Add function to calculate surface gravity using fixed point

Author: sam52796

src/galaxy/StarSystemGenerator.cpp

@@ -1481,7 +1481,7 @@ static fixed CalculateHabitability(SystemBody *sbody)
 	// Surface gravity - mean at 7.5 with standard deviation of 7.5, so slightly less than Earth gravity is a
 	// pleasure to work in, zero gravity is still habitable, but has a penalty, while heavier than Earth gravity
 	// gets progressively worse. Multiply the result a small amount so that Earth itself is back up to 1.0
-	const fixed gravity = fixed::FromDouble(sbody->CalcSurfaceGravity());
+	const fixed gravity = sbody->CalcSurfaceGravityAsFixed();
 	habitability *= FalloffFunction(gravity, fixed(15, 2), fixed(15, 2)) * fixed(106, 100);
 
 	// If the entire planet is covered in ice (ice coverage = 1) then that's hugely inconvenient for industry and
@@ -1502,20 +1502,21 @@ static fixed CalculateHabitability(SystemBody *sbody)
 	// too heavily. Fundamentally, if there is very low air pressure, then the temperature and composition don't matter.
 	// So first check air pressure, and if it's below a threshold then skip over the other two.
 
-	// Atmospheric pressure. 1.0 is Earth atmosphere density, so that's our baseline, with medium fall-off.
-	const fixed pressure = fixed::FromDouble(sbody->GetAtmSurfacePressure());
-	habitability *= FalloffFunction(pressure, fixed(1, 1), fixed(9, 10));
+	// For atmospheric pressure, we'll use the gas density, since it's already in fixed point and is good enough for this
+	// rough habitability calculation. 1.225 is Earth atmosphere density, so that's our baseline, with medium fall-off.
+	const fixed density = sbody->GetVolatileGasAsFixed();
+	habitability *= FalloffFunction(density, fixed(1225, 1000), fixed(11, 10));
 
-	if (pressure > 0.5) {
+	if (density > fixed(6, 10)) {
 		// Temperature is in Kelvin. Average temperature on Earth's service is 15 Celsius, so 273+15 is our mean.
 		// Humans live at extremes on Earth, plus we're dealing with the future, so 50 for a standard deviation seems
 		// reasonable, though narrow in astrophysical terms
-		const fixed temperature = fixed::FromDouble(sbody->GetAverageTemp());
+		const fixed temperature = fixed(sbody->GetAverageTemp(), 1);
 		habitability *= FalloffFunction(temperature, fixed(288, 1), fixed(50, 1));
 
 		// Breathable air is super convenient, while air that actively kills you is even worse than a vacuum. So let's heavily
 		// favour oxidising atmospheres over reducing onces.
-		const fixed air_type = fixed::FromDouble(sbody->GetAtmosOxidizing());
+		const fixed air_type = sbody->GetAtmosOxidizingAsFixed();
 		habitability *= FalloffFunction(air_type, fixed(1, 1), fixed(3, 10));
 	}
 

src/galaxy/SystemBody.cpp

@@ -969,6 +969,17 @@ double SystemBody::CalcSurfaceGravity() const
 	}
 }
 
+fixed SystemBody::CalcSurfaceGravityAsFixed() const
+{
+	if (m_radius <= fixed(0, 1))
+		return fixed(0, 1);
+	// m_radius and m_mass are already in native body units
+	// (Earth radius/mass for planets, Sol radius/mass for stars)
+	// so we don't need big G here - just multiply by the relevant value of g.
+	const fixed unitG = (GetSuperType() == SUPERTYPE_STAR) ? G_SOL : G_EARTH;
+	return unitG * ((m_mass / m_radius) / m_radius);
+}
+
 double SystemBody::CalcEscapeVelocity() const
 {
 	double r = GetRadius();

src/galaxy/SystemBody.h

@@ -14,6 +14,9 @@
 #include "galaxy/SystemPath.h"
 #include "gameconsts.h"
 
+static const fixed G_EARTH = fixed(980665, 100000); // 9.80665 m/s^2
+static const fixed G_SOL = fixed(2742, 10);	  // 274.2 m/s^2
+
 class StarSystem;
 
 struct AtmosphereParameters;
@@ -261,6 +264,7 @@ class SystemBody : public RefCounted, public SystemBodyType, protected SystemBod
 	double GetVolatileIces() const { return m_volatileIces.ToDouble(); }
 	fixed GetVolcanicityAsFixed() const { return m_volcanicity; }
 	double GetVolcanicity() const { return m_volcanicity.ToDouble(); }
+	fixed GetAtmosOxidizingAsFixed() const { return m_atmosOxidizing; }
 	double GetAtmosOxidizing() const { return m_atmosOxidizing.ToDouble(); }
 	fixed GetLifeAsFixed() const { return m_life; }
 	double GetLife() const { return m_life.ToDouble(); }
@@ -270,6 +274,7 @@ class SystemBody : public RefCounted, public SystemBodyType, protected SystemBod
 	fixed GetPopulationAsFixed() const { return m_population; }
 	double GetPopulation() const { return m_population.ToDouble(); }
 
+	fixed CalcSurfaceGravityAsFixed() const;
 	double CalcSurfaceGravity() const;
 	double CalcEscapeVelocity() const;
 	double CalcMeanDensity() const;