<?php

declare(strict_types=1);

namespace app\adminapi\service;

/**
 * 诊单 AI 助手 SSE 事件状态机：start -> delta* -> done|error。
 */
final class AssistantSseProtocol
{
    private int $seq = 0;

    private bool $started = false;

    private bool $terminal = false;

    /** @param array<string,mixed> $payload */
    public function encode(string $event, array $payload): ?string
    {
        if ($this->terminal || !in_array($event, ['start', 'delta', 'done', 'error'], true)) {
            return null;
        }
        if ((!$this->started && $event !== 'start') || ($this->started && $event === 'start')) {
            return null;
        }

        $nextSeq = $this->seq + 1;
        $encoded = json_encode(
            ['seq' => $nextSeq] + $payload,
            JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES | JSON_INVALID_UTF8_SUBSTITUTE
        );
        if (!is_string($encoded)) {
            return null;
        }

        $this->seq = $nextSeq;
        $this->started = true;
        if (in_array($event, ['done', 'error'], true)) {
            $this->terminal = true;
        }
        return 'event: ' . $event . "\n" . 'data: ' . $encoded . "\n\n";
    }

    public function isTerminal(): bool
    {
        return $this->terminal;
    }
}