<?php
/**
 * Entropy ordering: an even spatial spread for the feed.
 *
 * @package Mapsemble
 */

namespace Mapsemble\Feed;

use WP_Query;

/**
 * Replaces the feed query's ORDER BY with a round-robin over geographic
 * cells, so the first N features spread across the map instead of
 * clustering wherever the newest posts happen to be.
 *
 * A translation of the Drupal module's entropy hook: partition by the
 * 5.625° × 11.25° grid (exactly a 2-character geohash cell — computed
 * from the canonical lat/lng meta, no stored geohash needed), rank
 * within each cell by an MD5 tie-break, then interleave: every occupied
 * cell's first feature, then every second, and so on.
 */
class EntropyOrder {

	/**
	 * Hooks the ORDER BY filter. One instance for the request.
	 */
	public function register(): void {
		add_filter( 'posts_orderby', array( $this, 'filter_orderby' ), 10, 2 );
	}

	/**
	 * Swaps in the entropy ordering for flagged feed queries.
	 *
	 * @param string   $orderby The ORDER BY clause.
	 * @param WP_Query $query   The running query.
	 */
	public function filter_orderby( string $orderby, WP_Query $query ): string {
		$seed = (string) $query->get( 'mapsemble_entropy' );
		if ( '' === $seed ) {
			return $orderby;
		}

		// The coordinate aliases come from WP_Meta_Query itself, via the
		// named clauses the feed registers — no user input reaches the SQL.
		$clauses = isset( $query->meta_query ) && is_object( $query->meta_query )
			? $query->meta_query->get_clauses()
			: array();
		$lat     = (string) ( $clauses['mapsemble_lat']['alias'] ?? '' );
		$lng     = (string) ( $clauses['mapsemble_lng']['alias'] ?? '' );

		/**
		 * Filters whether the database can run the entropy window function.
		 *
		 * @param bool $supported True on MySQL 8+ / MariaDB 10.2+.
		 */
		$supported = (bool) apply_filters( 'mapsemble_entropy_supported', $this->server_supports() );

		if ( '' === $lat || '' === $lng || ! $supported ) {
			// Same spirit as Drupal's random fallback, but seeded by the map
			// so paging across offset requests stays coherent per map.
			return sprintf( 'RAND(%u)', crc32( $seed ) );
		}

		return self::entropy_sql( $lat, $lng );
	}

	/**
	 * Builds the entropy ORDER BY for two postmeta aliases.
	 *
	 * Every joined-column reference is wrapped in MIN(): WP_Query adds
	 * `GROUP BY posts.ID` whenever postmeta joins, and only_full_group_by
	 * rejects bare joined columns inside window functions. Lat/lng meta is
	 * single-valued, so MIN() is the value.
	 *
	 * @param string $lat_alias Alias of the latitude meta join.
	 * @param string $lng_alias Alias of the longitude meta join.
	 */
	public static function entropy_sql( string $lat_alias, string $lng_alias ): string {
		$lat_value = "MIN(CAST({$lat_alias}.meta_value AS DECIMAL(9,6)))";
		$lng_value = "MIN(CAST({$lng_alias}.meta_value AS DECIMAL(9,6)))";
		$cell      = "CONCAT(FLOOR(({$lat_value} + 90) / 5.625), '_', FLOOR(({$lng_value} + 180) / 11.25))";
		$tie       = "MD5(CONCAT(MIN({$lat_alias}.meta_value), ',', MIN({$lng_alias}.meta_value)))";

		return "ROW_NUMBER() OVER (PARTITION BY {$cell} ORDER BY {$tie}) ASC, MD5({$cell}) ASC";
	}

	/**
	 * Whether this database serves window functions.
	 */
	private function server_supports(): bool {
		global $wpdb;
		return self::version_supports( (string) $wpdb->db_server_info() );
	}

	/**
	 * Parses a server-info string for window-function support.
	 *
	 * MariaDB grew window functions in 10.2, MySQL in 8.0. MariaDB may hide
	 * behind a `5.5.5-` replication prefix, so its own version is read from
	 * around the word MariaDB rather than from the front of the string.
	 *
	 * @param string $server_info Raw `db_server_info()` string.
	 */
	public static function version_supports( string $server_info ): bool {
		if ( false !== stripos( $server_info, 'mariadb' ) ) {
			if ( preg_match( '/(\d+\.\d+(?:\.\d+)?)-MariaDB/i', $server_info, $m ) ) {
				return version_compare( $m[1], '10.2', '>=' );
			}
			return (bool) preg_match( '/MariaDB[^0-9]*(\d+\.\d+)/i', $server_info, $m )
				&& version_compare( $m[1], '10.2', '>=' );
		}
		if ( false !== stripos( $server_info, 'sqlite' ) ) {
			return false;
		}
		return (bool) preg_match( '/(\d+\.\d+(?:\.\d+)?)/', $server_info, $m )
			&& version_compare( $m[1], '8.0', '>=' );
	}
}