<?php
/**
 * WCPOS sync read surface.
 *
 * @package WCPOS\WooCommercePOS\API\V2
 */

namespace WCPOS\WooCommercePOS\API\V2;

use WC_Product_Variation;
use WC_REST_Product_Variations_Controller;
use WCPOS\WooCommercePOS\Services\Barcode_Field;
use WCPOS\WooCommercePOS\Sync\Api;
use WCPOS\WooCommercePOS\Sync\Collection_Rules;
use WCPOS\WooCommercePOS\Sync\Collection_Rules_Plan;
use WCPOS\WooCommercePOS\Sync\Digest_Index;
use WCPOS\WooCommercePOS\Sync\Endpoint_Permissions;
use WCPOS\WooCommercePOS\Sync\Pos_Visibility;
use WCPOS\WooCommercePOS\Sync\Product_Serializer;
use WP_Error;
use WP_Query;
use WP_REST_Request;
use WP_REST_Response;
use WP_REST_Server;

// phpcs:disable Squiz.Commenting, Generic.Commenting -- Ported lab documentation is preserved verbatim.

/**
 * Variations document endpoint — the collection's hydration AND list/seed lane (ADR 0034).
 *
 * Why a flat route: the change-signal yields BARE variation ids (no parent), and WooCommerce's
 * only variation routes are parent-mediated (`products/<parent>/variations`). One flat route
 * IS the cross-parent collection: bare pages seed the complete replica (the idle trickle),
 * `include=` is one filter on it (targeted hydration, no parent->child dance), and the
 * SKU/barcode discovery search is another.
 *
 * Why it EXTENDS WooCommerce's variations controller: because that is all the route ever needed.
 * WooCommerce's `get_objects()` already answers a cross-parent query — with no `product_id` in
 * the route there is no parent constraint, and `include`/`search`/`orderby`/pagination are its
 * own collection params. 1.9.x did exactly this: `parent::get_items( $request )`, one line
 * (`API\V1\Product_Variations_Controller::wcpos_get_all_items`).
 *
 * The previous version of this class extended a bare `WP_REST_Controller` and rebuilt the query
 * by hand — ~90 lines of raw postmeta SQL, five hand-declared args, no item schema — on the
 * stated grounds that "wc/v3 has no cross-parent variations?include=". That claim was false, and
 * the cost of acting on it was the payload: a variation hydrated through the PRODUCTS controller
 * carries `images[]` instead of `image`, which blanked every variation thumbnail in the POS on
 * 1.10.0 and wrote the parent's image onto every order line (#1710).
 *
 * What stays ours, and only this: the sync document envelope the engine reads
 * (`documents[].{id,parent_id,payload,_rxdb_digest}`), POS visibility, the barcode carrier
 * search, and the request bounds. Everything else is WooCommerce's.
 */
class Variations_Controller extends WC_REST_Product_Variations_Controller {
	/**
	 * Request keys the variation Collection Rules plan reads on this lane.
	 *
	 * @var array
	 */
	private const WCPOS_SORT_PARAM_MAP = array(
		'orderby' => 'orderby',
		'order'   => 'order',
	);

	/**
	 * The request whose declared sort `wcpos_posts_clauses()` applies.
	 *
	 * @var null|WP_REST_Request
	 */
	private $wcpos_sort_request = null;

	use Endpoint_Permissions;

	private const MAX_SKU_LENGTH    = 4096;
	private const MAX_SKU_TERMS     = 100;
	private const MAX_SEARCH_LENGTH = 256;
	private const MAX_SEARCH_TERMS  = 10;
	private const MAX_PAGE          = 1000;


	public function register_routes(): void {
		/*
		 * ONLY the flat sync route. `parent::register_routes()` is deliberately not called: the
		 * v2 namespace is a read/sync surface, and writes ride Write_Controller, which already
		 * pushes through WooCommerce's nested routes. Registering WC's CRUD routes here would
		 * widen the POS-marker-gated surface for no consumer.
		 *
		 * The args and the schema are WooCommerce's own, so `include`, `search`, `orderby`,
		 * `order`, `offset`, `page`, `per_page`, `status` … all behave exactly as they do on
		 * wc/v3, and the route documents itself in the REST index.
		 */
		register_rest_route(
			Api::ROUTE_NAMESPACE,
			'/variations',
			array(
				array(
					'methods'             => WP_REST_Server::READABLE,
					'callback'            => array( $this, 'get_variations' ),
					'permission_callback' => array( $this, 'permissions_check' ),
					'args'                => $this->get_collection_params(),
				),
				'schema' => array( $this, 'get_public_item_schema' ),
			)
		);
	}

