<?php
/**
 * How a file batch responds to a request that ran out of time.
 *
 * @package Shiftora\Migration
 */

declare(strict_types=1);

namespace Shiftora\Migration;

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

/**
 * A batch is sized by bytes but sent under a request timeout that shrinks with the
 * worker's remaining budget, so retrying the same bytes times out in the same place.
 * The answer to a timeout is to send fewer files.
 *
 * A failure the remote answered for its own reasons (rejected token, file outside
 * the manifest, server error) is not a sizing problem. A 413 means the body was too large, so it splits like a timeout.
 */
final class FileBatchSizer {

	/** A one-file request is the smallest there is; splitting it again only repeats the failure. */
	private const MIN_SPLITTABLE_FILES = 2;

	/**
	 * Whether a failure is evidence about the size of the request.
	 *
	 * Only no response, a 408/504 timeout or a 413 count. Any other answer is the
	 * remote deciding for its own reasons, and shrinking the batch would fix nothing.
	 *
	 * @param int|null $status HTTP status, or null/0 when no response arrived.
	 */
	public static function is_size_failure( ?int $status ): bool {
		// null and 0 mean the request never came back, same as a timeout.
		return null === $status || 0 === $status || 408 === $status || 413 === $status || 504 === $status;
	}

	/**
	 * Whether a failed batch should be retried as two smaller ones.
	 *
	 * @param int|null $status HTTP status, or null/0 when no response arrived.
	 */
	public static function should_split( int $file_count, ?int $status = null ): bool {
		return $file_count >= self::MIN_SPLITTABLE_FILES && self::is_size_failure( $status );
	}

	/**
	 * Split a batch in two, keeping every file once and the original order, so a
	 * resumed transfer sends the same sequence. An odd count puts the extra file in the second half.
	 *
	 * @param string[] $batch
	 * @return array{0: string[], 1: string[]}
	 */
	public static function halve( array $batch ): array {
		$batch = array_values( $batch );
		$half  = intdiv( count( $batch ), 2 );

		return [
			array_slice( $batch, 0, $half ),
			array_slice( $batch, $half ),
		];
	}
}