function wp_map_sidebars_widgets( $existing_sidebars_widgets ) {
	global $wp_registered_sidebars;

	$new_sidebars_widgets = array(
		'wp_inactive_widgets' => array(),
	);

	// Short-circuit if there are no sidebars to map.
	if ( ! is_array( $existing_sidebars_widgets ) || empty( $existing_sidebars_widgets ) ) {
		return $new_sidebars_widgets;
	}

	foreach ( $existing_sidebars_widgets as $sidebar => $widgets ) {
		if ( 'wp_inactive_widgets' === $sidebar || 'orphaned_widgets' === substr( $sidebar, 0, 16 ) ) {
			$new_sidebars_widgets['wp_inactive_widgets'] = array_merge( $new_sidebars_widgets['wp_inactive_widgets'], (array) $widgets );
			unset( $existing_sidebars_widgets[ $sidebar ] );
		}
	}

	// If old and new theme have just one sidebar, map it and we're done.
	if ( 1 === count( $existing_sidebars_widgets ) && 1 === count( $wp_registered_sidebars ) ) {
		$new_sidebars_widgets[ key( $wp_registered_sidebars ) ] = array_pop( $existing_sidebars_widgets );

		return $new_sidebars_widgets;
	}

	// Map locations with the same slug.
	$existing_sidebars = array_keys( $existing_sidebars_widgets );

	foreach ( $wp_registered_sidebars as $sidebar => $name ) {
		if ( in_array( $sidebar, $existing_sidebars, true ) ) {
			$new_sidebars_widgets[ $sidebar ] = $existing_sidebars_widgets[ $sidebar ];
			unset( $existing_sidebars_widgets[ $sidebar ] );
		} elseif ( ! array_key_exists( $sidebar, $new_sidebars_widgets ) ) {
			$new_sidebars_widgets[ $sidebar ] = array();
		}
	}

	// If there are more sidebars, try to map them.
	if ( ! empty( $existing_sidebars_widgets ) ) {

		/*
		 * If old and new theme both have sidebars that contain phrases
		 * from within the same group, make an educated guess and map it.
		 */
		$common_slug_groups = array(
			array( 'sidebar', 'primary', 'main', 'right' ),
			array( 'second', 'left' ),
			array( 'sidebar-2', 'footer', 'bottom' ),
			array( 'header', 'top' ),
		);

		// Go through each group...
		foreach ( $common_slug_groups as $slug_group ) {

			// ...and see if any of these slugs...
			foreach ( $slug_group as $slug ) {

				// ...and any of the new sidebars...
				foreach ( $wp_registered_sidebars as $new_sidebar => $args ) {

					// ...actually match!
					if ( false === stripos( $new_sidebar, $slug ) && false === stripos( $slug, $new_sidebar ) ) {
						continue;
					}

					// Then see if any of the existing sidebars...
					foreach ( $existing_sidebars_widgets as $sidebar => $widgets ) {

						// ...and any slug in the same group...
						foreach ( $slug_group as $slug ) {

							// ... have a match as well.
							if ( false === stripos( $sidebar, $slug ) && false === stripos( $slug, $sidebar ) ) {
								continue;
							}

							// Make sure this sidebar wasn't mapped and removed previously.
							if ( ! empty( $existing_sidebars_widgets[ $sidebar ] ) ) {

								// We have a match that can be mapped!
								$new_sidebars_widgets[ $new_sidebar ] = array_merge( $new_sidebars_widgets[ $new_sidebar ], $existing_sidebars_widgets[ $sidebar ] );

								// Remove the mapped sidebar so it can't be mapped again.
								unset( $existing_sidebars_widgets[ $sidebar ] );

								// Go back and check the next new sidebar.
								continue 3;
							}
						} // endforeach ( $slug_group as $slug )
					} // endforeach ( $existing_sidebars_widgets as $sidebar => $widgets )
				} // endforeach foreach ( $wp_registered_sidebars as $new_sidebar => $args )
			} // endforeach ( $slug_group as $slug )
		} // endforeach ( $common_slug_groups as $slug_group )
	}

	// Move any left over widgets to inactive sidebar.
	foreach ( $existing_sidebars_widgets as $widgets ) {
		if ( is_array( $widgets ) && ! empty( $widgets ) ) {
			$new_sidebars_widgets['wp_inactive_widgets'] = array_merge( $new_sidebars_widgets['wp_inactive_widgets'], $widgets );
		}
	}

	// Sidebars_widgets settings from when this theme was previously active.
	$old_sidebars_widgets = get_theme_mod( 'sidebars_widgets' );
	$old_sidebars_widgets = isset( $old_sidebars_widgets['data'] ) ? $old_sidebars_widgets['data'] : false;

	if ( is_array( $old_sidebars_widgets ) ) {

		// Remove empty sidebars, no need to map those.
		$old_sidebars_widgets = array_filter( $old_sidebars_widgets );

		// Only check sidebars that are empty or have not been mapped to yet.
		foreach ( $new_sidebars_widgets as $new_sidebar => $new_widgets ) {
			if ( array_key_exists( $new_sidebar, $old_sidebars_widgets ) && ! empty( $new_widgets ) ) {
				unset( $old_sidebars_widgets[ $new_sidebar ] );
			}
		}

		// Remove orphaned widgets, we're only interested in previously active sidebars.
		foreach ( $old_sidebars_widgets as $sidebar => $widgets ) {
			if ( 'orphaned_widgets' === substr( $sidebar, 0, 16 ) ) {
				unset( $old_sidebars_widgets[ $sidebar ] );
			}
		}

		$old_sidebars_widgets = _wp_remove_unregistered_widgets( $old_sidebars_widgets );

		if ( ! empty( $old_sidebars_widgets ) ) {

			// Go through each remaining sidebar...
			foreach ( $old_sidebars_widgets as $old_sidebar => $old_widgets ) {

				// ...and check every new sidebar...
				foreach ( $new_sidebars_widgets as $new_sidebar => $new_widgets ) {

					// ...for every widget we're trying to revive.
					foreach ( $old_widgets as $key => $widget_id ) {
						$active_key = array_search( $widget_id, $new_widgets, true );

						// If the widget is used elsewhere...
						if ( false !== $active_key ) {

							// ...and that elsewhere is inactive widgets...
							if ( 'wp_inactive_widgets' === $new_sidebar ) {

								// ...remove it from there and keep the active version...
								unset( $new_sidebars_widgets['wp_inactive_widgets'][ $active_key ] );
							} else {

								// ...otherwise remove it from the old sidebar and keep it in the new one.
								unset( $old_sidebars_widgets[ $old_sidebar ][ $key ] );
							}
						} // endif ( $active_key )
					} // endforeach ( $old_widgets as $key => $widget_id )
				} // endforeach ( $new_sidebars_widgets as $new_sidebar => $new_widgets )
			} // endforeach ( $old_sidebars_widgets as $old_sidebar => $old_widgets )
		} // endif ( ! empty( $old_sidebars_widgets ) )

		// Restore widget settings from when theme was previously active.
		$new_sidebars_widgets = array_merge( $new_sidebars_widgets, $old_sidebars_widgets );
	}

	return $new_sidebars_widgets;
}