	/**
	 * Narrow WooCommerce's variation query to what the POS may serve.
	 *
	 * Everything WooCommerce already understands — `include`, `offset`, `order`, pagination,
	 * status — comes from `parent::prepare_objects_query()`, which also applies
	 * `woocommerce_rest_product_variation_object_query` internally (wc/v3's CRUD controller fires
	 * it there, not in `get_items()`), so third-party query scoping reaches every lane built
	 * through this method. Layered on top — deliberately AFTER that filter, so a third party
	 * cannot widen what the POS may serve: POS visibility, the
	 * barcode-carrier search, and the sort keys the POS grids offer. This is the seam 1.9.x used
	 * for the same job (`API\V1\Product_Variations_Controller::prepare_objects_query`).
	 *
	 * @param WP_REST_Request $request Full details about the request.
	 *
	 * @return array
	 */
	protected function prepare_objects_query( $request ) {
		/*
		 * WooCommerce splits `sku` on commas without trimming, so `sku=A, B` looks for " B".
		 * Normalize before it sees the param rather than reimplementing its matching.
		 */
		$sku = (string) ( $request->get_param( 'sku' ) ?? '' );
		if ( '' !== $sku ) {
			$terms = array_values(
				array_filter(
					array_map( 'trim', explode( ',', $sku ) ),
					static function ( string $term ): bool {
						return '' !== $term;
					}
				)
			);
			$sku = implode( ',', $terms );
			$request->set_param( 'sku', $sku );
		}

		$args = parent::prepare_objects_query( $request );

		/*
		 * A product is not a variation document.
		 *
		 * WooCommerce widens `post_type` to `array( 'product', 'product_variation' )` whenever
		 * `sku` is set, because the two share one SKU space. On THIS route that would serve a
		 * simple product as a variation — and the client would file it into its variations
		 * collection, the mirror image of the misfiled-variation pollution it already carries a
		 * one-shot repair for. Type purity on a variations route is ours to enforce.
		 */
		$args['post_type'] = $this->post_type;

		/*
		 * `search` means the barcode CARRIERS here, not the post title.
		 *
		 * WooCommerce maps `search` onto `s`, which searches post_title/content — useless for a
		 * variation, whose title is a generated attribute string. The POS searches what a cashier
		 * actually types or scans: the SKU and whichever meta key the store configured as its
		 * barcode field (`Barcode_Field::search_keys()`). Every term must match at least one carrier.
		 *
		 * `sku` is left to WooCommerce: its own exact/comma-list handling is what the
		 * sku-beats-search precedence rule relies on.
		 */
		$search = (string) ( $request->get_param( 'search' ) ?? '' );
		if ( '' !== $sku ) {
			// SKU is an exact lookup and outranks a fuzzy one; leaving WooCommerce's post-title
			// `s` in place would AND the two and return nothing.
			unset( $args['s'] );
		}
		if ( '' !== $search && '' === $sku ) {
			unset( $args['s'] );
			$args['wcpos_variation_search'] = true;
			$terms = preg_split( '/[\s\p{Z}\p{C}]+/u', trim( $search ), -1, PREG_SPLIT_NO_EMPTY );
			if ( false === $terms ) {
				$terms = array();
			}
			$carriers = array( 'relation' => 'AND' );
			foreach ( $terms as $term ) {
				$term_carriers = array( 'relation' => 'OR' );
				foreach ( Barcode_Field::search_keys() as $key ) {
					$term_carriers[] = array(
						'key'     => $key,
						'value'   => $term,
						'compare' => 'LIKE',
					);
				}
				$carriers[] = $term_carriers;
			}
			if ( 1 < \count( $carriers ) ) {
				$args['meta_query'] = $this->add_meta_query( $args, $carriers ); // phpcs:ignore WordPress.DB.SlowDBQuery.slow_db_query_meta_query
			}
		}

