<?php
namespace Relay\Remote;

/**
 * How a signed answer travels: inside JSON that WordPress outputs itself, {"relay": the frame in base64url}
 * (a WP_Error's refusal carries it under data). The signature covers the frame, not the JSON around it. An older
 * SiteRelay answers with the raw frame, which is read as it is.
 */
final class Transport
{
    public const KEY = 'relay';

    /** @return array{relay: string} */
    public static function wrap(string $frame): array
    {
        return [self::KEY => rtrim(strtr(base64_encode($frame), '+/', '-_'), '=')];
    }

    /** The frame in an answer's body: out of its JSON, or the body itself when it isn't SiteRelay's JSON. */
    public static function unwrap(string $body): string
    {
        if ($body === '' || $body[0] !== '{') {
            return $body; // a raw frame (gzip) or someone else's page
        }
        $json = json_decode($body, true);
        $wrapped = is_array($json) ? ($json[self::KEY] ?? ($json['data'][self::KEY] ?? null)) : null;
        if (!is_string($wrapped) || !preg_match('/^[A-Za-z0-9_-]*$/', $wrapped)) {
            return $body;
        }
        $frame = base64_decode(strtr($wrapped, '-_', '+/') . str_repeat('=', (4 - strlen($wrapped) % 4) % 4), true);
        return $frame === false ? $body : $frame;
    }
}