mech nitrogen filtering 2 (#19868)

* target oxygen logic

* filter out nitrogen when low on oxygen

* vvrw and datafield for everything

* :trollface:

* bruh does work

* tagless chicken

* move into atmos, make it not depend on mech

* update mech prototype

---------

Co-authored-by: deltanedas <@deltanedas:kde.org>

Content.Server/Atmos/EntitySystems/AirFilterSystem.cs

@@ -0,0 +1,113 @@
+using Content.Server.Atmos;
+using Content.Server.Atmos.Components;
+using Content.Server.Atmos.Piping.Components;
+using Content.Shared.Atmos;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Map;
+using System.Diagnostics.CodeAnalysis;
+
+namespace Content.Server.Atmos.EntitySystems;
+
+/// <summary>
+/// Handles gas filtering and intake for <see cref="AirIntakeComponent"/> and <see cref="AirFilterComponent"/>.
+/// </summary>
+public sealed class AirFilterSystem : EntitySystem
+{
+    [Dependency] private readonly AtmosphereSystem _atmosphere = default!;
+    [Dependency] private readonly IMapManager _map = default!;
+    [Dependency] private readonly SharedTransformSystem _transform = default!;
+
+    public override void Initialize()
+    {
+        base.Initialize();
+
+        SubscribeLocalEvent<AirIntakeComponent, AtmosDeviceUpdateEvent>(OnIntakeUpdate);
+        SubscribeLocalEvent<AirFilterComponent, AtmosDeviceUpdateEvent>(OnFilterUpdate);
+    }
+
+    private void OnIntakeUpdate(EntityUid uid, AirIntakeComponent intake, AtmosDeviceUpdateEvent args)
+    {
+        if (!GetAir(uid, out var air))
+            return;
+
+        // if the volume is filled there is nothing to do
+        if (air.Pressure >= intake.Pressure)
+            return;
+
+        var environment = _atmosphere.GetContainingMixture(uid, true, true);
+        // nothing to intake from
+        if (environment == null)
+            return;
+
+        // absolute maximum pressure change
+        var pressureDelta = args.dt * intake.TargetPressureChange;
+        pressureDelta = MathF.Min(pressureDelta, intake.Pressure - air.Pressure);
+        if (pressureDelta <= 0)
+            return;
+
+        // how many moles to transfer to change internal pressure by pressureDelta
+        // ignores temperature difference because lazy
+        var transferMoles = pressureDelta * air.Volume / (environment.Temperature * Atmospherics.R);
+        _atmosphere.Merge(air, environment.Remove(transferMoles));
+    }
+
+    private void OnFilterUpdate(EntityUid uid, AirFilterComponent filter, AtmosDeviceUpdateEvent args)
+    {
+        if (!GetAir(uid, out var air))
+            return;
+
+        var ratio = MathF.Min(1f, args.dt * filter.TransferRate);
+        var removed = air.RemoveRatio(ratio);
+        // nothing left to remove from the volume
+        if (MathHelper.CloseToPercent(removed.TotalMoles, 0f))
+            return;
+
+        // when oxygen gets too low start removing overflow gases (nitrogen) to maintain oxygen ratio
+        var oxygen = air.GetMoles(filter.Oxygen) / air.TotalMoles;
+        var gases = oxygen >= filter.TargetOxygen ? filter.Gases : filter.OverflowGases;
+
+        var coordinates = Transform(uid).MapPosition;
+        GasMixture? destination = null;
+        if (_map.TryFindGridAt(coordinates, out _, out var grid))
+        {
+            var tile = grid.GetTileRef(coordinates);
+            destination = _atmosphere.GetTileMixture(tile.GridUid, null, tile.GridIndices, true);
+        }
+
+        if (destination != null)
+        {
+            _atmosphere.ScrubInto(removed, destination, gases);
+        }
+        else
+        {
+            // filtering into space/planet so just discard them
+            foreach (var gas in gases)
+            {
+                removed.SetMoles(gas, 0f);
+            }
+        }
+
+        _atmosphere.Merge(air, removed);
+    }
+
+    /// <summary>
+    /// Uses <see cref="GetFilterAirEvent"/> to get an internal volume of air on an entity.
+    /// Used for both filter and intake.
+    /// </summary>
+    public bool GetAir(EntityUid uid, [NotNullWhen(true)] out GasMixture? air)
+    {
+        air = null;
+
+        var ev = new GetFilterAirEvent();
+        RaiseLocalEvent(uid, ref ev);
+        air = ev.Air;
+        return air != null;
+    }
+}
+
+/// <summary>
+/// Get a reference to an entity's air volume to filter.
+/// Do not create a new mixture as this will be modified when filtering and intaking air.
+/// </summary>
+[ByRefEvent]
+public record struct GetFilterAirEvent(GasMixture? Air = null);