		/*
		 * This route only ever offers what the store owner has for sale — on EVERY lane, including
		 * `include`.
		 *
		 * WooCommerce's Enabled checkbox on the variation metabox writes `post_status = private`
		 * when unchecked ({@see \WC_Meta_Box_Product_Data::save_variations()}), and WooCommerce
		 * honours that everywhere a customer can reach: `get_visible_children()` and
		 * `get_available_variations()` both drop it. A cashier must not be able to sell a variation
		 * the owner switched off, so the POS behaves the same way.
		 *
		 * The `include` lane is NOT exempt. Being asked for an id by name is not evidence the owner
		 * wants it sold: the client learns those ids from the parent's `variations[]`, which
		 * WooCommerce fills from `get_children()` — publish AND private — and from the change
		 * signal, which journals a disabled variation like any other post. A disabled id simply is
		 * not hydrated, the client's targeted-pull shortfall prunes it, and it leaves every till.
		 * Re-enabling saves the variation, which journals it, and it comes back.
		 *
		 * Set after `parent::prepare_objects_query()` so an explicit `status` param cannot widen it.
		 */
		$args['post_status'] = 'publish';

		/*
		 * Leg-3 (ADR 0014 WP-M5): POS-hidden (`online_only`) variations are never served. As a
		 * query exclusion rather than a post-hoc filter of the result, so paging and totals count
		 * the same set the client is allowed to see.
		 *
		 * Through the helper, NOT a raw `post__not_in` merge: `parent::prepare_objects_query()`
		 * maps `include` to `post__in`, and WP_Query IGNORES `post__not_in` when `post__in` is
		 * present — so `?search=X&include=<hidden id>` would have served a hidden variation.
		 * `apply_to_wp_query_args()` already owns that trap: it intersects `post__in` with the
		 * hidden set and pins an empty intersection to `array( 0 )`.
		 */
		$args = ( new Pos_Visibility() )->apply_to_wp_query_args( $args, Pos_Visibility::VARIATIONS );

		/*
		 * The POS sorts on fields WooCommerce does not offer as orderby values. They are
		 * declared in Sync\Collection_Rules and projected into get_collection_params()
		 * below — without that, `orderby=sku` is rejected by REST argument validation
		 * before anything here runs.
		 *
		 * They are applied as SQL clauses, NOT as `meta_key` + `orderby => meta_value`:
		 * that pair INNER JOINs postmeta and drops every variation with no value for the
		 * key, so the sort silently filtered. `wcpos_posts_clauses()` LEFT JOINs instead
		 * and orders the meta-less rows last.
		 */
		$this->wcpos_sort_request = $request;
		add_filter( 'posts_clauses', array( $this, 'wcpos_posts_clauses' ), 10, 2 );

