using System.Text;
using System.Text.Json;
using System.Text.Json.Serialization.Metadata;
using Fleet.Core;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Routing;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Primitives;
using MoShiro.Backend.Application;
using MoShiro.Protocol;

namespace MoShiro.Backend.Api;

/// <summary>
/// 实时推送（SSE）：先推送当前快照，再推送增量（不补发历史）。SSE 只负责“更快地获得变化”，
/// 推送的内容都能由对应的 REST 查询重建。实时延迟取决于原始块的切块节奏（10 s）加上处理时间，典型值 10～12 s。
/// </summary>
public static class LiveEndpoints
{
    private static readonly TimeSpan s_tick = TimeSpan.FromMilliseconds(500);
    private static readonly TimeSpan s_debounce = TimeSpan.FromSeconds(1);
    private static readonly TimeSpan s_heartbeat = TimeSpan.FromSeconds(15);
    private static readonly TimeSpan s_periodicRefresh = TimeSpan.FromSeconds(10);

    public static void Map(RouteGroupBuilder group)
    {
        group.MapGet("/live", StreamAsync)
            .Api("live_stream", "Server-sent events: nodes, overview, series and test topics", "public")
            .Produces(StatusCodes.Status200OK, contentType: "text/event-stream")
            .ProducesProblems(400, 403, 404, 422, 429);
    }

    private static async Task<IResult> StreamAsync(
        HttpContext http, AgentService agents, HostStatusService hosts, LineService lineService, TestService tests, QueryService query, LiveHub hub, SseLimiter limiter,
        BackendOptions options, BackendCounters counters, TimeProvider time, IHostApplicationLifetime lifetime, string? ip_version)
    {
        string ipVersion = ip_version ?? IpVersions.V4;
        if (!IpVersions.IsValid(ipVersion))
        {
            return Problems.From(http, ValidationError.Single("ip_version", "must be ipv4 or ipv6"));
        }

        StringValues raw = http.Request.Query["topics"];
        Result<IReadOnlyList<LiveTopic>> parsed = LiveTopic.ParseAll(raw.Where(v => v is not null)!, options.MaxSseTopics);
        if (!parsed.TryGet(out IReadOnlyList<LiveTopic>? topics, out FleetError? error))
        {
            return Problems.From(http, error);
        }

        // 订阅的线路必须存在并且对访问者可见
        var lineKeys = new Dictionary<string, LineKey>(StringComparer.Ordinal);
        foreach (LiveTopic topic in topics.Where(t => t.Line is not null))
        {
            Result<LineView> found = await QueryHelpers.FindVisibleLineAsync(http, lineService, topic.Line!, http.RequestAborted);
            if (!found.TryGet(out LineView? resolved, out FleetError? lineError))
            {
                return Problems.From(http, lineError);
            }

            lineKeys[topic.Raw] = resolved.Key;
        }

        foreach (LiveTopic topic in topics.Where(t => t.Preset is not null))
        {
            Result<MetricSelection> selection = MetricSelection.Resolve(null, topic.Preset, options.MaxMetricsPerQuery);
            if (!selection.IsSuccess)
            {
                return Problems.From(http, selection.Error);
            }
        }

        string client = http.ClientAddress();
        if (!limiter.TryAcquire(client))
        {
            return Problems.Create(http, MoShiroErrors.TooManyStreams, $"at most {options.MaxSsePerClient} live connections per client");
        }

        counters.SseOpened();
        try
        {
            http.Response.ContentType = "text/event-stream";
            http.Response.Headers.CacheControl = "no-store";
            http.Response.Headers["X-Accel-Buffering"] = "no";
            await http.Response.Body.FlushAsync(http.RequestAborted);

            var session = new LiveSession(http, agents, hosts, lineService, tests, query, hub, options, time, topics, lineKeys, ipVersion);
            using var linked = CancellationTokenSource.CreateLinkedTokenSource(http.RequestAborted, lifetime.ApplicationStopping);
            await session.RunAsync(linked.Token);
        }
        catch (OperationCanceledException)
        {
            // 客户端断开或服务正在停止
        }
        finally
        {
            counters.SseClosed();
            limiter.Release(client);
        }

