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Edit: revision.php

<?php
/**
 * WordPress Administration Revisions API
 *
 * @package WordPress
 * @subpackage Administration
 * @since 3.6.0
 */

/**
 * Get the revision UI diff.
 *
 * @since 3.6.0
 *
 * @param WP_Post|int $post         The post object or post ID.
 * @param int         $compare_from The revision ID to compare from.
 * @param int         $compare_to   The revision ID to come to.
 * @return array|false Associative array of a post's revisioned fields and their diffs.
 *                    Or, false on failure.
 */
function wp_get_revision_ui_diff( $post, $compare_from, $compare_to ) {
	$post = get_post( $post );
	if ( ! $post ) {
		return false;
	}

if ( $compare_from ) {
		$compare_from = get_post( $compare_from );
		if ( ! $compare_from ) {
			return false;
		}
	} else {
		// If we're dealing with the first revision...
		$compare_from = false;
	}

$compare_to = get_post( $compare_to );
	if ( ! $compare_to ) {
		return false;
	}

// If comparing revisions, make sure we're dealing with the right post parent.
	// The parent post may be a 'revision' when revisions are disabled and we're looking at autosaves.
	if ( $compare_from && $compare_from->post_parent !== $post->ID && $compare_from->ID !== $post->ID ) {
		return false;
	}
	if ( $compare_to->post_parent !== $post->ID && $compare_to->ID !== $post->ID ) {
		return false;
	}

if ( $compare_from && strtotime( $compare_from->post_date_gmt ) > strtotime( $compare_to->post_date_gmt ) ) {
		$temp         = $compare_from;
		$compare_from = $compare_to;
		$compare_to   = $temp;
	}

// Add default title if title field is empty.
	if ( $compare_from && empty( $compare_from->post_title ) ) {
		$compare_from->post_title = __( '(no title)' );
	}
	if ( empty( $compare_to->post_title ) ) {
		$compare_to->post_title = __( '(no title)' );
	}

$return = array();

foreach ( _wp_post_revision_fields( $post ) as $field => $name ) {
		/**
		 * Contextually filter a post revision field.
		 *
		 * The dynamic portion of the hook name, `$field`, corresponds to each of the post
		 * fields of the revision object being iterated over in a foreach statement.
		 *
		 * @since 3.6.0
		 *
		 * @param string  $revision_field The current revision field to compare to or from.
		 * @param string  $field          The current revision field.
		 * @param WP_Post $compare_from   The revision post object to compare to or from.
		 * @param string  $context        The context of whether the current revision is the old
		 *                                or the new one. Values are 'to' or 'from'.
		 */
		$content_from = $compare_from ? apply_filters( "_wp_post_revision_field_{$field}", $compare_from->$field, $field, $compare_from, 'from' ) : '';

/** This filter is documented in wp-admin/includes/revision.php */
		$content_to = apply_filters( "_wp_post_revision_field_{$field}", $compare_to->$field, $field, $compare_to, 'to' );

$args = array(
			'show_split_view' => true,
			'title_left'      => __( 'Removed' ),
			'title_right'     => __( 'Added' ),
		);

/**
		 * Filters revisions text diff options.
		 *
		 * Filters the options passed to wp_text_diff() when viewing a post revision.
		 *
		 * @since 4.1.0
		 *
		 * @param array   $args {
		 *     Associative array of options to pass to wp_text_diff().
		 *
		 *     @type bool $show_split_view True for split view (two columns), false for
		 *                                 un-split view (single column). Default true.
		 * }
		 * @param string  $field        The current revision field.
		 * @param WP_Post $compare_from The revision post to compare from.
		 * @param WP_Post $compare_to   The revision post to compare to.
		 */
		$args = apply_filters( 'revision_text_diff_options', $args, $field, $compare_from, $compare_to );

$diff = wp_text_diff( $content_from, $content_to, $args );

if ( ! $diff && 'post_title' === $field ) {
			// It's a better user experience to still show the Title, even if it didn't change.
			// No, you didn't see this.
			$diff = '<table class="diff"><colgroup><col class="content diffsplit left"><col class="content diffsplit middle"><col class="content diffsplit right"></colgroup><tbody><tr>';

