<?php
/**
 * A REST response whose body is produced while it is written out.
 *
 * @package Shiftora\Api\Endpoints
 */

declare(strict_types=1);

namespace Shiftora\Api\Endpoints;

use Shiftora\Migration\FileExporter;
use WP_REST_Response;

// Prevent direct file access.
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

/**
 * Serves one file chunk by encoding it on the way out. Returned as data, the chunk
 * would exist raw, base64, gzip and JSON at once and can exhaust a shared host's
 * memory at the maximum size. The envelope goes first, then the payload in 128 KiB
 * pieces. get_data() decodes on demand; the streaming path never calls it.
 */
final class StreamedChunkResponse extends WP_REST_Response {

	/** @var array<string, mixed>|null Decoded body, once asked for. */
	private ?array $decoded = null;

	public function __construct(
		private readonly FileExporter $exporter,
		private readonly string $relative_path,
		private readonly int $offset,
		private readonly int $chunk_size,
		private readonly array $chunk
	) {
		// Left empty: decoding here would bring back the copies this class avoids.
		parent::__construct( [] );
	}

	/** @return array<string, mixed> */
	public function get_data() {
		if ( null !== $this->decoded ) {
			return $this->data;
		}

		$decoded = json_decode( $this->get_body(), true );

		$this->data    = is_array( $decoded ) ? $decoded : [];
		$this->decoded = $this->data;

		return $this->data;
	}

	/** @return string JSON. */
	public function get_body(): string {
		return implode( '', iterator_to_array( $this->body_fragments(), false ) );
	}

	/** @return \Generator<int, string> */
	public function body_fragments(): \Generator {
		yield from $this->exporter->chunk_body_fragments(
			$this->relative_path,
			$this->offset,
			$this->chunk_size,
			$this->chunk
		);
	}
}