        return TypedResults.Empty;
    }

    private sealed class LiveSession
    {
        private readonly HttpContext _http;
        private readonly AgentService _agents;
        private readonly HostStatusService _hosts;
        private readonly LineService _lines;
        private readonly IReadOnlyDictionary<string, LineKey> _lineKeys;
        private readonly string _ipVersion;
        private readonly TestService _tests;
        private readonly QueryService _query;
        private readonly LiveHub _hub;
        private readonly BackendOptions _options;
        private readonly TimeProvider _time;
        private readonly IReadOnlyList<LiveTopic> _topics;
        private readonly object _gate = new();
        private readonly Dictionary<string, long> _lastSent = new(StringComparer.Ordinal);
        private readonly HashSet<(NodeId Agent, string Series)> _dirtyLines = [];
        private readonly HashSet<TestId> _dirtyTests = [];
        private readonly Queue<NodeSignal> _nodeSignals = new();
        private readonly Dictionary<NodeId, HostSignal> _hostSignals = [];
        private long _pushAtMs;
        private long _lastWriteMs;
        private long _lastPeriodicMs;

        public LiveSession(HttpContext http, AgentService agents, HostStatusService hosts, LineService lines, TestService tests, QueryService query, LiveHub hub, BackendOptions options, TimeProvider time, IReadOnlyList<LiveTopic> topics, IReadOnlyDictionary<string, LineKey> lineKeys, string ipVersion)
        {
            _http = http;
            _agents = agents;
            _hosts = hosts;
            _lines = lines;
            _lineKeys = lineKeys;
            _ipVersion = ipVersion;
            _tests = tests;
            _query = query;
            _hub = hub;
            _options = options;
            _time = time;
            _topics = topics;
        }

        public async Task RunAsync(CancellationToken ct)
        {
            using LiveSubscription subscription = _hub.Subscribe();
            Task drain = DrainAsync(subscription, ct);
            long now = Now();
            _lastWriteMs = now;
            _lastPeriodicMs = now;
            await PushSnapshotAsync(ct);

            using var timer = new PeriodicTimer(s_tick, _time);
            while (await timer.WaitForNextTickAsync(ct))
            {
                await FlushPendingAsync(ct);
            }

            await drain;
        }

        private async Task DrainAsync(LiveSubscription subscription, CancellationToken ct)
        {
            try
            {
                await foreach (LiveSignal signal in subscription.Reader.ReadAllAsync(ct))
                {
                    lock (_gate)
                    {
                        switch (signal)
                        {
                            case NodeSignal node when _topics.Any(t => t.Kind == LiveTopicKind.Nodes):
                                _nodeSignals.Enqueue(node);
                                break;
                            case HostSignal host when _topics.Any(t => t.Kind == LiveTopicKind.Hosts):
                                _hostSignals[host.Node] = host;
                                break;
                            case ChunkSignal chunk:
                                _dirtyLines.Add((chunk.AgentId, chunk.Series));
                                _pushAtMs = _pushAtMs == 0 ? Now() + (long)s_debounce.TotalMilliseconds : _pushAtMs;
                                break;
                            case TestSignal test when _topics.Any(t => t.Test == test.TestId):
                                _dirtyTests.Add(test.TestId);
                                break;
                        }
                    }
                }
            }
            catch (OperationCanceledException)
            {
                // 连接结束
            }
        }

