Solution precision fixes (#25199)

* Add test for two chemistry issues

1. rounding issue with reaction processing when making chloral hydrate
2. reliable assert trip due to the ValidateSolution() heat capacity issue.

* Fix FixedPoint2 arithmetic

Fix internal floating point arithmetic in places where it could be avoided.

Fix incorrect rounding mode used in other places (it should always floor, like regular int arithmetic).

I had to add an explicit epsilon value for float -> FixedPoint2 because something like 1.05 is actually like 1.04999 and that'd cause it to be rounded down to 1.04.

This fixes reaction reagent processing in cases where the reagent inputs can't cleanly divide. Previously, when making 30u chloral hydrate by adding the chlorine in 10u increments you'd end up with 0.04 chlorine left over. This was caused by division in the reaction code rounding up in some cases. Changing division here to always round down fixes it.

* Attempt to fix heat capacity precision assert issues.

Fixes #22126

First, we just increase the tolerance of the assert. It was way too low.

Second, actually put a cap on float drift from one-off _heatCapacity changes.

* Fix float -> FixedPoint2 epsilon for negative number, fix tests.

* Fix DamageableTest

* Oh yeah I need to call CleanReturnAsync

Content.IntegrationTests/Tests/Chemistry/SolutionRoundingTest.cs

@@ -0,0 +1,122 @@
+using Content.Server.Chemistry.Containers.EntitySystems;
+using Content.Shared.Chemistry.Components;
+using Content.Shared.Chemistry.Reaction;
+using Content.Shared.Chemistry.Reagent;
+using Content.Shared.FixedPoint;
+using Robust.Shared.GameObjects;
+
+namespace Content.IntegrationTests.Tests.Chemistry;
+
+[TestFixture]
+[TestOf(typeof(ChemicalReactionSystem))]
+public sealed class SolutionRoundingTest
+{
+    // This test tests two things:
+    // * A rounding error in reaction code while I was making chloral hydrate
+    // * An assert with solution heat capacity calculations that I found a repro for while testing the above.
+
+    [TestPrototypes]
+    private const string Prototypes = @"
+- type: entity
+  id: SolutionRoundingTestContainer
+  components:
+  - type: SolutionContainerManager
+    solutions:
+      beaker:
+        maxVol: 100
+
+# This is the Chloral Hydrate recipe fyi.
+- type: reagent
+  id: SolutionRoundingTestReagentA
+  name: reagent-name-nothing
+  desc: reagent-desc-nothing
+  physicalDesc: reagent-physical-desc-nothing
+
+- type: reagent
+  id: SolutionRoundingTestReagentB
+  name: reagent-name-nothing
+  desc: reagent-desc-nothing
+  physicalDesc: reagent-physical-desc-nothing
+
+- type: reagent
+  id: SolutionRoundingTestReagentC
+  name: reagent-name-nothing
+  desc: reagent-desc-nothing
+  physicalDesc: reagent-physical-desc-nothing
+
+- type: reagent
+  id: SolutionRoundingTestReagentD
+  name: reagent-name-nothing
+  desc: reagent-desc-nothing
+  physicalDesc: reagent-physical-desc-nothing
+
+- type: reaction
+  id: SolutionRoundingTestReaction
+  impact: Medium
+  reactants:
+    SolutionRoundingTestReagentA:
+      amount: 3
+    SolutionRoundingTestReagentB:
+      amount: 1
+    SolutionRoundingTestReagentC:
+      amount: 1
+  products:
+    SolutionRoundingTestReagentD: 1
+";
+
+    [Test]
+    public async Task Test()
+    {
+        await using var pair = await PoolManager.GetServerClient();
+        var server = pair.Server;
+        var testMap = await pair.CreateTestMap();
+
+        Solution solution = default;
+        Entity<SolutionComponent> solutionEnt = default;
+
+        await server.WaitPost(() =>
+        {
+            var system = server.System<SolutionContainerSystem>();
+            var beaker = server.EntMan.SpawnEntity("SolutionRoundingTestContainer", testMap.GridCoords);
+
+            system.TryGetSolution(beaker, "beaker", out var newSolutionEnt, out var newSolution);
+
+            solutionEnt = newSolutionEnt!.Value;
+            solution = newSolution!;
+
+            system.TryAddSolution(solutionEnt, new Solution("SolutionRoundingTestReagentC", 50));
+            system.TryAddSolution(solutionEnt, new Solution("SolutionRoundingTestReagentB", 30));
+
+            for (var i = 0; i < 9; i++)
+            {
+                system.TryAddSolution(solutionEnt, new Solution("SolutionRoundingTestReagentA", 10));
+            }
+        });
+
+        await server.WaitAssertion(() =>
+        {
+            Assert.Multiple(() =>
+            {
+                Assert.That(
+                    solution.ContainsReagent("SolutionRoundingTestReagentA", null),
+                    Is.False,
+                    "Solution should not contain reagent A");
+
+                Assert.That(
+                    solution.ContainsReagent("SolutionRoundingTestReagentB", null),
+                    Is.False,
+                    "Solution should not contain reagent B");
+
+                Assert.That(
+                    solution![new ReagentId("SolutionRoundingTestReagentC", null)].Quantity,
+                    Is.EqualTo((FixedPoint2) 20));
+
+                Assert.That(
+                    solution![new ReagentId("SolutionRoundingTestReagentD", null)].Quantity,
+                    Is.EqualTo((FixedPoint2) 30));
+            });
+        });
+
+        await pair.CleanReturnAsync();
+    }
+}

