Source

File: wp-admin/includes/revision.php

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,
		);

		/**
		 * 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 );

}