space-game/apps/backend/Simulation/WorldService.cs

using System.Threading.Channels;
using SpaceGame.Simulation.Api.Contracts;

namespace SpaceGame.Simulation.Api.Simulation;

public sealed class WorldService(IWebHostEnvironment environment)
{
  private const int DeltaHistoryLimit = 256;

  private readonly object _sync = new();
  private readonly ScenarioLoader _loader = new(environment.ContentRootPath);
  private readonly SimulationEngine _engine = new();
  private readonly Dictionary<Guid, SubscriptionState> _subscribers = [];
  private readonly Queue<WorldDelta> _history = [];
  private SimulationWorld _world = new ScenarioLoader(environment.ContentRootPath).Load();
  private long _sequence;

  public WorldSnapshot GetSnapshot()
  {
    lock (_sync)
    {
      return _engine.BuildSnapshot(_world, _sequence);
    }
  }

  public (long Sequence, DateTimeOffset GeneratedAtUtc) GetStatus()
  {
    lock (_sync)
    {
      return (_sequence, _world.GeneratedAtUtc);
    }
  }

  public ChannelReader<WorldDelta> Subscribe(ObserverScope scope, long afterSequence, CancellationToken cancellationToken)
  {
    var channel = Channel.CreateUnbounded<WorldDelta>(new UnboundedChannelOptions
    {
      SingleReader = true,
      SingleWriter = false,
    });

    Guid subscriberId;
    lock (_sync)
    {
      subscriberId = Guid.NewGuid();
      _subscribers.Add(subscriberId, new SubscriptionState(scope, channel));

      foreach (var delta in _history.Where((candidate) => candidate.Sequence > afterSequence))
      {
        var filtered = FilterDeltaForScope(delta, scope);
        if (HasMeaningfulDelta(filtered))
        {
          channel.Writer.TryWrite(filtered);
        }
      }
    }

    cancellationToken.Register(() => Unsubscribe(subscriberId));
    return channel.Reader;
  }

  public void Tick(float deltaSeconds)
  {
    WorldDelta? delta = null;
    lock (_sync)
    {
      delta = _engine.Tick(_world, deltaSeconds, ++_sequence);
      if (!HasMeaningfulDelta(delta))
      {
        return;
      }

      _history.Enqueue(delta);
      while (_history.Count > DeltaHistoryLimit)
      {
        _history.Dequeue();
      }

      foreach (var subscriber in _subscribers.Values.ToList())
      {
        var filtered = FilterDeltaForScope(delta, subscriber.Scope);
        if (HasMeaningfulDelta(filtered))
        {
          subscriber.Channel.Writer.TryWrite(filtered);
        }
      }
    }
  }

  public WorldSnapshot Reset()
  {
    lock (_sync)
    {
      _world = _loader.Load();
      _sequence += 1;
      _history.Clear();

      var resetDelta = new WorldDelta(
        _sequence,
        _world.TickIntervalMs,
        DateTimeOffset.UtcNow,
        true,
        [new SimulationEventRecord("world", "world", "reset", "World reset requested", DateTimeOffset.UtcNow, "world", "universe", "world")],
        [],
        [],
        [],
        [],
        [],
        [],
        [],
        [],
        [],
        []);

      _history.Enqueue(resetDelta);
      foreach (var subscriber in _subscribers.Values.ToList())
      {
        subscriber.Channel.Writer.TryWrite(FilterDeltaForScope(resetDelta, subscriber.Scope));
      }

      return _engine.BuildSnapshot(_world, _sequence);
    }
  }

  private static bool HasMeaningfulDelta(WorldDelta delta) =>
    delta.RequiresSnapshotRefresh
    || delta.Events.Count > 0
    || delta.SpatialNodes.Count > 0
    || delta.LocalBubbles.Count > 0
    || delta.Nodes.Count > 0
    || delta.Stations.Count > 0
    || delta.Claims.Count > 0
    || delta.ConstructionSites.Count > 0
    || delta.MarketOrders.Count > 0
    || delta.Policies.Count > 0
    || delta.Ships.Count > 0
    || delta.Factions.Count > 0;

  private void Unsubscribe(Guid subscriberId)
  {
    lock (_sync)
    {
      if (!_subscribers.Remove(subscriberId, out var subscription))
      {
        return;
      }

      subscription.Channel.Writer.TryComplete();
    }
  }

  private WorldDelta FilterDeltaForScope(WorldDelta delta, ObserverScope scope)
  {
    if (string.Equals(scope.ScopeKind, "universe", StringComparison.OrdinalIgnoreCase))
    {
      return delta with
      {
        Events = delta.Events.Select((evt) => EnrichEventScope(evt)).ToList(),
        Scope = scope,
      };
    }

    var systemFilter = scope.SystemId;
    if (string.Equals(scope.ScopeKind, "local-bubble", StringComparison.OrdinalIgnoreCase) && systemFilter is null && scope.BubbleId is not null)
    {
      systemFilter = ResolveBubbleSystemId(scope.BubbleId);
    }