        private async Task FlushPendingAsync(CancellationToken ct)
        {
            long now = Now();
            NodeSignal[] nodes;
            HostSignal[] hostSignals;
            TestId[] testIds;
            (NodeId Agent, string Series)[] dirtyLines = [];
            bool refreshAll = now - _lastPeriodicMs >= (long)s_periodicRefresh.TotalMilliseconds;
            lock (_gate)
            {
                nodes = [.. _nodeSignals];
                _nodeSignals.Clear();
                hostSignals = [.. _hostSignals.Values];
                _hostSignals.Clear();
                testIds = [.. _dirtyTests];
                _dirtyTests.Clear();
                if (_pushAtMs != 0 && now >= _pushAtMs)
                {
                    dirtyLines = [.. _dirtyLines];
                    _dirtyLines.Clear();
                    _pushAtMs = 0;
                }
            }

            foreach (HostSignal host in hostSignals)
            {
                AgentView? view = await _agents.GetAsync(host.Node, ct);
                if (view is not null && IsVisible(view) && view.Agent.Status == Fleet.Server.NodeStatus.Active)
                {
                    await WriteAsync("host", new LiveHostDto(host.Node.Value, view.Online, Mappers.ToDto(host.Host)), ApiJsonContext.Default.LiveHostDto, ct);
                }
            }

            foreach (NodeSignal node in nodes)
            {
                AgentView? view = await _agents.GetAsync(new NodeId(node.Id), ct);
                if (view is not null && IsVisible(view))
                {
                    await WriteAsync("node", new LiveNodeDto(node.Kind, node.Id, node.Online, Mappers.Time(node.AtMs)), ApiJsonContext.Default.LiveNodeDto, ct);
                }
            }

            foreach (TestId testId in testIds)
            {
                await PushTestAsync(testId, ct);
            }

            if (dirtyLines.Length > 0)
            {
                foreach (LiveTopic topic in _topics.Where(t => t.Kind == LiveTopicKind.Series && dirtyLines.Contains((_lineKeys[t.Raw].Listener, _lineKeys[t.Raw].Series))))
                {
                    await PushSeriesAsync(topic, ct);
                }

                foreach (LiveTopic topic in _topics.Where(t => t.Kind == LiveTopicKind.Overview))
                {
                    await PushOverviewAsync(topic, ct);
                }

                _lastPeriodicMs = now;
            }
            else if (refreshAll)
            {
                // 没有新块也定期刷新（例如线路断了之后汇总需要更新）
                foreach (LiveTopic topic in _topics.Where(t => t.Kind is LiveTopicKind.Overview or LiveTopicKind.Series))
                {
                    await (topic.Kind == LiveTopicKind.Overview ? PushOverviewAsync(topic, ct) : PushSeriesAsync(topic, ct));
                }

                _lastPeriodicMs = now;
            }

            if (Now() - _lastWriteMs >= (long)s_heartbeat.TotalMilliseconds)
            {
                await WriteRawAsync(": keepalive\n\n", ct);
            }
        }

        private async Task PushSnapshotAsync(CancellationToken ct)
        {
            foreach (LiveTopic topic in _topics)
            {
                switch (topic.Kind)
                {
                    case LiveTopicKind.Nodes:
                        foreach (AgentView view in (await _agents.ListAsync(ct)).Where(v => v.Agent.Status == Fleet.Server.NodeStatus.Active && IsVisible(v)))
                        {
                            await WriteAsync("node", new LiveNodeDto("agent", view.Agent.Id.Value, view.Online, Mappers.Time(Now())), ApiJsonContext.Default.LiveNodeDto, ct);
                        }

                        break;
                    case LiveTopicKind.Hosts:
                        foreach (AgentView view in (await _agents.ListAsync(ct)).Where(v => v.Agent.Status == Fleet.Server.NodeStatus.Active && IsVisible(v) && v.Host is not null))
                        {
                            await WriteAsync("host", new LiveHostDto(view.Agent.Id.Value, view.Online, Mappers.ToDto(view.Host!)), ApiJsonContext.Default.LiveHostDto, ct);
                        }