		return $args;
	}

	/**
	 * GET /variations — the flat collection's three lanes, one response shape.
	 *
	 * `?sku=`/`?search=` discovers by barcode carrier; a bare request serves one
	 * collection page (the trickle's seed lane); `?include=12,34` hydrates the
	 * named ids. All three resolve ids through WooCommerce's collection query,
	 * then hydrate through the shared assembly line below. Mirrors the wc/v3
	 * `products?include=` shape; the parent is resolved server-side off the
	 * loaded variation object (get_parent_id), so the client never needs to know
	 * parents. Unknown / non-variation ids are skipped (deletes are handled by
	 * the change-signal tombstone path, not here).
	 */
	public function get_variations( WP_REST_Request $request ) {
		$started     = microtime( true );
		$search_meta = null;
		if ( $request->has_param( 'sku' ) || $request->has_param( 'search' ) ) {
			$validation = $this->validate_search_request( $request );
			if ( is_wp_error( $validation ) ) {
				return $validation;
			}
			list( $ids, $search_meta ) = $this->search_variation_ids( $request );
		} elseif ( array() === array_filter( (array) $request->get_param( 'include' ) ) ) {
			/*
			 * A bare collection request — no `include`, no discovery term — answers page one of the
			 * POS-servable set with WooCommerce's own pagination, exactly as its `get_items()` would.
			 *
			 * This used to be a 400. That refusal is why the client still counts variations on the
			 * FROZEN `wcpos/v1` lane — the single remaining v1 call in the app — because the census
			 * probes a collection route and reads `X-WP-Total`, and no v2 variations route could
			 * answer "how many". Refusing the question was never a safety property: `include` is a
			 * filter, and a collection route with no filter is a collection.
			 */
			list( $ids, $search_meta ) = $this->collection_page( $request );
		} else {
			/*
			 * The ask runs through the SAME query WooCommerce's own collection read builds
			 * (#1751): `parent::prepare_objects_query()` maps `include` to `post__in` and — in
			 * wc/v3's CRUD controller — applies `woocommerce_rest_product_variation_object_query`
			 * internally, so third-party query scoping reaches this lane like every other
			 * (hook-parity contract #1738). The collection and discovery lanes always had that
			 * property; this lane loaded ids directly and bypassed it. POS visibility and the
			 * publish gate ride the same args (layered in our override).
			 *
			 * The paging/ordering params are PINNED, not honoured: this lane answers a named
			 * ask, so `per_page` covers the whole ask, `offset`/`page` cannot skip any of it
			 * (a skipped id is absent from documents, which the client reads as "prune this
			 * id"), and `orderby=include` keeps WooCommerce from ordering by a meta key whose
			 * EXISTS join would silently drop every variation lacking that meta row. Pinning
			 * `orderby` also keeps the args complete for direct (non-dispatched) invocations,
			 * which carry no route defaults. Served order is the include order either way —
			 * the intersect below is the final authority.
			 */
			$include_ids = array_values( array_unique( array_map( 'intval', (array) $request->get_param( 'include' ) ) ) );
			// Pins live on a QUERY-ONLY clone: the dispatched request stays exactly
			// as the client sent it, for the serializer's prepare-filters and for
			// anything downstream reading it after dispatch.
			$query_request = clone $request;
			$query_request->set_param( 'per_page', max( 1, count( $include_ids ) ) );
			$query_request->set_param( 'page', 1 );
			$query_request->set_param( 'offset', 0 );
			$query_request->set_param( 'orderby', 'include' );
			$query_request->set_param( 'order', 'asc' );
			$args = $this->prepare_objects_query( $query_request );

			/*
			 * The ask is a CEILING. WooCommerce's variations controller UNIONS some
			 * collection params into `post__in` (`on_sale=true` array-unions every on-sale
			 * id on top of the ask), so without this intersection a stray param would
			 * hydrate the whole store into the till. No request param or filter may widen
			 * the served set beyond the named ids — narrowing is fine, that is what the
			 * object_query filter and the visibility exclusion are for. An emptied ask pins
			 * to `array( 0 )`, the same never-matches sentinel Pos_Visibility uses.
			 */
			$post_in = array_values( array_intersect( array_map( 'intval', (array) ( $args['post__in'] ?? array() ) ), $include_ids ) );
			$args['post__in'] = array() === $post_in ? array( 0 ) : $post_in;
			$results = $this->get_objects( $args );

			$allowed_ids = array();
			foreach ( $results['objects'] as $object ) {
				if ( $object instanceof WC_Product_Variation ) {
					$allowed_ids[] = $object->get_id();
				}
			}
			$ids = array_values( array_intersect( $include_ids, $allowed_ids ) );
		}
		_prime_post_caches( $ids, true, true );

		// Hydrate through THE product assembly line (Product_Serializer), the same
		// seam resolve/changes use (ADR 0003 — values come from the REST
		// representation, never raw SQL). wc_get_product() returns a
		// WC_Product_Variation for a variation id; the instanceof guard keeps a
		// product id from being hydrated through this lane.
		// Leg-3 (ADR 0014): attach each variation's stored 64-bit digest as `_rxdb_digest` so the client
		// seeds its existence-reconcile manifest from this pull too (products get theirs via the proxy
		// filter). Bulk-read once for the whole include set. A string — the digest exceeds int range.
		// ::class, never the bare string: from inside this namespace
		// class_exists( 'Digest_Index' ) probes the GLOBAL namespace and is
		// forever false, so variation digests would never emit (review finding 3).
		// Variations read the PRODUCTS id-space — one registry row owns both
		// object types, so this lane cannot drift from the proxy lane's answer.
		$digests = class_exists( Digest_Index::class )
			? ( new Digest_Index() )->read_digests( 'products', $ids )
			: array();

