WordPress at a glance
function is not described

wp_map_sidebars_widgets() WP 4.9.0

Compares a list of sidebars with their widgets against an allowed list.

No Hooks.

Return

Array. Mapped sidebars widgets.

Usage

wp_map_sidebars_widgets( $existing_sidebars_widgets );
$existing_sidebars_widgets(array) (required)
List of sidebars and their widget instance IDs.

Changelog

Since 4.9.0 Introduced.
Since 4.9.2 Always tries to restore widget assignments from previous data, not just if sidebars needed mapping.

Code of wp_map_sidebars_widgets() WP 5.5.1

<?php
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;
							}
						} // End foreach ( $slug_group as $slug ).
					} // End foreach ( $existing_sidebars_widgets as $sidebar => $widgets ).
				} // End foreach ( $wp_registered_sidebars as $new_sidebar => $args ).
			} // End foreach ( $slug_group as $slug ).
		} // End foreach ( $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 ] );
							}
						} // End if ( $active_key ).
					} // End foreach ( $old_widgets as $key => $widget_id ).
				} // End foreach ( $new_sidebars_widgets as $new_sidebar => $new_widgets ).
			} // End foreach ( $old_sidebars_widgets as $old_sidebar => $old_widgets ).
		} // End if ( ! 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;
}