<?php
declare(strict_types=1);

namespace Operanse\Agent\Telemetry;

defined( 'ABSPATH' ) || exit;

use Operanse\Agent\Config\Settings;
use Operanse\Agent\Http\Client;
use Operanse\Agent\Telemetry\Spool\SpoolInterface;

/**
 * Uploads spooled events in gzip batches with exponential backoff + jitter.
 *
 * Runs from WP-Cron and, on a Mode C site with a connected daemon, from `wp operanse stream` (ADR-018, milestone 4).
 * Both paths are the same code against the same endpoint with the same batch ids and the same `event_id` dedupe;
 * `$via` only labels which one uploaded, and the backoff state is shared so the two can never fight over a site the
 * cloud is rejecting.
 */
final class Flusher {
	public const MAX_EVENTS       = 500;
	public const MAX_BATCH_BYTES  = 1_000_000;
	public const MAX_BATCHES_RUN  = 10;
	public const BACKOFF_MIN_S    = 30;
	public const BACKOFF_MAX_S    = 3600;
	public const TIME_BUDGET_S    = 20;

	public function __construct( private SpoolInterface $spool, private Client $client, private Settings $settings, private string $via = 'cron' ) {}

	/** @return array{batches:int,accepted:int,error:string|null} */
	public function flush(): array {
		$now = time();
		if ( (int) $this->settings->get( 'backoff_until', 0 ) > $now ) {
			return array( 'batches' => 0, 'accepted' => 0, 'error' => 'backoff' );
		}
		$start    = microtime( true );
		$batches  = 0;
		$accepted = 0;
		for ( $i = 0; $i < self::MAX_BATCHES_RUN; $i++ ) {
			if ( microtime( true ) - $start > self::TIME_BUDGET_S ) {
				break;
			}
			$batch = $this->spool->read_batch( self::MAX_EVENTS, self::MAX_BATCH_BYTES );
			if ( $batch['events'] === array() ) {
				break;
			}
			$stats   = $this->spool->stats();
			$payload = array(
				'schema_version' => Event::SCHEMA_VERSION,
				'batch_id'       => Event::uuid7(),
				'sent_at'        => gmdate( 'Y-m-d\TH:i:s\Z' ),
				'agent_version'  => OPERANSE_AGENT_VERSION,
				'dropped'        => (object) $stats['dropped'],
				'via'            => $this->via === 'stream' ? 'stream' : 'cron',
				'events'         => $batch['events'],
			);
			$res = $this->client->post_json( '/v1/agent/telemetry', $payload, true );
			if ( $res['ok'] ) {
				$this->spool->ack( $batch['handle'] );
				$this->spool->reset_dropped();
				$accepted += (int) ( $res['body']['accepted'] ?? 0 );
				$batches++;
				$this->settings->set_many( array( 'last_flush_at' => time(), 'last_flush_ok' => true, 'backoff_seconds' => 0, 'backoff_until' => 0, 'last_error' => '' ) );
				continue;
			}
			if ( ! $res['retryable'] && $res['status'] >= 400 ) {
				// Permanent rejection by the cloud (schema/auth): drop this batch locally so a poison batch cannot block the queue,
				// but keep a bounded dead-letter copy for diagnostics.
				$this->dead_letter( $batch['events'], (string) $res['error'] );
				$this->spool->ack( $batch['handle'] );
				$this->settings->set_many( array( 'last_flush_at' => time(), 'last_flush_ok' => false, 'last_error' => 'telemetry rejected: ' . $res['error'], 'last_error_at' => time() ) );
				$batches++;
				continue;
			}
			// Retryable (network, 5xx, 429) or local error (status 0): keep the data, back off.
			$this->backoff( (string) $res['error'] );
			return array( 'batches' => $batches, 'accepted' => $accepted, 'error' => $res['error'] );
		}
		return array( 'batches' => $batches, 'accepted' => $accepted, 'error' => null );
	}

	private function backoff( string $error ): void {
		$prev = (int) $this->settings->get( 'backoff_seconds', 0 );
		$next = $prev === 0 ? self::BACKOFF_MIN_S : min( self::BACKOFF_MAX_S, $prev * 2 );
		$jitter = random_int( 0, (int) max( 1, $next / 4 ) );
		$this->settings->set_many(
			array(
				'backoff_seconds' => $next,
				'backoff_until'   => time() + $next + $jitter,
				'last_flush_at'   => time(),
				'last_flush_ok'   => false,
				'last_error'      => 'telemetry upload failed: ' . $error,
				'last_error_at'   => time(),
			)
		);
	}

	/** @param list<array<string,mixed>> $events */
	private function dead_letter( array $events, string $reason ): void {
		$existing = get_option( 'operanse_agent_dead_letters', array() );
		$existing = is_array( $existing ) ? $existing : array();
		$existing[] = array( 'at' => time(), 'reason' => substr( $reason, 0, 200 ), 'count' => count( $events ), 'sample' => array_slice( $events, 0, 3 ) );
		update_option( 'operanse_agent_dead_letters', array_slice( $existing, -20 ), false );
	}
}