lead to near-instant page load times. This is also referred to as speculative loading.
     *
     * There are two aspects to the configuration:
     * * The "mode" (whether to "prefetch" or "prerender" URLs).
     * * The "eagerness" (whether to speculatively load URLs in an "eager", "moderate", or "conservative" way).
     *
     * By default, the speculation rules configuration is decided by WordPress Core ("auto"). This filter can be used
     * to force a certain configuration, which could for instance load URLs more or less eagerly.
     *
     * For logged-in users or for sites that are not configured to use pretty permalinks, the default value is `null`,
     * indicating that speculative loading is entirely disabled.
     *
     * @since 6.8.0
     * @see https://developer.chrome.com/docs/web-platform/prerender-pages
     *
     * @param array<string, string>|null $config Associative array with 'mode' and 'eagerness' keys, or `null`. The
     *                                           default value for both of the keys is 'auto'. Other possible values
     *                                           for 'mode' are 'prefetch' and 'prerender'. Other possible values for
     *                                           'eagerness' are 'eager', 'moderate', and 'conservative'. The value
     *                                           `null` is used to disable speculative loading entirely.
     */
    $config = apply_filters( 'wp_speculation_rules_configuration', $config );

    // Allow the value `null` to indicate that speculative loading is disabled.
    if ( null === $config ) {
        return null;
    }

    // Sanitize the configuration and replace 'auto' with current defaults.
    $default_mode      = 'prefetch';
    $default_eagerness = 'conservative';
    if ( ! is_array( $config ) ) {
        return array(
            'mode'      => $default_mode,
            'eagerness' => $default_eagerness,
        );
    }
    if (
        ! isset( $config['mode'] ) ||
        'auto' === $config['mode'] ||
        ! WP_Speculation_Rules::is_valid_mode( $config['mode'] )
    ) {
        $config['mode'] = $default_mode;
    }
    if (
        ! isset( $config['eagerness'] ) ||
        'auto' === $config['eagerness'] ||
        ! WP_Speculation_Rules::is_valid_eagerness( $config['eagerness'] ) ||
        // 'immediate' is a valid eagerness, but for safety WordPress does not allow it for document-level rules.
        'immediate' === $config['eagerness']
    ) {
        $config['eagerness'] = $default_eagerness;
    }

    return array(
        'mode'      => $config['mode'],
        'eagerness' => $config['eagerness'],
    );
}

/**
 * Returns the full speculation rules data based on the configuration.
 *
 * Plugins with features that rely on frontend URLs to exclude from prefetching or prerendering should use the
 * {@see 'wp_speculation_rules_href_exclude_paths'} filter to ensure those URL patterns are excluded.
 *
 * Additional speculation rules other than the default rule from WordPress Core can be provided by using the
 * {@see 'wp_load_speculation_rules'} action and amending the passed WP_Speculation_Rules object.
 *
 * @since 6.8.0
 * @access private
 *
 * @return WP_Speculation_Rules|null Object representing the speculation rules to use, or null if speculative loading
 *                                   is disabled in the current context.
 */
function wp_get_speculation_rules(): ?WP_Speculation_Rules {
    $configuration = wp_get_speculation_rules_configuration();
    if ( null === $configuration ) {
        return null;
    }

    $mode      = $configuration['mode'];
    $eagerness = $configuration['eagerness'];

    $prefixer = new WP_URL_Pattern_Prefixer();

    $base_href_exclude_paths = array(
        $prefixer->prefix_path_pattern( '/wp-*.php', 'site' ),
        $prefixer->prefix_path_pattern( '/wp-admin/*', 'site' ),
        $prefixer->prefix_path_pattern( '/*', 'uploads' ),
        $prefixer->prefix_path_pattern( '/*', 'content' ),
        $prefixer->prefix_path_pattern( '/*', 'plugins' ),
        $prefixer->prefix_path_pattern( '/*', 'template' ),
        $prefixer->prefix_path_pattern( '/*', 'stylesheet' ),
    );