// In split screen mode, show the title before/after side by side.
			if ( true === $args['show_split_view'] ) {
				$diff .= '<td>' . esc_html( $compare_from->post_title ) . '</td><td></td><td>' . esc_html( $compare_to->post_title ) . '</td>';
			} else {
				$diff .= '<td>' . esc_html( $compare_from->post_title ) . '</td>';

// In single column mode, only show the title once if unchanged.
				if ( $compare_from->post_title !== $compare_to->post_title ) {
					$diff .= '</tr><tr><td>' . esc_html( $compare_to->post_title ) . '</td>';
				}
			}

$diff .= '</tr></tbody>';
			$diff .= '</table>';
		}

if ( $diff ) {
			$return[] = array(
				'id'   => $field,
				'name' => $name,
				'diff' => $diff,
			);
		}
	}

/**
	 * Filters the fields displayed in the post revision diff UI.
	 *
	 * @since 4.1.0
	 *
	 * @param array[] $return       Array of revision UI fields. Each item is an array of id, name, and diff.
	 * @param WP_Post $compare_from The revision post to compare from.
	 * @param WP_Post $compare_to   The revision post to compare to.
	 */
	return apply_filters( 'wp_get_revision_ui_diff', $return, $compare_from, $compare_to );

}

/**
 * Prepare revisions for JavaScript.
 *
 * @since 3.6.0
 *
 * @param WP_Post|int $post                 The post object or post ID.
 * @param int         $selected_revision_id The selected revision ID.
 * @param int         $from                 Optional. The revision ID to compare from.
 * @return array An associative array of revision data and related settings.
 */
function wp_prepare_revisions_for_js( $post, $selected_revision_id, $from = null ) {
	$post    = get_post( $post );
	$authors = array();
	$now_gmt = time();

$revisions = wp_get_post_revisions(
		$post->ID,
		array(
			'order'         => 'ASC',
			'check_enabled' => false,
		)
	);
	// If revisions are disabled, we only want autosaves and the current post.
	if ( ! wp_revisions_enabled( $post ) ) {
		foreach ( $revisions as $revision_id => $revision ) {
			if ( ! wp_is_post_autosave( $revision ) ) {
				unset( $revisions[ $revision_id ] );
			}
		}
		$revisions = array( $post->ID => $post ) + $revisions;
	}

$show_avatars = get_option( 'show_avatars' );

cache_users( wp_list_pluck( $revisions, 'post_author' ) );

$can_restore = current_user_can( 'edit_post', $post->ID );
	$current_id  = false;

foreach ( $revisions as $revision ) {
		$modified     = strtotime( $revision->post_modified );
		$modified_gmt = strtotime( $revision->post_modified_gmt . ' +0000' );
		if ( $can_restore ) {
			$restore_link = str_replace(
				'&amp;',
				'&',
				wp_nonce_url(
					add_query_arg(
						array(
							'revision' => $revision->ID,
							'action'   => 'restore',
						),
						admin_url( 'revision.php' )
					),
					"restore-post_{$revision->ID}"
				)
			);
		}

if ( ! isset( $authors[ $revision->post_author ] ) ) {
			$authors[ $revision->post_author ] = array(
				'id'     => (int) $revision->post_author,
				'avatar' => $show_avatars ? get_avatar( $revision->post_author, 32 ) : '',
				'name'   => get_the_author_meta( 'display_name', $revision->post_author ),
			);
		}

$autosave = (bool) wp_is_post_autosave( $revision );
		$current  = ! $autosave && $revision->post_modified_gmt === $post->post_modified_gmt;
		if ( $current && ! empty( $current_id ) ) {
			// If multiple revisions have the same post_modified_gmt, highest ID is current.
			if ( $current_id < $revision->ID ) {
				$revisions[ $current_id ]['current'] = false;
				$current_id                          = $revision->ID;
			} else {
				$current = false;
			}
		} elseif ( $current ) {
			$current_id = $revision->ID;
		}

$revisions_data = array(
			'id'         => $revision->ID,
			'title'      => get_the_title( $post->ID ),
			'author'     => $authors[ $revision->post_author ],
			'date'       => date_i18n( __( 'M j, Y @ H:i' ), $modified ),
			'dateShort'  => date_i18n( _x( 'j M @ H:i', 'revision date short format' ), $modified ),
			/* translators: %s: Human-readable time difference. */
			'timeAgo'    => sprintf( __( '%s ago' ), human_time_diff( $modified_gmt, $now_gmt ) ),
			'autosave'   => $autosave,
			'current'    => $current,
			'restoreUrl' => $can_restore ? $restore_link : false,
		);

