qbsp mathlib: add helper for getting angle between vectors

qbsp/mathlib.cc

@@ -139,3 +139,26 @@ VectorScale(const vec3_t v, const vec_t scale, vec3_t out)
     out[1] = v[1] * scale;
     out[2] = v[2] * scale;
 }
+
+/// Returns degrees of clockwise rotation from start to end, assuming `normal` is pointing towards the viewer
+float SignedDegreesBetweenUnitVectors(const vec3_t start, const vec3_t end, const vec3_t normal)
+{
+    const float cosangle = qmax(-1.0, qmin(1.0, DotProduct(start, end)));
+    const float unsigned_degrees = acos(cosangle) * (360.0 / (2.0 * Q_PI));
+    
+    if (unsigned_degrees < ANGLEEPSILON)
+        return 0;
+    
+    // get a normal for the rotation plane using the right-hand rule
+    vec3_t rotationNormal;
+    CrossProduct(start, end, rotationNormal);
+    VectorNormalize(rotationNormal);
+    
+    const float normalsCosAngle = DotProduct(rotationNormal, normal);
+    if (normalsCosAngle >= 0) {
+        // counterclockwise rotation
+        return -unsigned_degrees;
+    }
+    // clockwise rotation
+    return unsigned_degrees;
+}

qbsp/qbsp.hh

@@ -235,6 +235,8 @@ vec_t VectorNormalize(vec3_t v);
 void VectorInverse(vec3_t v);
 void VectorScale(const vec3_t v, const vec_t scale, vec3_t out);
 
+float SignedDegreesBetweenUnitVectors(const vec3_t start, const vec3_t end, const vec3_t normal);
+
 #ifdef __GNUC__
 /* min and max macros with type checking */
 #define qmax(a,b) ({      \