C: Added Astronomy_VectorFromSphere, astro_spherical_t.

Added new data type astro_spherical_t that represents generic spherical coordinates.
Implemented Astronomy_VectorFromSphere to convert spherical coordinates
to Cartesian coordinates. Included unit test to verify it is working as expected.

generate/ctest.c

@@ -1835,11 +1835,61 @@ fail:
     return error;
 }
 
+static int TestVectorFromAngles(double lat, double lon, double x, double y, double z)
+{
+    astro_spherical_t sphere;
+    astro_vector_t vector;
+    astro_time_t time;
+    double diff, dx, dy, dz;
+
+    /* Confirm the expected vector really is a unit vector. */
+    diff = fabs((x*x + y*y + z*z) - 1.0);
+    if (diff > 1.0e-16)
+    {
+        fprintf(stderr, "TestVectorFromAngles: EXCESSIVE unit error = %lg\n", diff);
+        return 1;
+    }
+
+    sphere.status = ASTRO_SUCCESS;
+    sphere.lat = lat;
+    sphere.lon = lon;
+    sphere.dist = 1.0;
+
+    time = Astronomy_MakeTime(2015, 3, 5, 12, 0, 0.0);
+    vector = Astronomy_VectorFromSphere(sphere, time);
+
+    if (vector.status != ASTRO_SUCCESS)
+    {
+        fprintf(stderr, "ERROR(TestVectorFromAngles): vector.status = %d\n", vector.status);
+        return 1;
+    }
+
+    dx = x - vector.x;
+    dy = y - vector.y;
+    dz = z - vector.z;
+    diff = sqrt(dx*dx + dy*dy + dz*dz);
+
+    printf("TestVectorFromAngles(%lf, %lf): diff = %lg\n", lat, lon, diff);
+    if (diff > 2.0e-16)
+    {
+        fprintf(stderr, "TestVectorFromAngles: EXCESSIVE ERROR.\n");
+        return 1;
+    }
+    return 0;
+}
+
 static int RotationTest(void)
 {
     int error;
     CHECK(Rotation_MatrixInverse());
     CHECK(Rotation_MatrixMultiply());
+    CHECK(TestVectorFromAngles(0.0, 0.0, 1.0, 0.0, 0.0));
+    CHECK(TestVectorFromAngles(0.0, 90.0, 0.0, 1.0, 0.0));
+    CHECK(TestVectorFromAngles(0.0, 180.0, -1.0, 0.0, 0.0));
+    CHECK(TestVectorFromAngles(0.0, 270.0, 0.0, -1.0, 0.0));
+    CHECK(TestVectorFromAngles(+90.0, 0.0, 0.0, 0.0, 1.0));
+    CHECK(TestVectorFromAngles(-90.0, 0.0, 0.0, 0.0, -1.0));
+    CHECK(TestVectorFromAngles(-30.0, +60.0, 0.43301270189221946, 0.75, -0.5));
     error = 0;
 fail:
     return error;

generate/template/astronomy.c

@@ -4143,6 +4143,44 @@ astro_rotation_t Astronomy_CombineRotation(astro_rotation_t a, astro_rotation_t
     return c;
 }
 
+/**
+ * @brief Converts spherical coordinates to Cartesian coordinates.
+ *
+ * Given spherical coordinates and a time at which they are valid,
+ * returns a vector of Cartesian coordinates. The returned value
+ * includes the time, as required by the type #astro_vector_t.
+ *
+ * @param sphere
+ *      Spherical coordinates to be converted.
+ *
+ * @param time
+ *      The time that should be included in the return value.
+ *
+ * @return
+ *      The vector form of the supplied spherical coordinates.
+ */
+astro_vector_t Astronomy_VectorFromSphere(astro_spherical_t sphere, astro_time_t time)
+{
+    astro_vector_t vector;
+    double radlat, radlon, rcoslat;
+
+    if (sphere.status != ASTRO_SUCCESS)
+        return VecError(ASTRO_INVALID_PARAMETER, time);
+
+    radlat = sphere.lat * DEG2RAD;
+    radlon = sphere.lon * DEG2RAD;
+    rcoslat = sphere.dist * cos(radlat);
+
+    vector.status = ASTRO_SUCCESS;
+    vector.t = time;
+    vector.x = rcoslat * cos(radlon);
+    vector.y = rcoslat * sin(radlon);
+    vector.z = sphere.dist * sin(radlat);
+
+    return vector;
+}
+
+
 #if 0
 /**
  * @brief

source/c/README.md

@@ -1136,6 +1136,31 @@ After calculating the date and time of an astronomical event in the form of an [
 