                        break;
                    case LiveTopicKind.Overview:
                        await PushOverviewAsync(topic, ct);
                        break;
                    case LiveTopicKind.Series:
                        await PushSeriesAsync(topic, ct);
                        break;
                    case LiveTopicKind.Test:
                        await PushTestAsync(topic.Test!.Value, ct);
                        break;
                }
            }
        }

        private async Task PushOverviewAsync(LiveTopic topic, CancellationToken ct)
        {
            MetricSelection selection = MetricSelection.Resolve(null, topic.Preset, _options.MaxMetricsPerQuery).Value;
            IReadOnlyList<LineView> visible = await _lines.ListAsync(a => a.Agent.Status == Fleet.Server.NodeStatus.Active && IsVisible(a), ct);
            Result<OverviewResult> result = await _query.OverviewAsync([.. visible.Where(l => l.IpVersion == _ipVersion)], new RangeRequest(topic.Range, null, null, null, null), selection, ct);
            if (result.TryGet(out OverviewResult? overview, out _))
            {
                var dto = new OverviewDto(
                    topic.Range!, Mappers.Time(overview.Range.FromMs), Mappers.Time(overview.Range.ToMs), Steps.Format(overview.Range.StepMs),
                    overview.Plan.Name, overview.Plan.Degraded, [.. overview.Lines.Select(Mappers.ToDto)]);
                await WriteAsync("overview", dto, ApiJsonContext.Default.OverviewDto, ct);
            }
        }

        private async Task PushSeriesAsync(LiveTopic topic, CancellationToken ct)
        {
            MetricSelection selection = MetricSelection.Resolve(null, topic.Preset, _options.MaxMetricsPerQuery).Value;
            string step = Steps.Format(topic.StepMs);
            long now = Now();
            long from = _lastSent.TryGetValue(topic.Raw, out long last) ? last : now - 20_000;

            Result<SeriesResult> latest = await _query.SeriesAsync(_lineKeys[topic.Raw], new RangeRequest(null, DateTimeOffset.FromUnixTimeMilliseconds(from), null, step, null), selection, ct);
            Result<SeriesResult> summary = await _query.SeriesAsync(_lineKeys[topic.Raw], new RangeRequest("15m", null, null, step, null), selection, ct);
            if (!latest.TryGet(out SeriesResult? series, out _) || !summary.TryGet(out SeriesResult? whole, out _))
            {
                return;
            }

            if (series.T.Count > 0)
            {
                _lastSent[topic.Raw] = series.T[^1];
            }

            SeriesDto dto = Mappers.ToDto(series, live: true) with { Summary = Mappers.ToDto(whole.Summary) };
            await WriteAsync("series", dto, ApiJsonContext.Default.SeriesDto, ct);
        }

        private async Task PushTestAsync(TestId testId, CancellationToken ct)
        {
            TestDetail? detail = await _tests.GetAsync(testId, ct);
            if (detail is null)
            {
                return;
            }

            AgentView? view = await _agents.GetAsync(detail.Test.AgentId, ct);
            if (view is null || !IsVisible(view))
            {
                return;
            }

            await WriteAsync("test", new LiveTestDto(testId.Value, detail.Test.Status, detail.Test.Phase, detail.Test.PhaseCount), ApiJsonContext.Default.LiveTestDto, ct);
        }

        private bool IsVisible(AgentView view) => view.Agent.PublicVisible || PublicGuard.CanSeeHidden(_http);

        private async Task WriteAsync<T>(string name, T payload, JsonTypeInfo<T> typeInfo, CancellationToken ct)
        {
            string json = JsonSerializer.Serialize(payload, typeInfo);
            await WriteRawAsync($"event: {name}\ndata: {json}\n\n", ct);
        }

        private async Task WriteRawAsync(string text, CancellationToken ct)
        {
            await _http.Response.Body.WriteAsync(Encoding.UTF8.GetBytes(text), ct);
            await _http.Response.Body.FlushAsync(ct);
            _lastWriteMs = Now();
        }

        private long Now() => _time.GetUtcNow().ToUnixTimeMilliseconds();
    }
}