namespace TinfoilVibeServer.Utilities;

public static class FileSystemExtensions
{
    /// <summary>
    /// Returns the most recent last‑write time (UTC) of any file under the supplied
    /// root directories, traversing all sub‑directories. If no files are found,
    /// <c>null</c> is returned.
    /// </summary>
    /// <param name="rootDirectories">
    /// A collection of absolute paths that must point to existing directories.
    /// Paths that do not exist or are inaccessible are silently skipped.
    /// </param>
    /// <returns>
    /// The UTC <see cref="DateTime"/> of the newest file, or <c>null</c> if there are none.
    /// </returns>
    public static DateTime? GetLatestModifiedUtc(IEnumerable<string> rootDirectories)
    {
        if (rootDirectories == null) throw new ArgumentNullException(nameof(rootDirectories));

        // We keep a mutable variable because we don't want to materialise the entire
        // sequence into memory.
        DateTime? latest = null;

        foreach (var root in rootDirectories)
        {
            if (string.IsNullOrWhiteSpace(root) || !Directory.Exists(root))
                continue; // skip bad paths

            try
            {
                // Enumerate lazily and process each file as soon as it’s yielded.
                foreach (var filePath in Directory.EnumerateFiles(
                             root,
                             "*",
                             SearchOption.AllDirectories))
                {
                    try
                    {
                        // Using FileSystemInfo to fetch only the property we need.
                        var fsi = new FileInfo(filePath);
                        var lastWrite = fsi.LastWriteTimeUtc;

                        if (!latest.HasValue || lastWrite > latest.Value)
                            latest = lastWrite;
                    }
                    catch (FileNotFoundException) // file vanished while we were enumerating
                    {
                        // ignore and keep going
                    }
                    catch (UnauthorizedAccessException)
                    {
                        // file exists but we can’t read its attributes – skip it
                    }
                }
            }
            catch (UnauthorizedAccessException)
            {
                // the root directory itself is inaccessible – skip it
            }
        }

        return latest;
    }

    /// <summary>
    /// Parallelised version that may be faster on very large directory trees.
    /// </summary>
    public static DateTime? GetLatestModifiedUtcParallel(IEnumerable<string> rootDirectories)
    {
        if (rootDirectories == null) throw new ArgumentNullException(nameof(rootDirectories));

        // Flatten all file paths into a single stream first (this is the only
        // part that needs to be thread‑safe).
        var allFiles = rootDirectories
            .Where(r => !string.IsNullOrWhiteSpace(r) && Directory.Exists(r))
            .SelectMany(r => Directory.EnumerateFiles(r, "*", SearchOption.AllDirectories))
            .ToArray(); // materialise once, then parallelise

        // Now fetch the dates in parallel.  The LINQ overload of Max() that takes
        // an async selector is not available, so we just use Parallel.ForEach.
        DateTime? latest = null;
        var lockObj = new object();

        Parallel.ForEach(allFiles, filePath =>
        {
            try
            {
                var lastWrite = new FileInfo(filePath).LastWriteTimeUtc;
                lock (lockObj)
                {
                    if (!latest.HasValue || lastWrite > latest.Value)
                        latest = lastWrite;
                }
            }
            catch (Exception)
            {
                // swallow all exceptions – the caller only cares about the max date
            }
        });

        return latest;
    }

    /// <summary>
    /// Creates the directory (and all missing parent directories) if it does not already exist.
    /// </summary>
    /// <param name="path">Absolute or relative path to the directory to create.</param>
    /// <exception cref="ArgumentNullException">Thrown if <paramref name="path"/> is null.</exception>
    /// <exception cref="ArgumentException">Thrown if <paramref name="path"/> is empty or contains only whitespace.</exception>
    /// <exception cref="UnauthorizedAccessException">Thrown if the caller does not have permission.</exception>
    /// <exception cref="IOException">Thrown if a file exists at the target path or the directory cannot be created.</exception>
    public static void EnsureDirectoryExists(string? path)
    {
        if (path is null)
            throw new ArgumentNullException(nameof(path));

        if (string.IsNullOrWhiteSpace(path))
            throw new ArgumentException("Path must not be empty or whitespace.", nameof(path));

        // Directory.CreateDirectory is already idempotent – it only creates missing parts.
        Directory.CreateDirectory(path);
    }
}