<?php
/**
 * Answer cache.
 *
 * In an FAQ chatbot the same question is asked over and over again. Caching sharply reduces
 * the number of LLM calls — the most effective way to reduce load on shared hosting.
 *
 * When a persistent object cache (Redis/Memcached) is available it is used; otherwise a dedicated
 * table is used instead of transients, so wp_options does not bloat.
 *
 * @package AI House
 */

defined( 'ABSPATH' ) || exit;

/**
 * Class MYBOT_Cache
 */
class MYBOT_Cache {

	/**
	 * Cache group.
	 *
	 * @var string
	 */
	const GROUP = 'mybot';

	/**
	 * Table name.
	 *
	 * @return string
	 */
	public static function table() {
		global $wpdb;

		return $wpdb->prefix . 'mybot_cache';
	}

	/**
	 * Builds the cache key from the question and the context.
	 *
	 * @param string $question Normalized question.
	 * @param array  $extra    Data that affects the answer.
	 * @return string
	 */
	public static function key( $question, $extra = array() ) {
		return hash( 'sha256', $question . '|' . wp_json_encode( $extra ) );
	}

	/**
	 * Normalizes the question to increase the cache hit rate.
	 *
	 * @param string $question Raw question.
	 * @return string
	 */
	public static function normalize( $question ) {
		$question = function_exists( 'mb_strtolower' ) ? mb_strtolower( (string) $question, 'UTF-8' ) : strtolower( (string) $question );
		$question = preg_replace( '/[^\p{L}\p{N}\s]/u', ' ', $question );
		$question = preg_replace( '/\s+/u', ' ', (string) $question );

		return trim( (string) $question );
	}

	/**
	 * Reads from the cache.
	 *
	 * @param string $key Cache key.
	 * @return string|null
	 */
	public function get( $key ) {
		if ( wp_using_ext_object_cache() ) {
			$value = wp_cache_get( $key, self::GROUP );

			return is_string( $value ) ? $value : null;
		}

		global $wpdb;

		$table = self::table();

		// phpcs:ignore WordPress.DB.DirectDatabaseQuery.DirectQuery, WordPress.DB.DirectDatabaseQuery.NoCaching
		$row = $wpdb->get_var(
			$wpdb->prepare(
				'SELECT cache_value FROM %i WHERE cache_key = %s AND expires_at > %s',
				$table,
				$key,
				current_time( 'mysql' )
			)
		);

		return is_string( $row ) ? $row : null;
	}

	/**
	 * Writes to the cache.
	 *
	 * @param string $key   Cache key.
	 * @param string $value Value.
	 * @param int    $ttl   Time to live in seconds. 0 means do not cache.
	 * @return void
	 */
	public function set( $key, $value, $ttl ) {
		$ttl = (int) $ttl;

		if ( $ttl <= 0 ) {
			return;
		}

		if ( wp_using_ext_object_cache() ) {
			wp_cache_set( $key, $value, self::GROUP, $ttl );

			return;
		}

		global $wpdb;

		$table      = self::table();
		$expires_at = gmdate( 'Y-m-d H:i:s', current_time( 'timestamp' ) + $ttl );

		// phpcs:ignore WordPress.DB.DirectDatabaseQuery.DirectQuery, WordPress.DB.DirectDatabaseQuery.NoCaching
		$wpdb->query(
			$wpdb->prepare(
				'INSERT INTO %i (cache_key, cache_value, expires_at) VALUES (%s, %s, %s)
				 ON DUPLICATE KEY UPDATE cache_value = VALUES(cache_value), expires_at = VALUES(expires_at)',
				$table,
				$key,
				$value,
				$expires_at
			)
		);
	}

	/**
	 * Deletes a single key.
	 *
	 * @param string $key Cache key.
	 * @return void
	 */
	public function delete( $key ) {
		if ( wp_using_ext_object_cache() ) {
			wp_cache_delete( $key, self::GROUP );

			return;
		}

		global $wpdb;

		// phpcs:ignore WordPress.DB.DirectDatabaseQuery.DirectQuery, WordPress.DB.DirectDatabaseQuery.NoCaching
		$wpdb->delete( self::table(), array( 'cache_key' => $key ), array( '%s' ) );
	}

	/**
	 * Flushes the entire cache.
	 *
	 * @return int Number of deleted rows.
	 */
	public function flush() {
		if ( wp_using_ext_object_cache() ) {
			wp_cache_flush_group( self::GROUP );

			return 0;
		}

		global $wpdb;

		$table = self::table();

		// phpcs:ignore WordPress.DB.DirectDatabaseQuery.DirectQuery, WordPress.DB.DirectDatabaseQuery.NoCaching
		return (int) $wpdb->query( $wpdb->prepare( 'DELETE FROM %i', $table ) );
	}

	/**
	 * Deletes expired entries.
	 *
	 * @return int Number of deleted rows.
	 */
	public function purge_expired() {
		if ( wp_using_ext_object_cache() ) {
			return 0;
		}

		global $wpdb;

		$table = self::table();

		// phpcs:ignore WordPress.DB.DirectDatabaseQuery.DirectQuery, WordPress.DB.DirectDatabaseQuery.NoCaching
		return (int) $wpdb->query(
			$wpdb->prepare( 'DELETE FROM %i WHERE expires_at < %s', $table, current_time( 'mysql' ) )
		);
	}

	/**
	 * Number of entries currently in the cache.
	 *
	 * @return int
	 */
	public function count() {
		if ( wp_using_ext_object_cache() ) {
			return 0;
		}

		global $wpdb;

		$table = self::table();

		// phpcs:ignore WordPress.DB.DirectDatabaseQuery.DirectQuery, WordPress.DB.DirectDatabaseQuery.NoCaching
		return (int) $wpdb->get_var( $wpdb->prepare( 'SELECT COUNT(*) FROM %i', $table ) );
	}
}