Content.IntegrationTests/Tests/Damageable/DamageableTest.cs

@@ -170,19 +170,19 @@ await server.WaitAssertion(() =>
 
                 // Check that damage works properly if it is NOT perfectly divisible among group members
                 types = group3.DamageTypes;
-                damageToDeal = FixedPoint2.New(types.Count * 5 - 1);
-                damage = new DamageSpecifier(group3, damageToDeal);
+
+                Assert.That(types, Has.Count.EqualTo(3));
+
+                damage = new DamageSpecifier(group3, 14);
                 sDamageableSystem.TryChangeDamage(uid, damage, true);
 
                 Assert.Multiple(() =>
                 {
-                    Assert.That(sDamageableComponent.TotalDamage, Is.EqualTo(damageToDeal));
-                    Assert.That(sDamageableComponent.DamagePerGroup[group3.ID], Is.EqualTo(damageToDeal));
-                    Assert.That(sDamageableComponent.Damage.DamageDict[type3a.ID], Is.EqualTo(damageToDeal / types.Count));
-                    Assert.That(sDamageableComponent.Damage.DamageDict[type3b.ID], Is.EqualTo(damageToDeal / types.Count));
-
-                    // last one will get 0.01 less, since its not perfectly divisble by 3
-                    Assert.That(sDamageableComponent.Damage.DamageDict[type3c.ID], Is.EqualTo(damageToDeal / types.Count - 0.01));
+                    Assert.That(sDamageableComponent.TotalDamage, Is.EqualTo(FixedPoint2.New(14)));
+                    Assert.That(sDamageableComponent.DamagePerGroup[group3.ID], Is.EqualTo(FixedPoint2.New(14)));
+                    Assert.That(sDamageableComponent.Damage.DamageDict[type3a.ID], Is.EqualTo(FixedPoint2.New(4.66f)));
+                    Assert.That(sDamageableComponent.Damage.DamageDict[type3b.ID], Is.EqualTo(FixedPoint2.New(4.67f)));
+                    Assert.That(sDamageableComponent.Damage.DamageDict[type3c.ID], Is.EqualTo(FixedPoint2.New(4.67f)));
                 });
 
