From 9b3eff2ee85ca6627342cfbbe1ac3ba988dc377f Mon Sep 17 00:00:00 2001
From: Dan Goodliffe <dan@randomdan.homeip.net>
Date: Fri, 20 Sep 2024 20:27:51 +0100
Subject: Move getSunPos to Environment

---
 game/environment.cpp | 67 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 game/environment.h   |  2 ++
 2 files changed, 69 insertions(+)

(limited to 'game')

diff --git a/game/environment.cpp b/game/environment.cpp
index fd2bfd4..665c11b 100644
--- a/game/environment.cpp
+++ b/game/environment.cpp
@@ -16,3 +16,70 @@ Environment::render(const SceneRenderer & renderer, const SceneProvider & scene)
 	renderer.setAmbientLight({0.5F, 0.5F, 0.5F});
 	renderer.setDirectionalLight({0.6F, 0.6F, 0.6F}, {-1, 1, -1}, scene);
 }
+
+// Based on the C++ code published at https://www.psa.es/sdg/sunpos.htm
+// Linked from https://www.pveducation.org/pvcdrom/properties-of-sunlight/suns-position-to-high-accuracy
+Direction2D
+Environment::getSunPos(const Direction2D position, const time_t time)
+{
+	auto & longitude = position.x;
+	auto & latitude = position.y;
+	using std::acos;
+	using std::asin;
+	using std::atan2;
+	using std::cos;
+	using std::floor;
+	using std::sin;
+	using std::tan;
+	static const auto JD2451545 = "2000-01-01T12:00:00"_time_t;
+
+	// Calculate difference in days between the current Julian Day
+	// and JD 2451545.0, which is noon 1 January 2000 Universal Time
+	// Calculate time of the day in UT decimal hours
+	const auto dDecimalHours = static_cast<float>(time % 86400) / 3600.F;
+	const auto dElapsedJulianDays = static_cast<float>(time - JD2451545) / 86400.F;
+
+	// Calculate ecliptic coordinates (ecliptic longitude and obliquity of the
+	// ecliptic in radians but without limiting the angle to be less than 2*Pi
+	// (i.e., the result may be greater than 2*Pi)
+	const auto dOmega = 2.1429F - 0.0010394594F * dElapsedJulianDays;
+	const auto dMeanLongitude = 4.8950630F + 0.017202791698F * dElapsedJulianDays; // Radians
+	const auto dMeanAnomaly = 6.2400600F + 0.0172019699F * dElapsedJulianDays;
+	const auto dEclipticLongitude = dMeanLongitude + 0.03341607F * sin(dMeanAnomaly)
+			+ 0.00034894F * sin(2 * dMeanAnomaly) - 0.0001134F - 0.0000203F * sin(dOmega);
+	const auto dEclipticObliquity = 0.4090928F - 6.2140e-9F * dElapsedJulianDays + 0.0000396F * cos(dOmega);
+
+	// Calculate celestial coordinates ( right ascension and declination ) in radians
+	// but without limiting the angle to be less than 2*Pi (i.e., the result may be
+	// greater than 2*Pi)
+	const auto dSin_EclipticLongitude = sin(dEclipticLongitude);
+	const auto dY = cos(dEclipticObliquity) * dSin_EclipticLongitude;
+	const auto dX = cos(dEclipticLongitude);
+	auto dRightAscension = atan2(dY, dX);
+	if (dRightAscension < 0) {
+		dRightAscension = dRightAscension + two_pi;
+	}
+	const auto dDeclination = asin(sin(dEclipticObliquity) * dSin_EclipticLongitude);
+
+	// Calculate local coordinates ( azimuth and zenith angle ) in degrees
+	const auto dGreenwichMeanSiderealTime = 6.6974243242F + 0.0657098283F * dElapsedJulianDays + dDecimalHours;
+	const auto dLocalMeanSiderealTime
+			= (dGreenwichMeanSiderealTime * 15.0F + (longitude / degreesToRads)) * degreesToRads;
+	const auto dHourAngle = dLocalMeanSiderealTime - dRightAscension;
+	const auto dLatitudeInRadians = latitude;
+	const auto dCos_Latitude = cos(dLatitudeInRadians);
+	const auto dSin_Latitude = sin(dLatitudeInRadians);
+	const auto dCos_HourAngle = cos(dHourAngle);
+	Direction2D udtSunCoordinates;
+	udtSunCoordinates.y
+			= (acos(dCos_Latitude * dCos_HourAngle * cos(dDeclination) + sin(dDeclination) * dSin_Latitude));
+	udtSunCoordinates.x = atan2(-sin(dHourAngle), tan(dDeclination) * dCos_Latitude - dSin_Latitude * dCos_HourAngle);
+	if (udtSunCoordinates.x < 0) {
+		udtSunCoordinates.x = udtSunCoordinates.x + two_pi;
+	}
+	// Parallax Correction
+	const auto dParallax = (earthMeanRadius / astronomicalUnit) * sin(udtSunCoordinates.y);
+	udtSunCoordinates.y = half_pi - (udtSunCoordinates.y + dParallax);
+
+	return udtSunCoordinates;
+}
diff --git a/game/environment.h b/game/environment.h
index 62eedb8..a6f3036 100644
--- a/game/environment.h
+++ b/game/environment.h
@@ -1,5 +1,6 @@
 #pragma once
 
+#include "config/types.h"
 #include "worldobject.h"
 
 class SceneRenderer;
@@ -10,6 +11,7 @@ public:
 	Environment();
 	void tick(TickDuration elapsed) override;
 	void render(const SceneRenderer &, const SceneProvider &) const;
+	static Direction2D getSunPos(const Direction2D position, const time_t time);
 
 private:
 	time_t worldTime;
-- 
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