using Fleet.Core;
using MoShiro.Backend.Analysis;
using MoShiro.Protocol;

namespace MoShiro.Backend.Application;

public readonly record struct SourceRow(long Start, MetricSet Set, bool Partial);

/// <summary>从原始块现算的查询全局最多同时 2 个。</summary>
public sealed class RawQueryGate : IDisposable
{
    private readonly SemaphoreSlim _slots = new(2, 2);

    public bool TryEnter() => _slots.Wait(0);

    public void Exit() => _slots.Release();

    public void Dispose() => _slots.Dispose();
}

/// <summary>按数据源计划读取一条序列的累加器行（热层、10s/1m/5m/1h 层，或从原始块现算）。</summary>
public sealed class RowLoader
{
    private readonly IMetricStore _metrics;
    private readonly HotLayer _hot;
    private readonly WindowComputer _computer;
    private readonly RawQueryGate _gate;

    public RowLoader(IMetricStore metrics, HotLayer hot, WindowComputer computer, RawQueryGate gate)
    {
        _metrics = metrics;
        _hot = hot;
        _computer = computer;
        _gate = gate;
    }

    /// <summary>
    /// 这条序列最后一个样本的时刻：先看热层（1 秒精度，保留 1 小时），没有再看 1 分钟层最近 7 天（精度是窗口宽度）。没有任何数据返回 null。
    /// </summary>
    public async ValueTask<long?> LatestSampleMsAsync(NodeId agent, string series, long nowMs, CancellationToken ct)
    {
        long? hot = _hot.GetRange(agent, series, nowMs - Steps.Hour, nowMs + 10_000).Select(r => (long?)r.Key).Max();
        if (hot is { } at)
        {
            return at + 1000;
        }

        // 热层空了（Backend 刚重启、热层还没攒起来）就看持久层：各层里最晚的窗口取最大。只看 1 分钟层会落后——它比 5 分钟层更新得慢时，
        // 窗口就被定在好几个小时以前，页面显示的全是旧数据。
        long? latest = null;
        foreach (MetricLevel level in new[] { MetricLevel.S10, MetricLevel.M1, MetricLevel.M5 })
        {
            if (await _metrics.LatestAsync(level, agent, series, ct) is { } start)
            {
                long end = Math.Min(start + level.WidthMs(), nowMs);
                latest = latest is null ? end : Math.Max(latest.Value, end);
            }
        }

        return latest;
    }

    /// <summary>
    /// 这条序列最早的数据在什么时候：各层（10 秒、1 分钟、5 分钟、1 小时）里最早的窗口，加上热层里还没落到持久层的部分。
    /// 展示页据此只提供“有数据”的时间范围。没有任何数据返回 null。
    /// </summary>
    public async ValueTask<long?> EarliestSampleMsAsync(NodeId agent, string series, long nowMs, CancellationToken ct)
    {
        // 从粗到细：粗的层窗口很宽（最宽 1 小时），起点会比真实的第一个样本早；
        // 如果更细的层的起点落在粗层第一个窗口里面，就用更细的（更准确）；落在窗口之外说明更早的数据只剩粗层里有。
        long? earliest = null;
        long width = 0;
        foreach (MetricLevel level in new[] { MetricLevel.H1, MetricLevel.M5, MetricLevel.M1, MetricLevel.S10 })
        {
            if (await _metrics.EarliestAsync(level, agent, series, ct) is not { } at)
            {
                continue;
            }

            if (earliest is null || at < earliest.Value + width)
            {
                earliest = at;
                width = level.WidthMs();
            }
        }

        long? hot = _hot.GetRange(agent, series, nowMs - Steps.Hour, nowMs + 10_000).Select(r => (long?)r.Key).Min();
        if (hot is { } h && (earliest is null || h < earliest.Value + width))
        {
            earliest = h;
        }