    return delta with
    {
      Events = delta.Events
        .Select((evt) => EnrichEventScope(evt))
        .Where((evt) => IsEventVisibleToScope(evt, scope, systemFilter))
        .ToList(),
      SpatialNodes = delta.SpatialNodes.Where((node) => systemFilter is null || node.SystemId == systemFilter).ToList(),
      LocalBubbles = delta.LocalBubbles.Where((bubble) => systemFilter is null || bubble.SystemId == systemFilter).ToList(),
      Nodes = delta.Nodes.Where((node) => systemFilter is null || node.SystemId == systemFilter).ToList(),
      Stations = delta.Stations.Where((station) => systemFilter is null || station.SystemId == systemFilter).ToList(),
      Claims = delta.Claims.Where((claim) => systemFilter is null || claim.SystemId == systemFilter).ToList(),
      ConstructionSites = delta.ConstructionSites.Where((site) => systemFilter is null || site.SystemId == systemFilter).ToList(),
      MarketOrders = delta.MarketOrders.Where((order) => IsOrderVisibleToScope(order, systemFilter)).ToList(),
      Policies = string.Equals(scope.ScopeKind, "universe", StringComparison.OrdinalIgnoreCase) ? delta.Policies : [],
      Ships = delta.Ships.Where((ship) => systemFilter is null || ship.SystemId == systemFilter).ToList(),
      Factions = string.Equals(scope.ScopeKind, "universe", StringComparison.OrdinalIgnoreCase) ? delta.Factions : [],
      Scope = scope,
    };
  }

  private SimulationEventRecord EnrichEventScope(SimulationEventRecord evt)
  {
    if (!string.Equals(evt.ScopeKind, "universe", StringComparison.OrdinalIgnoreCase) || evt.ScopeEntityId is not null)
    {
      return evt;
    }

    return evt.EntityKind switch
    {
      "ship" => WithEntityScope(evt, "system", _world.Ships.FirstOrDefault((ship) => ship.Id == evt.EntityId)?.SystemId),
      "station" => WithEntityScope(evt, "system", _world.Stations.FirstOrDefault((station) => station.Id == evt.EntityId)?.SystemId),
      "node" => WithEntityScope(evt, "system", _world.Nodes.FirstOrDefault((node) => node.Id == evt.EntityId)?.SystemId),
      "spatial-node" => WithEntityScope(evt, "system", _world.SpatialNodes.FirstOrDefault((node) => node.Id == evt.EntityId)?.SystemId),
      "local-bubble" => WithEntityScope(evt, "local-bubble", _world.LocalBubbles.FirstOrDefault((bubble) => bubble.Id == evt.EntityId)?.Id),
      "claim" => WithEntityScope(evt, "system", _world.Claims.FirstOrDefault((claim) => claim.Id == evt.EntityId)?.SystemId),
      "construction-site" => WithEntityScope(evt, "system", _world.ConstructionSites.FirstOrDefault((site) => site.Id == evt.EntityId)?.SystemId),
      "market-order" => WithEntityScope(evt, "system", ResolveMarketOrderSystemId(evt.EntityId)),
      _ => evt,
    };
  }

  private static SimulationEventRecord WithEntityScope(SimulationEventRecord evt, string scopeKind, string? scopeEntityId) =>
    evt with
    {
      Family = evt.Kind.Contains("power", StringComparison.Ordinal) ? "power" :
        evt.Kind.Contains("construction", StringComparison.Ordinal) ? "construction" :
        evt.Kind.Contains("population", StringComparison.Ordinal) ? "population" :
        evt.Kind.Contains("claim", StringComparison.Ordinal) ? "claim" :
        "simulation",
      ScopeKind = scopeKind,
      ScopeEntityId = scopeEntityId,
    };

  private string? ResolveBubbleSystemId(string bubbleId) =>
    _world.LocalBubbles.FirstOrDefault((bubble) => bubble.Id == bubbleId)?.SystemId;

  private string? ResolveMarketOrderSystemId(string orderId)
  {
    var order = _world.MarketOrders.FirstOrDefault((candidate) => candidate.Id == orderId);
    if (order?.StationId is not null)
    {
      return _world.Stations.FirstOrDefault((station) => station.Id == order.StationId)?.SystemId;
    }

    if (order?.ConstructionSiteId is not null)
    {
      return _world.ConstructionSites.FirstOrDefault((site) => site.Id == order.ConstructionSiteId)?.SystemId;
    }

    return null;
  }

  private bool IsOrderVisibleToScope(MarketOrderDelta order, string? systemFilter)
  {
    if (systemFilter is null)
    {
      return true;
    }

    if (order.StationId is not null)
    {
      return _world.Stations.Any((station) => station.Id == order.StationId && station.SystemId == systemFilter);
    }

    if (order.ConstructionSiteId is not null)
    {
      return _world.ConstructionSites.Any((site) => site.Id == order.ConstructionSiteId && site.SystemId == systemFilter);
    }

    return false;
  }

  private static bool IsEventVisibleToScope(SimulationEventRecord evt, ObserverScope scope, string? systemFilter)
  {
    return scope.ScopeKind switch
    {
      "universe" => true,
      "system" => evt.ScopeKind == "universe" || evt.ScopeEntityId == systemFilter,
      "local-bubble" => evt.ScopeKind == "universe" || evt.ScopeEntityId == systemFilter,
      _ => true,
    };
  }

  private sealed record SubscriptionState(ObserverScope Scope, Channel<WorldDelta> Channel);
}