		$serializer = new Product_Serializer();
		// A CLONE of the live request, not a synthetic bare one (so prepare-filters
		// see the real request context), and not the live request itself (the
		// serializer stamps store scope and a per-variation `product_id` onto
		// whatever it is handed; the dispatched request must leave this method as
		// the client sent it).
		$serialization_request = clone $request;
		$documents             = array();
		foreach ( $ids as $id ) {
			$variation = wc_get_product( $id );
			if ( ! $variation instanceof WC_Product_Variation ) {
				continue;
			}
			/*
			 * DISABLED variations are never hydrated — see the `post_status` note in
			 * prepare_objects_query(). The query-level publish gate covers ALL lanes, including
			 * `include`; this check only guards a status change between the id query and object load.
			 *
			 * `meta.requested` now counts the query-eligible ask: a disabled or query-filtered id is
			 * absent from $ids. The client's targeted-pull shortfall — absence from documents — is
			 * unchanged.
			 */
			if ( 'publish' !== $variation->get_status() ) {
				continue;
			}
			$payload  = $serializer->serialize( $variation, $serialization_request );
			$document = array(
				'id'        => $id,
				'parent_id' => (int) $variation->get_parent_id(),
				'payload'   => $payload,
			);
			if ( isset( $digests[ $id ] ) ) {
				$document['_rxdb_digest'] = $digests[ $id ];
			}
			$documents[] = $document;
		}

		$meta = array(
			'duration_ms' => round( ( microtime( true ) - $started ) * 1000, 3 ),
			'requested'   => \count( $ids ),
			'returned'    => \count( $documents ),
		);
		if ( null !== $search_meta ) {
			$meta = array_merge( $meta, $search_meta );
		}

		$response = rest_ensure_response(
			array(
				'documents' => $documents,
				'meta'      => $meta,
			)
		);

		/*
		 * The pagination WooCommerce would have sent.
		 *
		 * No v2 route emitted `X-WP-Total`/`X-WP-TotalPages` — including this one, the only one
		 * that paginates. The client asks for them on every v2 GET (the response envelope mirrors
		 * them into the body), so it has been receiving an empty mirror and falling back to
		 * short-page detection, which cannot tell "last page" from "the server truncated".
		 */
		if ( null !== $search_meta && $response instanceof WP_REST_Response ) {
			$response->header( 'X-WP-Total', (string) $search_meta['total'] );
			$response->header(
				'X-WP-TotalPages',
				(string) ( $search_meta['per_page'] > 0 ? (int) ceil( $search_meta['total'] / $search_meta['per_page'] ) : 0 )
			);
		}

		return $response;
	}

	/**
	 * Reject search requests that could build excessively large SQL queries or offsets.
	 *
	 * @return true|WP_Error
	 */
	private function validate_search_request( WP_REST_Request $request ) {
		$sku  = (string) ( $request->get_param( 'sku' ) ?? '' );
		$skus = array_filter(
			array_map( 'trim', explode( ',', $sku ) ),
			static function ( string $term ): bool {
				return '' !== $term;
			}
		);
		if ( array() !== $skus ) {
			$sku = implode( ',', $skus );
			if ( self::MAX_SKU_LENGTH < \strlen( $sku ) ) {
				return new WP_Error( 'woocommerce_pos_variations_search_limit_exceeded', 'sku must not exceed 4096 bytes', array( 'status' => 400 ) );
			}
			if ( self::MAX_SKU_TERMS < \count( $skus ) ) {
				return new WP_Error( 'woocommerce_pos_variations_search_limit_exceeded', 'sku must not contain more than 100 comma-separated terms', array( 'status' => 400 ) );
			}
		} else {
			$search = (string) $request->get_param( 'search' );
			// Unlike mb_strlen(), PCRE is independent of blog_charset and detects malformed UTF-8.
			$characters = preg_match_all( '/./us', $search );
			if ( false === $characters ) {
				return new WP_Error( 'woocommerce_pos_variations_search_invalid', 'search must be valid UTF-8', array( 'status' => 400 ) );
			}
			if ( self::MAX_SEARCH_LENGTH < $characters ) {
				return new WP_Error( 'woocommerce_pos_variations_search_limit_exceeded', 'search must not exceed 256 characters', array( 'status' => 400 ) );
			}
			$terms = preg_split( '/[\s\p{Z}\p{C}]+/u', trim( $search ), -1, PREG_SPLIT_NO_EMPTY );
			if ( false === $terms ) {
				return new WP_Error( 'woocommerce_pos_variations_search_invalid', 'search must be valid UTF-8', array( 'status' => 400 ) );
			}
			if ( self::MAX_SEARCH_TERMS < \count( $terms ) ) {
				return new WP_Error( 'woocommerce_pos_variations_search_limit_exceeded', 'search must not contain more than 10 whitespace-separated terms', array( 'status' => 400 ) );
			}
		}