        return earliest;
    }

    public async ValueTask<Result<IReadOnlyList<SourceRow>>> LoadAsync(SourcePlan plan, QueryRange range, NodeId agent, string series, CancellationToken ct)
    {
        switch (plan.Source)
        {
            case DataSource.Hot:
                return Result<IReadOnlyList<SourceRow>>.Ok([.. _hot.GetRange(agent, series, range.FromMs, range.ToMs).Select(r => new SourceRow(r.Key, r.Value, false))]);
            case DataSource.Raw:
                if (!_gate.TryEnter())
                {
                    return Result<IReadOnlyList<SourceRow>>.Fail(MoShiroErrors.TooManyRequests, "too many queries are reading raw data; retry shortly or use a coarser step");
                }

                try
                {
                    WindowComputer.Computed computed = await _computer.ComputeAsync(agent, series, range.FromMs, range.ToMs, plan.StepMs, null, ct);
                    return Result<IReadOnlyList<SourceRow>>.Ok([.. computed.Buckets.Select(b => new SourceRow(b.Key, b.Value, computed.AnyPartial))]);
                }
                finally
                {
                    _gate.Exit();
                }

            default:
                IReadOnlyList<MetricRow> rows = await _metrics.ReadAsync(ToLevel(plan.Source), agent, series, range.FromMs, range.ToMs, ct);
                return Result<IReadOnlyList<SourceRow>>.Ok(Decode(rows));
        }
    }

    /// <summary>多条线路一起读取（首页排行）。热层与原始块现算逐条读取。</summary>
    public async ValueTask<Result<Dictionary<LineKey, IReadOnlyList<SourceRow>>>> LoadManyAsync(
        SourcePlan plan, QueryRange range, IReadOnlyCollection<LineKey> lines, CancellationToken ct)
    {
        var result = new Dictionary<LineKey, IReadOnlyList<SourceRow>>();
        if (plan.Source is DataSource.Hot or DataSource.Raw)
        {
            foreach (LineKey line in lines)
            {
                Result<IReadOnlyList<SourceRow>> one = await LoadAsync(plan, range, line.Listener, line.Series, ct);
                if (!one.TryGet(out IReadOnlyList<SourceRow>? rows, out FleetError? error))
                {
                    return Result<Dictionary<LineKey, IReadOnlyList<SourceRow>>>.Fail(error);
                }

                result[line] = rows;
            }

            return Result<Dictionary<LineKey, IReadOnlyList<SourceRow>>>.Ok(result);
        }

        // 每个拨号方一个序列：按序列分组，每组一次查询
        foreach (IGrouping<string, LineKey> bySeries in lines.GroupBy(l => l.Series))
        {
            NodeId[] agents = [.. bySeries.Select(l => l.Listener).Distinct()];
            IReadOnlyList<MetricRow> all = await _metrics.ReadAgentsAsync(ToLevel(plan.Source), agents, bySeries.Key, range.FromMs, range.ToMs, ct);
            foreach (IGrouping<NodeId, MetricRow> group in all.GroupBy(r => r.AgentId))
            {
                LineKey key = bySeries.First(l => l.Listener == group.Key);
                result[key] = Decode([.. group]);
            }
        }

        foreach (LineKey line in lines)
        {
            result.TryAdd(line, []);
        }

        return Result<Dictionary<LineKey, IReadOnlyList<SourceRow>>>.Ok(result);
    }

    private static MetricLevel ToLevel(DataSource source) => source switch
    {
        DataSource.S10 => MetricLevel.S10,
        DataSource.M1 => MetricLevel.M1,
        DataSource.M5 => MetricLevel.M5,
        _ => MetricLevel.H1,
    };

    private static List<SourceRow> Decode(IReadOnlyList<MetricRow> rows)
    {
        var result = new List<SourceRow>(rows.Count);
        foreach (MetricRow row in rows)
        {
            // 损坏的行跳过（体现为缺口），不让一行坏数据拖垮整个查询
            if (AccumulatorCodec.Decode(row.Metrics).TryGet(out MetricSet? set, out _))
            {
                result.Add(new SourceRow(row.WindowStart, set, row.Partial));
            }
        }

        return result;
    }
}