fix: normalize vreckon output longitude to [-180, 180] range

src/pymap3d/tests/test_vincenty_vreckon.py

@@ -23,11 +23,11 @@
     [
         (0, 0, 0, 0, 0, 0),
         (0, 0, 1.001875e7, 90, 0, 90),
-        (0, 0, 1.001875e7, 270, 0, 270),
-        (0, 0, 1.001875e7, -90, 0, 270),
+        (0, 0, 1.001875e7, 270, 0, -90),
+        (0, 0, 1.001875e7, -90, 0, -90),
         (0, 0, 2.00375e7, 90, 0, 180),
-        (0, 0, 2.00375e7, 270, 0, 180),
-        (0, 0, 2.00375e7, -90, 0, 180),
+        (0, 0, 2.00375e7, 270, 0, -180),
+        (0, 0, 2.00375e7, -90, 0, -180),
     ],
 )
 def test_vreckon_unit(deg, lat, lon, srange, az, lato, lono):
@@ -43,6 +43,15 @@ def test_vreckon_unit(deg, lat, lon, srange, az, lato, lono):
     assert isinstance(lon1, float)
 
 
+def test_negative_lon_stays_negative():
+    """Regression test for issue #88: vreckon with negative start longitude
+    must return a negative (or near-zero) destination longitude, not 358+."""
+    lat2, lon2 = vincenty.vreckon(52.22610277777778, -1.2696583333333333, 839.63, 63.02)
+    assert lat2 == approx(52.22952562862266, rel=1e-6)
+    assert -180 <= lon2 <= 0, f"lon2={lon2} should be negative for a point west of prime meridian"
+    assert lon2 == approx(-1.258707, abs=1e-3)
+
+
 def test_az_vector():
     np = pytest.importorskip("numpy")
     az = np.array(az1)

src/pymap3d/vincenty.py

@@ -433,7 +433,7 @@ def vreckon(
     #    lon2 = pi*((absolute(lon2)/pi) -
     #       2*ceil(((absolute(lon2)/pi)-1)/2)) * sign(lon2)
 
-    lon2 %= tau  # follow [0, 360) convention
+    lon2 = (lon2 + pi) % tau - pi  # normalize to [-180, 180) convention
 
     if deg:
         lat2 = degrees(lat2)