WordPress/wp-admin/includes/revision.php

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

/**
 * Get the revision UI diff.
 *
 * @param object $post The post object.
 * @param int $compare_from The revision id to compare from.
 * @param int $compare_to The revision id to come to.
 *
 * @return array|bool 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 ) {
	if ( ! $post = get_post( $post ) )
		return false;

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

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

	// If comparing revisions, make sure we're dealing with the right post parent.
	if ( $compare_from && $compare_from->post_parent !== $post->ID )
		return false;
	if ( $compare_to->post_parent !== $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() as $field => $name ) {
		$content_from = $compare_from ? apply_filters( "_wp_post_revision_field_$field", $compare_from->$field, $field, $compare_from, 'left' ) : '';
		$content_to = apply_filters( "_wp_post_revision_field_$field", $compare_to->$field, $field, $compare_to, 'right' );

		$diff = wp_text_diff( $content_from, $content_to, array( 'show_split_view' => true ) );

		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>';
			$diff .= '<td>' . esc_html( $compare_from->post_title ) . '</td><td></td><td>' . esc_html( $compare_to->post_title ) . '</td>';
			$diff .= '</tr></tbody>';
			$diff .= '</table>';
		}

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

/**
 * Prepare revisions for JavaScript.
 *
 * @param object $post The post object.
 * @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 daIta and related settings.
 */
function wp_prepare_revisions_for_js( $post, $selected_revision_id, $from = null ) {
	$post = get_post( $post );
	$revisions = $authors = array();
	$now_gmt = time();

	$revisions = wp_get_post_revisions( $post->ID, array( 'order' => 'ASC' ) );
	$show_avatars = get_option( 'show_avatars' );

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

	foreach ( $revisions as $revision ) {
		$modified = strtotime( $revision->post_modified );
		$modified_gmt = strtotime( $revision->post_modified_gmt );
		$restore_link = 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, 24 ) : '',
				'name' => get_the_author_meta( 'display_name', $revision->post_author ),
			);
		}

		$revisions[ $revision->ID ] = array(
			'id'         => $revision->ID,
			'title'      => get_the_title( $post->ID ),
			'author'     => $authors[ $revision->post_author ],
			'date'       => date_i18n( __( 'M j, Y @ G:i' ), $modified ),
			'dateShort'  => date_i18n( _x( 'j M @ G:i', 'revision date short format' ), $modified ),
			'timeAgo'    => sprintf( __( '%s ago' ), human_time_diff( $modified_gmt, $now_gmt ) ),
			'autosave'   => wp_is_post_autosave( $revision ),
			'current'    => $revision->post_modified_gmt === $post->post_modified_gmt,
			'restoreUrl' => urldecode( $restore_link ),
		);
	}

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