using System.Text;
using System.Text.Json;
using Fleet.Core;
using Monitor.Protocol;

namespace Monitor.Measurement;

/// <summary>
/// 节点进程和宿主机更新器之间的文件交换（容器里 <c>/etc/monitor</c>，也就是数据目录）：
/// 节点把面板发来的清单和签名写进 <c>update/</c>，把「更新」请求写成 <c>update/apply.request</c>；更新器验签并更新，状态写在 root 拥有的 <c>update-status/status.json</c>。
/// 节点进程不执行任何命令，也不信任清单的内容（验签在宿主机上）。没有安装更新器时目录写不了或没人读，调用只是返回 false。
/// </summary>
public sealed class UpdateBridge
{
    private const int MaxManifestBytes = 4096;
    private const int MaxSignatureBytes = 512;
    private const int MaxStatusBytes = 4096;

    private readonly string _updateDirectory;
    private readonly string _statusFile;
    private readonly Action<string, byte[]> _write;
    private readonly INodeLog _log;

    /// <param name="write">写数据目录里的文件（相对路径，原子替换）：由节点传入 <c>ConfigStore.Write</c>——常驻进程里只有它可以写文件。</param>
    public UpdateBridge(string dataDirectory, Action<string, byte[]> write, INodeLog log)
    {
        _updateDirectory = Path.Combine(dataDirectory, "update");
        _statusFile = Path.Combine(dataDirectory, "update-status", "status.json");
        _write = write;
        _log = log;
    }

    /// <summary>放下面板发来的清单和签名（和已有的一样就不动，免得每次重连都惊动更新器）。先写清单、后写签名：更新器监视的是签名文件。</summary>
    public bool WriteOffer(UpdateOfferParams offer)
    {
        byte[] manifest = Encoding.UTF8.GetBytes(offer.Manifest);
        byte[] signature = Encoding.UTF8.GetBytes(offer.Signature);
        if (manifest.Length is 0 or > MaxManifestBytes || signature.Length is 0 or > MaxSignatureBytes)
        {
            _log.Warn("ignoring an update offer of an unexpected size");
            return false;
        }

        try
        {
            string manifestPath = Path.Combine(_updateDirectory, "offer.manifest");
            string signaturePath = Path.Combine(_updateDirectory, "offer.sig");
            if (Same(manifestPath, manifest) && Same(signaturePath, signature))
            {
                return true;
            }

            _write("update/offer.manifest", manifest);
            _write("update/offer.sig", signature);
            _log.Info("update offer received");
            return true;
        }
        catch (Exception ex) when (ex is IOException or UnauthorizedAccessException)
        {
            _log.Warn("could not store the update offer (is the updater installed?)", ("err", ex.Message));
            return false;
        }
    }

    /// <summary>请求更新：写一个空的 <c>apply.request</c>（更新器验签通过、清单更新才会真的动手）。</summary>
    public bool RequestApply()
    {
        try
        {
            _write("update/apply.request", []);
            _log.Info("update requested");
            return true;
        }
        catch (Exception ex) when (ex is IOException or UnauthorizedAccessException)
        {
            _log.Warn("could not request the update (is the updater installed?)", ("err", ex.Message));
            return false;
        }
    }

    /// <summary>更新器的状态；没有安装更新器、文件不存在或内容不对时返回 null。</summary>
    public UpdateStatus? ReadStatus()
    {
        try
        {
            if (!File.Exists(_statusFile) || new FileInfo(_statusFile).Length > MaxStatusBytes)
            {
                return null;
            }

            return JsonSerializer.Deserialize(File.ReadAllBytes(_statusFile), MonitorJsonContext.Default.UpdateStatus);
        }
        catch (Exception ex) when (ex is IOException or UnauthorizedAccessException or JsonException)
        {
            return null;
        }
    }

    private static bool Same(string path, byte[] content) => File.Exists(path) && File.ReadAllBytes(path).AsSpan().SequenceEqual(content);
}