Facet query var helper doesn't support 3+ terms

[mboynes]

Basic example

add_filter( 'sp_search_wp_query_args', function( $wp_args ) {
	$wp_args['facets'] = [
		'Category' => [
			'type' => 'taxonomy',
			'taxonomy' => 'category',
			'count' => 10,
		],
	];

	return $wp_args;
} );

When outputting the facets, e.g. as links, the query_vars entry in the processed facet data contains the query vars needed to generate a link to filter the results:

$facet_data = SP_Integration()->search_obj->get_facet_data();
if ( ! empty( $facet_data['Category']['items'] ) ) :
	foreach ( $facet_data['Category']['items'] as $cat ) :
		?>
		<li>
			<a href="<?php echo esc_url( add_query_arg( $cat['query_vars'] ) ) ?>">
				<?php echo esc_html( $cat['name'] ) ?>
			</a>
		</li>
		<?php
	endforeach;
endif;

If you run a search and click a category, this works great. If you click a second category, it still works great. If you click a third category, the second is replaced with the third.

Problem and Proposed Solution

This is a problem with core, where core replaces the query var value with only the first in the list.

To fix this, we should be using $GLOBALS['wp_query']->tax_query->queries. It's important to note that union queries (?taxonomy=term1+term2) will add multiple arrays to $GLOBALS['wp_query']->tax_query->queries, whereas intersection queries (?taxonomy=term1,term2) will add multiple terms a single array entry. Here's a var_export() dump illustrating this:

?s=markup&category_name=antiquarianism,aciform
$GLOBALS['wp_query']->tax_query->queries
array (
  0 => 
  array (
    'taxonomy' => 'category',
    'terms' => 
    array (
      0 => 'antiquarianism',
      1 => 'aciform',
    ),
    'field' => 'slug',
    'operator' => 'IN',
    'include_children' => true,
  ),
)

‌‌?s=markup&category_name=antiquarianism+aciform
$GLOBALS['wp_query']->tax_query->queries
array (
  0 => 
  array (
    'taxonomy' => 'category',
    'terms' => 
    array (
      0 => 'antiquarianism',
    ),
    'field' => 'slug',
    'operator' => 'IN',
    'include_children' => true,
  ),
  1 => 
  array (
    'taxonomy' => 'category',
    'terms' => 
    array (
      0 => 'aciform',
    ),
    'field' => 'slug',
    'operator' => 'IN',
    'include_children' => true,
  ),
)

In addition, it looks like we're not fully accounting for union queries when building the query var arrays in the facet data. Relevant code in core to mimic for consistency.