 
 
+---
+
+<a name="Astronomy_VectorFromSphere"></a>
+### Astronomy_VectorFromSphere(sphere, time) &#8658; [`astro_vector_t`](#astro_vector_t)
+
+**Converts spherical coordinates to Cartesian coordinates.** 
+
+
+
+Given spherical coordinates and a time at which they are valid, returns a vector of Cartesian coordinates. The returned value includes the time, as required by the type [`astro_vector_t`](#astro_vector_t).
+
+
+
+**Returns:**  The vector form of the supplied spherical coordinates. 
+
+
+
+| Type | Parameter | Description |
+| --- | --- | --- |
+| [`astro_spherical_t`](#astro_spherical_t) | `sphere` |  Spherical coordinates to be converted. | 
+| [`astro_time_t`](#astro_time_t) | `time` |  The time that should be included in the return value. | 
+
+
+
+
 ---
 
 <a name="Astronomy_VectorLength"></a>
@@ -1588,6 +1613,23 @@ You can create this structure directly, or you can call the convenience function
 | [`astro_time_t`](#astro_time_t) | `dec_solstice` |  The date and time of the December solstice for the specified year.  |
 
 
+---
+
+<a name="astro_spherical_t"></a>
+### `astro_spherical_t`
+
+**Spherical coordinates: latitude, longitude, distance.** 
+
+
+
+| Type | Member | Description |
+| ---- | ------ | ----------- |
+| [`astro_status_t`](#astro_status_t) | `status` |  ASTRO_SUCCESS if this struct is valid; otherwise an error code.  |
+| `double` | `lat` |  The latitude angle: -90..+90 degrees.  |
+| `double` | `lon` |  The longitude angle: 0..360 degrees.  |
+| `double` | `dist` |  Distance in AU.  |
+
+
 ---
 
 <a name="astro_time_t"></a>

source/c/astronomy.c

@@ -5382,6 +5382,44 @@ astro_rotation_t Astronomy_CombineRotation(astro_rotation_t a, astro_rotation_t
     return c;
 }
 
+/**
+ * @brief Converts spherical coordinates to Cartesian coordinates.
+ *
+ * Given spherical coordinates and a time at which they are valid,
+ * returns a vector of Cartesian coordinates. The returned value
+ * includes the time, as required by the type #astro_vector_t.
+ *
+ * @param sphere
+ *      Spherical coordinates to be converted.
+ *
+ * @param time
+ *      The time that should be included in the return value.
+ *
+ * @return
+ *      The vector form of the supplied spherical coordinates.
+ */
+astro_vector_t Astronomy_VectorFromSphere(astro_spherical_t sphere, astro_time_t time)
+{
+    astro_vector_t vector;
+    double radlat, radlon, rcoslat;
+
+    if (sphere.status != ASTRO_SUCCESS)
+        return VecError(ASTRO_INVALID_PARAMETER, time);
+
+    radlat = sphere.lat * DEG2RAD;
+    radlon = sphere.lon * DEG2RAD;
+    rcoslat = sphere.dist * cos(radlat);
+
+    vector.status = ASTRO_SUCCESS;
+    vector.t = time;
+    vector.x = rcoslat * cos(radlon);
+    vector.y = rcoslat * sin(radlon);
+    vector.z = sphere.dist * sin(radlat);
+
+    return vector;
+}
+
+
 #if 0
 /**
  * @brief

source/c/astronomy.h

@@ -170,6 +170,18 @@ typedef struct
 }
 astro_vector_t;
 
+/**
+ * @brief Spherical coordinates: latitude, longitude, distance.
+ */
+typedef struct
+{
+    astro_status_t status;  /**< ASTRO_SUCCESS if this struct is valid; otherwise an error code. */
+    double lat;             /**< The latitude angle: -90..+90 degrees. */
+    double lon;             /**< The longitude angle: 0..360 degrees. */
+    double dist;            /**< Distance in AU. */
+}
+astro_spherical_t;
+
 /**
  * @brief An angular value expressed in degrees.
  */
@@ -602,6 +614,7 @@ astro_apsis_t Astronomy_NextLunarApsis(astro_apsis_t apsis);
 
 astro_rotation_t Astronomy_InverseRotation(astro_rotation_t rotation);
 astro_rotation_t Astronomy_CombineRotation(astro_rotation_t a, astro_rotation_t b);
+astro_vector_t Astronomy_VectorFromSphere(astro_spherical_t sphere, astro_time_t time);
 
 #if 0
 astro_rotation_t Astronomy_Rotation_EQD_EQJ(astro_time_t time);