From fae9b35aaac5b50fbb87dc689e5260fd70c0ecf4 Mon Sep 17 00:00:00 2001
From: ArtisticRoomba <145879011+ArtisticRoomba@users.noreply.github.com>
Date: Mon, 2 Feb 2026 01:31:59 -0800
Subject: [PATCH] Clarify documentation on Atmospherics heat capacity APIs
 (#42747)

* clarify heat capacity APIs

* more docs
---
 Content.Shared/Atmos/Atmospherics.cs          |  4 ++-
 .../SharedAtmosphereSystem.Gases.cs           | 35 +++++++++++++++----
 Content.Shared/CCVar/CCVars.Atmos.cs          | 13 +++++--
 3 files changed, 42 insertions(+), 10 deletions(-)

diff --git a/Content.Shared/Atmos/Atmospherics.cs b/Content.Shared/Atmos/Atmospherics.cs
index 4f5c0939ea5..4e64b6b4d18 100644
--- a/Content.Shared/Atmos/Atmospherics.cs
+++ b/Content.Shared/Atmos/Atmospherics.cs
@@ -159,7 +159,9 @@ namespace Content.Shared.Atmos
         public const float MinimumHeatCapacity = 0.0003f;
 
         /// <summary>
-        ///     For the purposes of making space "colder"
+        /// Allows Atmospherics to cool down rooms during spacing
+        /// by assigning a fake heat capacity to space,
+        /// making space "actually cold" for gameplay reasons.
         /// </summary>
         public const float SpaceHeatCapacity = 7000f;
 
diff --git a/Content.Shared/Atmos/EntitySystems/SharedAtmosphereSystem.Gases.cs b/Content.Shared/Atmos/EntitySystems/SharedAtmosphereSystem.Gases.cs
index 956c1aa8279..98472b7ca27 100644
--- a/Content.Shared/Atmos/EntitySystems/SharedAtmosphereSystem.Gases.cs
+++ b/Content.Shared/Atmos/EntitySystems/SharedAtmosphereSystem.Gases.cs
@@ -1,5 +1,7 @@
 using System.Runtime.CompilerServices;
 using Content.Shared.Atmos.Prototypes;
+using Content.Shared.CCVar;
+using JetBrains.Annotations;
 
 namespace Content.Shared.Atmos.EntitySystems;
 
@@ -40,17 +42,27 @@ public abstract partial class SharedAtmosphereSystem
 
         for (var i = 0; i < GasPrototypes.Length; i++)
         {
+            /*
+             As an optimization routine we pre-divide the specific heat by the heat scale here,
+             so we don't have to do it every time we calculate heat capacity.
+             Most usages are going to want the scaled value anyway.
+
+             If you would like the unscaled specific heat, you'd need to multiply by HeatScale again.
+             TODO ATMOS: please just make this 2 separate arrays instead of invoking multiplication every time.
+             */
             _gasSpecificHeats[i] = GasPrototypes[i].SpecificHeat / HeatScale;
         }
     }
 
     /// <summary>
-    ///     Calculates the heat capacity for a gas mixture.
+    /// Calculates the heat capacity for a <see cref="GasMixture"/>.
     /// </summary>
-    /// <param name="mixture">The mixture whose heat capacity should be calculated</param>
-    /// <param name="applyScaling"> Whether the internal heat capacity scaling should be applied. This should not be
-    /// used outside of atmospheric related heat transfer.</param>
-    /// <returns></returns>
+    /// <param name="mixture">The <see cref="GasMixture"/> to calculate the heat capacity for.</param>
+    /// <param name="applyScaling">Whether to apply the heat capacity scaling factor.
+    /// This is an extremely important boolean to consider or else you will get heat transfer wrong.
+    /// See <see cref="CCVars.AtmosHeatScale"/> for more info.</param>
+    /// <returns>The heat capacity of the <see cref="GasMixture"/>.</returns>
+    [PublicAPI]
     public float GetHeatCapacity(GasMixture mixture, bool applyScaling)
     {
         var scale = GetHeatCapacityCalculation(mixture.Moles, mixture.Immutable);
@@ -60,6 +72,14 @@ public abstract partial class SharedAtmosphereSystem
         return applyScaling ? scale : scale * HeatScale;
     }
 
+    /// <summary>
+    /// Gets the heat capacity for a <see cref="GasMixture"/>.
+    /// </summary>
+    /// <param name="mixture">The <see cref="GasMixture"/> to calculate the heat capacity for.</param>
+    /// <returns>The heat capacity of the <see cref="GasMixture"/>.</returns>
+    /// <remarks>Note that the heat capacity of the mixture may be slightly different from
+    /// "real life" as we intentionally fake a heat capacity for space in <see cref="Atmospherics.SpaceHeatCapacity"/>
+    /// in order to allow Atmospherics to cool down space.</remarks>
     protected float GetHeatCapacity(GasMixture mixture)
     {
         return GetHeatCapacityCalculation(mixture.Moles, mixture.Immutable);
@@ -70,8 +90,9 @@ public abstract partial class SharedAtmosphereSystem
     /// </summary>
     /// <param name="moles">The moles array of the <see cref="GasMixture"/></param>
     /// <param name="space">Whether this <see cref="GasMixture"/> represents space,
-    /// and thus experiences space-specific mechanics (we cheat and make it a bit cooler).</param>
-    /// <returns></returns>
+    /// and thus experiences space-specific mechanics (we cheat and make it a bit cooler).
+    /// See <see cref="Atmospherics.SpaceHeatCapacity"/>.</param>
+    /// <returns>The heat capacity of the <see cref="GasMixture"/>.</returns>
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     protected abstract float GetHeatCapacityCalculation(float[] moles, bool space);
 }
diff --git a/Content.Shared/CCVar/CCVars.Atmos.cs b/Content.Shared/CCVar/CCVars.Atmos.cs
index 9b3ef5388f2..833938b765f 100644
--- a/Content.Shared/CCVar/CCVars.Atmos.cs
+++ b/Content.Shared/CCVar/CCVars.Atmos.cs
@@ -138,9 +138,18 @@ public sealed partial class CCVars
         CVarDef.Create("atmos.speedup", 8f, CVar.SERVERONLY);
 
     /// <summary>
-    ///     Like atmos.speedup, but only for gas and reaction heat values. 64x means
-    ///     gases heat up and cool down 64x faster than real life.
+    /// Like atmos.speedup, but only for gas and reaction heat values. 64x means
+    /// gases heat up and cool down 64x faster than real life.
     /// </summary>
+    /// <remarks><para>This is only supposed to facilitate the speedup of
+    /// heat transfer between bodies by making the heat capacities appear smaller.
+    /// In the context of a heat-only reaction like gas reactions creating heat,
+    /// you shouldn't be using the scaled specific heat.</para>
+    /// <para>So things like thermomachines and the TEG, which work based on heat transfer, would need to use the scaled value,
+    /// whereas heat-only reactions like gas reactions and anything else that "generates" or "consumes" heat would use the unscaled value.</para>
+    /// </remarks>
+    /// <example>Mechanics like the thermomachine or TEG should be affected by this CVAR change,
+    /// whereas gas reactions and heat-only interactions should stay the same.</example>
     public static readonly CVarDef<float> AtmosHeatScale =
         CVarDef.Create("atmos.heat_scale", 8f, CVar.REPLICATED | CVar.SERVER);