Kotlin: true solar time demo

demo/kotlin/correct/solar_time.txt

@@ -0,0 +1 @@
+True solar time = 11.6286 hours (11:37:42.979)

demo/kotlin/demotest

@@ -17,6 +17,7 @@ rm -rf build test
 mkdir -p test
 ./gradlew jar || Fail "Error building Kotlin demo application."
 
+TestDemo solar_time +38.88 -77.03 2023-02-12T17:00:00Z
 TestDemo constellation 2021-06-01T00:00:00Z
 TestDemo jupiter_moons 2021-04-16T00:26:18Z
 TestDemo lunar_eclipse 1988-01-01T00:00:00Z

demo/kotlin/demotest.bat

@@ -12,6 +12,7 @@ call gradlew.bat jar || exit /b 1
 
 REM ----------------------------------------------------------------------------------
 
+call :TestDemo solar_time +38.88 -77.03 2023-02-12T17:00:00Z || exit /b 1
 call :TestDemo constellation 2021-06-01T00:00:00Z || exit /b 1
 call :TestDemo jupiter_moons 2021-04-16T00:26:18Z || exit /b 1
 call :TestDemo lunar_eclipse 1988-01-01T00:00:00Z || exit /b 1

demo/kotlin/src/main/kotlin/Main.kt

@@ -52,6 +52,10 @@ Command line arguments:
         Given an integer year number, displays the solstices and
         equinoxes for that year. The year must be in the range 0000..9999.
 
+    solar_time latitude longitude [yyyy-mm-ddThh:mm:ssZ]
+        Displays the true solar time for the observer at the
+        given geographic coordinates.
+
 """
 
 private fun printUsage(): Int {
@@ -172,5 +176,11 @@ internal val demoList = listOf(
         `Seasons demo`(
             parseYear(args[1])
         )
-    }
+    },
+    Demo("solar_time", 3, 4) { args ->
+        `Solar true time`(
+            parseObserver(args, 1),
+            parseTime(args, 3)
+        )
+    },
 )

demo/kotlin/src/main/kotlin/SolarTime.kt

@@ -0,0 +1,29 @@
+import io.github.cosinekitty.astronomy.*
+import kotlin.math.roundToInt
+
+/**
+ * Calculate true solar time for a given observer and UTC time.
+ *
+ * @param observer
+ * The geographic location for which to calculate apparent celestial body positions.
+ *
+ * @param time
+ * The date and time for which to calculate positions.
+ */
+internal fun `Solar true time`(observer: Observer, time: Time): Int {
+    val ha = hourAngle(Body.Sun, time, observer)
+    val solarTimeHours = (ha + 12.0) % 24.0
+    var milli = (solarTimeHours * 3.6e+6).roundToInt()
+    var second = milli / 1000
+    milli %= 1000
+    var minute = second / 60
+    second %= 60
+    var hour = minute / 60
+    minute %= 60
+    hour %= 24
+
+    println("True solar time = %7.4f hours (%02d:%02d:%02d.%03d)".format(
+        solarTimeHours, hour, minute, second, milli))
+
+    return 0
+}