                 // Heal
@@ -224,7 +224,7 @@ await server.WaitAssertion(() =>
 
                 Assert.Multiple(() =>
                 {
-                    Assert.That(sDamageableComponent.Damage.DamageDict[type3a.ID], Is.EqualTo(FixedPoint2.New(1.33)));
+                    Assert.That(sDamageableComponent.Damage.DamageDict[type3a.ID], Is.EqualTo(FixedPoint2.New(1.34)));
                     Assert.That(sDamageableComponent.Damage.DamageDict[type3b.ID], Is.EqualTo(FixedPoint2.New(3.33)));
                     Assert.That(sDamageableComponent.Damage.DamageDict[type3c.ID], Is.EqualTo(FixedPoint2.New(0)));
                 });

Content.Shared/Chemistry/Components/Solution.cs

@@ -92,6 +92,12 @@ public bool CanAddSolution(Solution solution)
         /// </summary>
         [ViewVariables] private bool _heatCapacityDirty = true;
 
+        [ViewVariables(VVAccess.ReadWrite)]
+        private int _heatCapacityUpdateCounter;
+
+        // This value is arbitrary btw.
+        private const int HeatCapacityUpdateInterval = 15;
+
         public void UpdateHeatCapacity(IPrototypeManager? protoMan)
         {
             IoCManager.Resolve(ref protoMan);
@@ -103,6 +109,8 @@ public void UpdateHeatCapacity(IPrototypeManager? protoMan)
                 _heatCapacity += (float) quantity *
                                     protoMan.Index<ReagentPrototype>(reagent.Prototype).SpecificHeat;
             }
+
+            _heatCapacityUpdateCounter = 0;
         }
 
         public float GetHeatCapacity(IPrototypeManager? protoMan)
@@ -112,6 +120,15 @@ public float GetHeatCapacity(IPrototypeManager? protoMan)
             return _heatCapacity;
         }
 
+        public void CheckRecalculateHeatCapacity()
+        {
+            // For performance, we have a few ways for heat capacity to get modified without a full recalculation.
+            // To avoid these drifting too much due to float error, we mark it as dirty after N such operations,
+            // so it will be recalculated.
+            if (++_heatCapacityUpdateCounter >= HeatCapacityUpdateInterval)
+                _heatCapacityDirty = true;
+        }
+
         public float GetThermalEnergy(IPrototypeManager? protoMan)
         {
             return GetHeatCapacity(protoMan) * Temperature;
@@ -165,6 +182,7 @@ public Solution(Solution solution)
             Temperature = solution.Temperature;
             _heatCapacity = solution._heatCapacity;
             _heatCapacityDirty = solution._heatCapacityDirty;
+            _heatCapacityUpdateCounter = solution._heatCapacityUpdateCounter;
             ValidateSolution();
         }
 
@@ -193,7 +211,7 @@ public void ValidateSolution()
                 var cur = _heatCapacity;
                 _heatCapacityDirty = true;
                 UpdateHeatCapacity(null);
-                DebugTools.Assert(MathHelper.CloseTo(_heatCapacity, cur));
+                DebugTools.Assert(MathHelper.CloseTo(_heatCapacity, cur, tolerance: 0.01));
             }
     #endif
         }
@@ -380,7 +398,9 @@ public void AddReagent(ReagentId id, FixedPoint2 quantity, bool dirtyHeatCap = t
         public void AddReagent(ReagentPrototype proto, ReagentId reagentId, FixedPoint2 quantity)
         {
             AddReagent(reagentId, quantity, false);
+
             _heatCapacity += quantity.Float() * proto.SpecificHeat;
+            CheckRecalculateHeatCapacity();
         }
 
         public void AddReagent(ReagentQuantity reagentQuantity)
@@ -419,6 +439,7 @@ public void ScaleSolution(int scale)
 
             _heatCapacity *= scale;
             Volume *= scale;
+            CheckRecalculateHeatCapacity();
 
             for (int i = 0; i < Contents.Count; i++)
             {
@@ -752,6 +773,7 @@ public void AddSolution(Solution otherSolution, IPrototypeManager? protoMan)
             }
 
             _heatCapacity += otherSolution._heatCapacity;
+            CheckRecalculateHeatCapacity();
             if (closeTemps)
                 _heatCapacityDirty |= otherSolution._heatCapacityDirty;
             else