		if ( self::MAX_PAGE < (int) $request->get_param( 'page' ) ) {
			return new WP_Error( 'woocommerce_pos_variations_search_limit_exceeded', 'page must not exceed 1000', array( 'status' => 400 ) );
		}

		return true;
	}

	/**
	 * Apply the declared POS variation sorts to the SQL clauses.
	 *
	 * `posts_clauses` fires for EVERY WP_Query, so the body is guarded by post type and by
	 * the plan itself — it contributes nothing unless this request claimed one of the
	 * declared sorts.
	 *
	 * @param array    $clauses  Associative array of the clauses for the query.
	 * @param WP_Query $wp_query The WP_Query instance.
	 *
	 * @return array
	 */
	public function wcpos_posts_clauses( array $clauses, WP_Query $wp_query ): array {
		if ( null === $this->wcpos_sort_request ) {
			return $clauses;
		}

		$post_type = $wp_query->query_vars['post_type'] ?? null;
		if ( 'product_variation' !== $post_type && ( ! \is_array( $post_type ) || ! \in_array( 'product_variation', $post_type, true ) ) ) {
			return $clauses;
		}

		$plan = Collection_Rules::for_request( 'variations', $this->wcpos_sort_request, self::WCPOS_SORT_PARAM_MAP );

		return $plan->filter( Collection_Rules_Plan::HOOK_POSTS_CLAUSES, $clauses, $wp_query );
	}

	/**
	 * WooCommerce's collection params, plus the sort keys the POS grids offer.
	 *
	 * `orderby` is a validated enum. Appending here is what lets `prepare_objects_query()` act on
	 * these four — otherwise the request 400s during argument validation and the switch is dead
	 * code. 1.9.x extended the same enum for the same reason.
	 */
	public function get_collection_params() {
		$params = parent::get_collection_params();
		$params['search']['sanitize_callback'] = 'rest_sanitize_request_arg';

		if ( isset( $params['orderby']['enum'] ) && \is_array( $params['orderby']['enum'] ) ) {
			$params['orderby']['enum'] = array_values(
				array_unique(
					array_merge(
						$params['orderby']['enum'],
						Collection_Rules::orderby_enum( 'variations' )
					)
				)
			);
		}

		return $params;
	}

	/**
	 * De-duplicate variation searches joined through matching meta rows.
	 *
	 * @param string   $groupby Existing GROUP BY clause.
	 * @param WP_Query $query   Query being filtered.
	 */
	public function group_search_results( string $groupby, WP_Query $query ): string {
		global $wpdb;

		return ! empty( $query->query_vars['wcpos_variation_search'] ) ? "{$wpdb->posts}.ID" : $groupby;
	}

	/**
	 * Does this discovery request still carry a term after normalization?
	 *
	 * `has_param()` is what selects discovery mode, and an empty or whitespace-only value passes
	 * it. This is the check that decides whether a query would actually be constrained.
	 */
	private function has_discovery_constraint( WP_REST_Request $request ): bool {
		$sku = (string) ( $request->get_param( 'sku' ) ?? '' );
		if ( '' !== trim( $sku, " \t\n\r\0\x0B," ) ) {
			return true;
		}

