using AcDream.Content.Pak;

namespace AcDream.Bake;

/// <summary>
/// Publishes a bake as one same-volume filesystem transaction. Work and
/// validation happen against a unique adjacent temporary file; failure or
/// cancellation leaves an existing destination untouched.
/// </summary>
public static class BakeOutputTransaction
{
    public static TResult WriteValidateAndPublish<TResult>(
        string destinationPath,
        Func<string, TResult> writeTemporary,
        Action<string, TResult> validateTemporary,
        CancellationToken cancellationToken = default)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(destinationPath);
        ArgumentNullException.ThrowIfNull(writeTemporary);
        ArgumentNullException.ThrowIfNull(validateTemporary);

        string fullDestination = Path.GetFullPath(destinationPath);
        string? directory = Path.GetDirectoryName(fullDestination);
        if (string.IsNullOrEmpty(directory))
            throw new InvalidOperationException("destination has no parent directory");
        Directory.CreateDirectory(directory);

        string temporaryPath = Path.Combine(
            directory,
            $".{Path.GetFileName(fullDestination)}.{Guid.NewGuid():N}.tmp");

        try
        {
            cancellationToken.ThrowIfCancellationRequested();
            TResult result = writeTemporary(temporaryPath);
            cancellationToken.ThrowIfCancellationRequested();
            validateTemporary(temporaryPath, result);
            cancellationToken.ThrowIfCancellationRequested();

            // Same-volume MoveFileEx/rename is the publication primitive.
            // File.Replace additionally performs destination metadata/backup
            // semantics and proved brittle for a validated 28 GiB artifact on
            // Windows. Move(overwrite:true) changes only the directory entry:
            // readers observe the complete old file or the complete new file.
            File.Move(temporaryPath, fullDestination, overwrite: true);

            return result;
        }
        finally
        {
            try
            {
                if (File.Exists(temporaryPath))
                    File.Delete(temporaryPath);
            }
            catch
            {
                // Preserve the original exception. An adjacent .tmp is
                // recognizable and never mistaken for a published pak.
            }
        }
    }
}