Content.Shared/FixedPoint/FixedPoint2.cs

@@ -20,6 +20,9 @@ public struct FixedPoint2 : ISelfSerialize, IComparable<FixedPoint2>, IEquatable
         public static FixedPoint2 Epsilon { get; } = new(1);
         public static FixedPoint2 Zero { get; } = new(0);
 
+        // This value isn't picked by any proper testing, don't @ me.
+        private const float FloatEpsilon = 0.00001f;
+
 #if DEBUG
         static FixedPoint2()
         {
@@ -47,7 +50,17 @@ public static FixedPoint2 New(int value)
 
         public static FixedPoint2 New(float value)
         {
-            return new((int) MathF.Round(value * ShiftConstant, MidpointRounding.AwayFromZero));
+            return new((int) ApplyFloatEpsilon(value * ShiftConstant));
+        }
+
+        private static float ApplyFloatEpsilon(float value)
+        {
+            return value + FloatEpsilon * Math.Sign(value);
+        }
+
+        private static double ApplyFloatEpsilon(double value)
+        {
+            return value + FloatEpsilon * Math.Sign(value);
         }
 
         /// <summary>
@@ -60,17 +73,12 @@ public static FixedPoint2 NewCeiling(float value)
 
         public static FixedPoint2 New(double value)
         {
-            return new((int) Math.Round(value * ShiftConstant, MidpointRounding.AwayFromZero));
+            return new((int) ApplyFloatEpsilon(value * ShiftConstant));
         }
 
         public static FixedPoint2 New(string value)
         {
-            return New(FloatFromString(value));
-        }
-
-        private static float FloatFromString(string value)
-        {
-            return float.Parse(value, CultureInfo.InvariantCulture);
+            return New(Parse.Float(value));
         }
 
         public static FixedPoint2 operator +(FixedPoint2 a) => a;
@@ -85,17 +93,17 @@ private static float FloatFromString(string value)
 
         public static FixedPoint2 operator *(FixedPoint2 a, FixedPoint2 b)
         {
-            return new((int) MathF.Round(b.Value * a.Value / (float) ShiftConstant, MidpointRounding.AwayFromZero));
+            return new(b.Value * a.Value / ShiftConstant);
         }
 
         public static FixedPoint2 operator *(FixedPoint2 a, float b)
         {
-            return new((int) MathF.Round(a.Value * b, MidpointRounding.AwayFromZero));
+            return new((int) ApplyFloatEpsilon(a.Value * b));
         }
 
         public static FixedPoint2 operator *(FixedPoint2 a, double b)
         {
-            return new((int) Math.Round(a.Value * b, MidpointRounding.AwayFromZero));
+            return new((int) ApplyFloatEpsilon(a.Value * b));
         }
 
         public static FixedPoint2 operator *(FixedPoint2 a, int b)
@@ -105,12 +113,12 @@ private static float FloatFromString(string value)
 
         public static FixedPoint2 operator /(FixedPoint2 a, FixedPoint2 b)
         {
-            return new((int) MathF.Round((ShiftConstant * a.Value) / (float) b.Value, MidpointRounding.AwayFromZero));
+            return new((int) (ShiftConstant * (long) a.Value / b.Value));
         }
 
         public static FixedPoint2 operator /(FixedPoint2 a, float b)
         {
-            return new((int) MathF.Round(a.Value / b, MidpointRounding.AwayFromZero));
+            return new((int) ApplyFloatEpsilon(a.Value / b));
         }
 
         public static bool operator <=(FixedPoint2 a, int b)
@@ -185,7 +193,7 @@ public readonly double Double()
 
         public readonly int Int()
         {
-            return (int) ShiftDown();
+            return Value / ShiftConstant;
         }
 
         // Implicit operators ftw
@@ -266,7 +274,7 @@ public void Deserialize(string value)
             if (value == "MaxValue")
                 Value = int.MaxValue;
             else
-                this = New(FloatFromString(value));
+                this = New(Parse.Float(value));
         }
 
         public override readonly string ToString() => $"{ShiftDown().ToString(CultureInfo.InvariantCulture)}";