		$search = (string) ( $request->get_param( 'search' ) ?? '' );
		$terms  = preg_split( '/[\s\p{Z}\p{C}]+/u', trim( $search ), -1, PREG_SPLIT_NO_EMPTY );
		if ( false === $terms ) {
			return false;
		}

		return array() !== $terms;
	}

	/**
	 * One page of the POS-servable variation collection, with its total.
	 *
	 * WooCommerce's query pair, same as {@see search_variation_ids()} — the only difference is that
	 * a bare collection request carries no discovery constraint to normalize away, so the
	 * blank-scan guard that turns `?search=%20` into zero rows must NOT apply here. Visibility and
	 * `post_status` narrowing both ride `prepare_objects_query()`, so this page counts exactly what
	 * the client is allowed to receive.
	 *
	 * @return array{0: array<int, int>, 1: array{total: int, page: int, per_page: int}}
	 */
	private function collection_page( WP_REST_Request $request ): array {
		$per_page = max( 1, min( 100, (int) ( $request->get_param( 'per_page' ) ?? 10 ) ) );
		$page     = max( 1, (int) ( $request->get_param( 'page' ) ?? 1 ) );
		$request->set_param( 'per_page', $per_page );
		$request->set_param( 'page', $page );

		$results = $this->get_objects( $this->prepare_objects_query( $request ) );

		$ids = array();
		foreach ( $results['objects'] as $object ) {
			if ( $object instanceof WC_Product_Variation ) {
				$ids[] = $object->get_id();
			}
		}

		return array(
			$ids,
			array(
				'total'    => (int) ( $results['total'] ?? \count( $ids ) ),
				'page'     => $page,
				'per_page' => $per_page,
			),
		);
	}

	/**
	 * Discover a page of published, POS-visible variation ids by SKU/barcode.
	 *
	 * The query is WooCommerce's — `prepare_objects_query()` + `get_objects()`, the same pair its
	 * own `get_items()` uses. This method previously hand-built the SQL: a `wp_posts`/`wp_postmeta`
	 * INNER JOIN with `LIKE` predicates assembled per (field, term) pair, a second COUNT(DISTINCT)
	 * query for the total, and the hidden-id exclusion spliced into the same placeholder list. All
	 * of it duplicated `WP_Query` — which is where such copies go wrong, quietly and later.
	 *
	 * @return array{0: array<int, int>, 1: array{total: int, page: int, per_page: int}}
	 */
	private function search_variation_ids( WP_REST_Request $request ): array {
		$per_page = max( 1, min( 100, (int) ( $request->get_param( 'per_page' ) ?? 10 ) ) );
		$page     = max( 1, (int) ( $request->get_param( 'page' ) ?? 1 ) );
		$request->set_param( 'per_page', $per_page );
		$request->set_param( 'page', $page );

		$query_args = $this->prepare_objects_query( $request );

		/*
		 * A discovery request whose terms normalize away — `?sku=`, `?sku=,%20`, `?search=%20` —
		 * has no constraint left. Inherited, that query would return the FIRST PAGE OF EVERY
		 * VARIATION and advertise the catalogue-wide total; the replaced SQL deliberately used
		 * `1 = 0`. A blank scan must hydrate nothing, not everything.
		 */
		if ( ! $this->has_discovery_constraint( $request ) ) {
			return array(
				array(),
				array(
					'total'    => 0,
					'page'     => $page,
					'per_page' => $per_page,
				),
			);
		}

		add_filter( 'posts_groupby', array( $this, 'group_search_results' ), 10, 2 );
		try {
			$results = $this->get_objects( $query_args );
		} finally {
			remove_filter( 'posts_groupby', array( $this, 'group_search_results' ), 10 );
		}

		$ids = array();
		foreach ( $results['objects'] as $object ) {
			if ( $object instanceof WC_Product_Variation ) {
				$ids[] = $object->get_id();
			}
		}

		return array(
			$ids,
			array(
				'total'    => (int) ( $results['total'] ?? \count( $ids ) ),
				'page'     => $page,
				'per_page' => $per_page,
			),
		);
	}
}