Merge pull request #366 from adrn/reorder-nbody

Fix inefficient nbody orbit reorder

CHANGES.rst

@@ -15,6 +15,8 @@ Bug fixes
   class to enable using the new (correct) parameter values, but the default will
   continue to use the Gala modified values (for backwards compatibility).
 
+- Improved internal efficiency of ``DirectNBody``.
+
 
 API changes
 -----------

gala/dynamics/nbody/core.py

@@ -259,13 +259,7 @@ def integrate_orbit(self, Integrator=None, Integrator_kwargs=dict(), **time_spec
                 frame=self.frame,
             )
 
-        # Reorder orbits:
-        remap_idx = np.zeros((orbits.shape[-1], orbits.shape[-1]), dtype=int)
-        remap_idx[idx, np.arange(orbits.shape[-1])] = 1
-        _, undo_idx = np.where(remap_idx == 1)
-
-        return orbits[..., undo_idx]
-        remap_idx[idx, np.arange(orbits.shape[-1])] = 1
-        _, undo_idx = np.where(remap_idx == 1)
+        # Reorder orbits to original order:
+        undo_idx = np.argsort(idx)
 
         return orbits[..., undo_idx]

gala/dynamics/nbody/tests/test_nbody.py

@@ -3,22 +3,23 @@
 import numpy as np
 import pytest
 
-# Custom
-from gala.potential import (
-    NullPotential,
-    NFWPotential,
-    HernquistPotential,
-    ConstantRotatingFrame,
-    StaticFrame,
-)
 from gala.dynamics import PhaseSpacePosition, combine
-from gala.units import UnitSystem, galactic
 from gala.integrate import (
     DOPRI853Integrator,
     LeapfrogIntegrator,
     Ruth4Integrator,
 )
 
+# Custom
+from gala.potential import (
+    ConstantRotatingFrame,
+    HernquistPotential,
+    NFWPotential,
+    NullPotential,
+    StaticFrame,
+)
+from gala.units import UnitSystem, galactic
+
 # Project
 from ..core import DirectNBody
 
@@ -220,3 +221,28 @@ def test_directnbody_integrate_rotframe(self, Integrator):
 
         assert u.allclose(orbits_static.xyz, orbits_static.xyz)
         assert u.allclose(orbits2.v_xyz, orbits2.v_xyz)
+
+    @pytest.mark.parametrize("Integrator", [DOPRI853Integrator])
+    def test_nbody_reorder(self, Integrator):
+        N = 16
+        rng = np.random.default_rng(seed=42)
+        w0 = PhaseSpacePosition(
+            pos=rng.normal(0, 5, size=(3, N)) * u.kpc,
+            vel=rng.normal(0, 50, size=(3, N)) * u.km / u.s,
+        )
+        pots = [
+            (
+                HernquistPotential(1e9 * u.Msun, 1.0 * u.pc, units=galactic)
+                if rng.uniform() > 0.5
+                else None
+            )
+            for _ in range(N)
+        ]
+        sim = DirectNBody(
+            w0,
+            pots,
+            external_potential=HernquistPotential(1e12, 10, units=galactic),
+            units=galactic,
+        )
+        orbits = sim.integrate_orbit(dt=1.0 * u.Myr, t1=0, t2=100 * u.Myr)
+        assert np.allclose(orbits.pos[0].xyz, w0.pos.xyz)