/**
		 * Filters the array of revisions used on the revisions screen.
		 *
		 * @since 4.4.0
		 *
		 * @param array   $revisions_data {
		 *     The bootstrapped data for the revisions screen.
		 *
		 *     @type int        $id         Revision ID.
		 *     @type string     $title      Title for the revision's parent WP_Post object.
		 *     @type int        $author     Revision post author ID.
		 *     @type string     $date       Date the revision was modified.
		 *     @type string     $dateShort  Short-form version of the date the revision was modified.
		 *     @type string     $timeAgo    GMT-aware amount of time ago the revision was modified.
		 *     @type bool       $autosave   Whether the revision is an autosave.
		 *     @type bool       $current    Whether the revision is both not an autosave and the post
		 *                                  modified date matches the revision modified date (GMT-aware).
		 *     @type bool|false $restoreUrl URL if the revision can be restored, false otherwise.
		 * }
		 * @param WP_Post $revision       The revision's WP_Post object.
		 * @param WP_Post $post           The revision's parent WP_Post object.
		 */
		$revisions[ $revision->ID ] = apply_filters( 'wp_prepare_revision_for_js', $revisions_data, $revision, $post );
	}

/**
	 * If we only have one revision, the initial revision is missing; This happens
	 * when we have an autsosave and the user has clicked 'View the Autosave'
	 */
	if ( 1 === count( $revisions ) ) {
		$revisions[ $post->ID ] = array(
			'id'         => $post->ID,
			'title'      => get_the_title( $post->ID ),
			'author'     => $authors[ $revision->post_author ],
			'date'       => date_i18n( __( 'M j, Y @ H:i' ), strtotime( $post->post_modified ) ),
			'dateShort'  => date_i18n( _x( 'j M @ H:i', 'revision date short format' ), strtotime( $post->post_modified ) ),
			/* translators: %s: Human-readable time difference. */
			'timeAgo'    => sprintf( __( '%s ago' ), human_time_diff( strtotime( $post->post_modified_gmt ), $now_gmt ) ),
			'autosave'   => false,
			'current'    => true,
			'restoreUrl' => false,
		);
		$current_id             = $post->ID;
	}

/*
	 * If a post has been saved since the last revision (no revisioned fields
	 * were changed), we may not have a "current" revision. Mark the latest
	 * revision as "current".
	 */
	if ( empty( $current_id ) ) {
		if ( $revisions[ $revision->ID ]['autosave'] ) {
			$revision = end( $revisions );
			while ( $revision['autosave'] ) {
				$revision = prev( $revisions );
			}
			$current_id = $revision['id'];
		} else {
			$current_id = $revision->ID;
		}
		$revisions[ $current_id ]['current'] = true;
	}

// Now, grab the initial diff.
	$compare_two_mode = is_numeric( $from );
	if ( ! $compare_two_mode ) {
		$found = array_search( $selected_revision_id, array_keys( $revisions ), true );
		if ( $found ) {
			$from = array_keys( array_slice( $revisions, $found - 1, 1, true ) );
			$from = reset( $from );
		} else {
			$from = 0;
		}
	}

$from = absint( $from );

$diffs = array(
		array(
			'id'     => $from . ':' . $selected_revision_id,
			'fields' => wp_get_revision_ui_diff( $post->ID, $from, $selected_revision_id ),
		),
	);

return array(
		'postId'         => $post->ID,
		'nonce'          => wp_create_nonce( 'revisions-ajax-nonce' ),
		'revisionData'   => array_values( $revisions ),
		'to'             => $selected_revision_id,
		'from'           => $from,
		'diffData'       => $diffs,
		'baseUrl'        => parse_url( admin_url( 'revision.php' ), PHP_URL_PATH ),
		'compareTwoMode' => absint( $compare_two_mode ), // Apparently booleans are not allowed.
		'revisionIds'    => array_keys( $revisions ),
	);
}

/**
 * Print JavaScript templates required for the revisions experience.
 *
 * @since 4.1.0
 *
 * @global WP_Post $post Global post object.
 */
function wp_print_revision_templates() {
	global $post;
	?><script id="tmpl-revisions-frame" type="text/html">
		<div class="revisions-control-frame"></div>
		<div class="revisions-diff-frame"></div>
	</script>

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