Updates a post with new post data.

The date does not have to be set for drafts. You can set the date and it will
not be overridden.

function wp_update_post( $postarr = array(), $wp_error = false, $fire_after_hooks = true ) {
	if ( is_object( $postarr ) ) {
		// Non-escaped post was passed.
		$postarr = get_object_vars( $postarr );
		$postarr = wp_slash( $postarr );
	}

	// First, get all of the original fields.
	$post = get_post( $postarr['ID'], ARRAY_A );

	if ( is_null( $post ) ) {
		if ( $wp_error ) {
			return new WP_Error( 'invalid_post', __( 'Invalid post ID.' ) );
		}
		return 0;
	}

	// Escape data pulled from DB.
	$post = wp_slash( $post );

	// Passed post category list overwrites existing category list if not empty.
	if ( isset( $postarr['post_category'] ) && is_array( $postarr['post_category'] )
		&& count( $postarr['post_category'] ) > 0
	) {
		$post_cats = $postarr['post_category'];
	} else {
		$post_cats = $post['post_category'];
	}

	// Drafts shouldn't be assigned a date unless explicitly done so by the user.
	if ( isset( $post['post_status'] )
		&& in_array( $post['post_status'], array( 'draft', 'pending', 'auto-draft' ), true )
		&& empty( $postarr['edit_date'] ) && ( '0000-00-00 00:00:00' === $post['post_date_gmt'] )
	) {
		$clear_date = true;
	} else {
		$clear_date = false;
	}

	// Merge old and new fields with new fields overwriting old ones.
	$postarr                  = array_merge( $post, $postarr );
	$postarr['post_category'] = $post_cats;
	if ( $clear_date ) {
		$postarr['post_date']     = current_time( 'mysql' );
		$postarr['post_date_gmt'] = '';
	}

	if ( 'attachment' === $postarr['post_type'] ) {
		return wp_insert_attachment( $postarr, false, 0, $wp_error );
	}

	// Discard 'tags_input' parameter if it's the same as existing post tags.
	if ( isset( $postarr['tags_input'] ) && is_object_in_taxonomy( $postarr['post_type'], 'post_tag' ) ) {
		$tags      = get_the_terms( $postarr['ID'], 'post_tag' );
		$tag_names = array();

		if ( $tags && ! is_wp_error( $tags ) ) {
			$tag_names = wp_list_pluck( $tags, 'name' );
		}

		if ( $postarr['tags_input'] === $tag_names ) {
			unset( $postarr['tags_input'] );
		}
	}

	return wp_insert_post( $postarr, $wp_error, $fire_after_hooks );
}