<?php
/** Hash-bound metadata parts. No post bodies or authenticated page rendering. */
if (!defined('ABSPATH')) { exit; }

final class Chat4U_Transfer {
    public const PART_BYTES = 262144;

    public static function response(WP_REST_Request $request, array $value): WP_REST_Response|WP_Error {
        if (Chat4U_Protocol::fits($value) && $request->get_param('part_offset') === null) {
            return Chat4U_Protocol::response($request, $value);
        }
        $raw = wp_json_encode($value, JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES);
        if ($raw === false) {
            return new WP_Error('chat4u_transfer_invalid', __('The public metadata could not be encoded. Retry the connection check.', 'chat4u'), ['status' => 503]);
        }
        $hash = hash('sha256', $raw);
        $descriptor = ['size' => strlen($raw), 'sha256' => $hash, 'part_bytes' => self::PART_BYTES];
        if ($request->get_param('part_offset') === null) {
            return Chat4U_Protocol::response($request, ['transfer' => $descriptor]);
        }
        $offset = (string) $request->get_param('part_offset');
        $expected = (string) $request->get_param('part_hash');
        if (!ctype_digit($offset) || (int) $offset >= strlen($raw) || !hash_equals($hash, $expected)) {
            return new WP_Error('chat4u_transfer_changed', __('The public metadata changed during transfer. Retry from the current version.', 'chat4u'), ['status' => 409]);
        }
        $part = substr($raw, (int) $offset, self::PART_BYTES);
        return Chat4U_Protocol::response($request, $descriptor + ['offset' => (int) $offset,
            'data' => base64_encode($part), 'part_sha256' => hash('sha256', $part)]);
    }
}