    /*
     * If pretty permalinks are enabled, exclude any URLs with query parameters.
     * Otherwise, exclude specifically the URLs with a `_wpnonce` query parameter or any other query parameter
     * containing the word `nonce`.
     */
    if ( get_option( 'permalink_structure' ) ) {
        $base_href_exclude_paths[] = $prefixer->prefix_path_pattern( '/*\\?(.+)', 'home' );
    } else {
        $base_href_exclude_paths[] = $prefixer->prefix_path_pattern( '/*\\?*(^|&)*nonce*=*', 'home' );
    }

    /**
     * Filters the paths for which speculative loading should be disabled.
     *
     * All paths should start in a forward slash, relative to the root document. The `*` can be used as a wildcard.
     * If the WordPress site is in a subdirectory, the exclude paths will automatically be prefixed as necessary.
     *
     * Note that WordPress always excludes certain path patterns such as `/wp-login.php` and `/wp-admin/*`, and those
     * cannot be modified using the filter.
     *
     * @since 6.8.0
     *
     * @param string[] $href_exclude_paths Additional path patterns to disable speculative loading for.
     * @param string   $mode               Mode used to apply speculative loading. Either 'prefetch' or 'prerender'.
     */
    $href_exclude_paths = (array) apply_filters( 'wp_speculation_rules_href_exclude_paths', array(), $mode );

    // Ensure that:
    // 1. There are no duplicates.
    // 2. The base paths cannot be removed.
    // 3. The array has sequential keys (i.e. array_is_list()).
    $href_exclude_paths = array_values(
        array_unique(
            array_merge(
                $base_href_exclude_paths,
                array_map(
                    static function ( string $href_exclude_path ) use ( $prefixer ): string {
                        return $prefixer->prefix_path_pattern( $href_exclude_path );
                    },
                    $href_exclude_paths
                )
            )
        )
    );

    $speculation_rules = new WP_Speculation_Rules();

    $main_rule_conditions = array(
        // Include any URLs within the same site.
        array(
            'href_matches' => $prefixer->prefix_path_pattern( '/*' ),
        ),
        // Except for excluded paths.
        array(
            'not' => array(
                'href_matches' => $href_exclude_paths,
            ),
        ),
        // Also exclude rel=nofollow links, as certain plugins use that on their links that perform an action.
        array(
            'not' => array(
                'selector_matches' => 'a[rel~="nofollow"]',
            ),
        ),
        // Also exclude links that are explicitly marked to opt out, either directly or via a parent element.
        array(
            'not' => array(
                'selector_matches' => ".no-{$mode}, .no-{$mode} a",
            ),
        ),
    );

    // If using 'prerender', also exclude links that opt out of 'prefetch' because it's part of 'prerender'.
    if ( 'prerender' === $mode ) {
        $main_rule_conditions[] = array(
            'not' => array(
                'selector_matches' => '.no-prefetch, .no-prefetch a',
            ),
        );
    }

    $speculation_rules->add_rule(
        $mode,
        'main',
        array(
            'source'    => 'document',
            'where'     => array(
                'and' => $main_rule_conditions,
            ),
            'eagerness' => $eagerness,
        )
    );

    /**
     * Fires when speculation rules data is loaded, allowing to amend the rules.
     *
     * @since 6.8.0
     *
     * @param WP_Speculation_Rules $speculation_rules Object representing the speculation rules to use.
     */
    do_action( 'wp_load_speculation_rules', $speculation_rules );

    return $speculation_rules;
}

/**
 * Prints the speculation rules.
 *
 * For browsers that do not support speculation rules yet, the `script[type="speculationrules"]` tag will be ignored.
 *
 * @since 6.8.0
 * @access private
 */
function wp_print_speculation_rules(): void {
    $speculation_rules = wp_get_speculation_rules();
    if ( null === $speculation_rules ) {
        return;
    }

    wp_print_inline_script_tag(
        (string) wp_json_encode(
            $speculation_rules,
            JSON_HEX_TAG | JSON_UNESCAPED_SLASHES
        ),
        array( 'type' => 'speculationrules' )
    );
}