using System.Globalization;
using System.Text.Json;
using Fleet.Core;
using MoShiro.Protocol;

namespace MoShiro.Measurement;

/// <summary>
/// 节点的「更新」：面板建议、请求，节点<b>只负责有序退出</b>，换新版本由外面的服务管理器完成——
/// 例如 Podman 的 Quadlet 单元写了 <c>Pull=always</c>，每次启动都先从镜像仓库拉最新的镜像，所以退出再被拉起就是更新。
/// 节点自己不下载、不安装、不执行任何东西，所以不需要验证什么：最多让面板（管理员）请求一次重启，和定时重启是同一种能力。
/// <list type="bullet">
/// <item>只有设了环境变量 <c>MOSHIRO_UPDATE=restart</c> 才启用（Quadlet 模板里写好了）：别的部署方式退出之后不会重新拉取，开了也不会更新，所以没有启用就不报告更新状态。</item>
/// <item>策略由环境变量 <c>MOSHIRO_UPDATE_POLICY</c> 决定：<c>auto</c>（默认）发现新版本就自己退出重启，<c>manual</c> 等管理端请求。自动更新有保护：
/// 进程至少运行 <see cref="MinimumUptime"/> 才会自动退出，新镜像没拉到（标签没变、仓库还没更新）最多每个这么久重启一次，不会陷入重启循环。</item>
/// <item>面板转来的发布清单（面板登记时已验过 CI 签名）只用来显示「有没有新版本」：组件对、比这个程序新（seq 晚于构建时间）、tag 和提交不同。</item>
/// </list>
/// </summary>
public sealed class RestartUpdater
{
    public const string EnvironmentVariable = "MOSHIRO_UPDATE";
    public const string PolicyVariable = "MOSHIRO_UPDATE_POLICY";

    /// <summary>自动更新之前，进程至少要运行这么久。</summary>
    public static readonly TimeSpan MinimumUptime = TimeSpan.FromMinutes(10);

    private const int MaxManifestBytes = 4096;

    private readonly string _component;
    private readonly Action<string> _requestRestart;
    private readonly TimeProvider _time;
    private readonly INodeLog _log;
    private readonly string _tag;
    private readonly string _commit;
    private readonly long _builtAt;
    private readonly object _gate = new();
    private readonly bool _auto;
    private int _autoScheduled;
    private readonly long _startedAt;
    private UpdateStatus _status;

    /// <param name="tag">这个程序的构建标签、提交和构建时间（秒）：默认取 <see cref="BuildInfo"/>，测试可以传入。</param>
    public RestartUpdater(string component, Action<string> requestRestart, TimeProvider time, INodeLog log, string? tag = null, string? commit = null, long? builtAtEpoch = null, bool auto = false)
    {
        _auto = auto;
        _startedAt = time.GetTimestamp();
        _component = component;
        _requestRestart = requestRestart;
        _time = time;
        _log = log;
        _tag = tag ?? BuildInfo.Tag;
        _commit = commit ?? BuildInfo.Commit;
        _builtAt = builtAtEpoch ?? (DateTimeOffset.TryParse(BuildInfo.BuiltAt, CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal, out DateTimeOffset built) ? built.ToUnixTimeSeconds() : 0);
        _status = new UpdateStatus { State = "up_to_date", At = Now() };
    }

    /// <summary>环境变量 <c>MOSHIRO_UPDATE=restart</c> 才启用；否则返回 null，节点不报告更新状态。</summary>
    public static RestartUpdater? FromEnvironment(string component, Action<string> requestRestart, TimeProvider time, INodeLog log) =>
        string.Equals(Environment.GetEnvironmentVariable(EnvironmentVariable), "restart", StringComparison.Ordinal)
            ? new RestartUpdater(component, requestRestart, time, log, auto: !string.Equals(Environment.GetEnvironmentVariable(PolicyVariable), "manual", StringComparison.OrdinalIgnoreCase))
            : null;

    /// <summary>面板转来的发布清单：只用来判断有没有新版本。</summary>
    public void Offer(UpdateOfferParams offer)
    {
        if (offer.Manifest.Length is 0 or > MaxManifestBytes)
        {
            _log.Warn("ignoring an update offer of an unexpected size");
            return;
        }

        try
        {
            using JsonDocument document = JsonDocument.Parse(offer.Manifest);
            JsonElement root = document.RootElement;
            if (root.ValueKind != JsonValueKind.Object || Text(root, "component") != _component || !root.TryGetProperty("seq", out JsonElement seqValue) || !seqValue.TryGetInt64(out long seq))
            {
                return;
            }

            string tag = Text(root, "tag");
            string commit = Text(root, "commit");
            bool same = (_tag.Length > 0 && tag == _tag) || (_commit.Length > 0 && commit.Length > 0 && commit == _commit);
            bool available;
            lock (_gate)
            {
                _status = same || seq <= _builtAt
                    ? new UpdateStatus { State = "up_to_date", At = Now() }
                    : new UpdateStatus
                    {
                        State = "available",
                        Available = new UpdateAvailable { Tag = tag, Version = Text(root, "version"), Digest = Text(root, "digest"), Seq = seq },
                        At = Now(),
                    };
                available = _status.State == "available";
            }

            if (available && _auto && Interlocked.Exchange(ref _autoScheduled, 1) == 0)
            {
                _ = ApplyAfterMinimumUptimeAsync();
            }
        }
        catch (JsonException)
        {
            _log.Warn("ignoring an update offer that is not valid JSON");
        }
    }

    /// <summary>自动更新：等进程运行够久再退出（刚启动就发现新版本，通常是重启之后标签还没变，立刻再退出会变成重启循环）。</summary>
    private async Task ApplyAfterMinimumUptimeAsync()
    {
        TimeSpan wait = MinimumUptime - _time.GetElapsedTime(_startedAt);
        if (wait > TimeSpan.Zero)
        {
            _log.Info("update available, restarting automatically later", ("in", wait.ToString("c", CultureInfo.InvariantCulture)));
            await Task.Delay(wait, _time);
        }

        RequestApply();
    }

    /// <summary>面板请求更新：有新版本才退出（由外面的服务管理器拉起并拉取最新镜像）。</summary>
    public bool RequestApply()
    {
        lock (_gate)
        {
            if (_status.State != "available")
            {
                return false;
            }

            _status = new UpdateStatus { State = "applying", Available = _status.Available, At = Now() };
        }

        _log.Info("update requested, restarting so the supervisor pulls the latest image");
        _requestRestart("update");
        return true;
    }

    /// <summary>随心跳上报的状态。</summary>
    public UpdateStatus Status()
    {
        lock (_gate)
        {
            return new UpdateStatus { State = _status.State, Current = _status.Current, Available = _status.Available, Error = _status.Error, At = _status.At };
        }
    }

    private long Now() => _time.GetUtcNow().ToUnixTimeSeconds();

    private static string Text(JsonElement root, string name) =>
        root.TryGetProperty(name, out JsonElement value) && value.ValueKind == JsonValueKind.String ? value.GetString() ?? "" : "";
}