From cc59145fc960951f2aecc8fff83ee135a5b60b9b Mon Sep 17 00:00:00 2001
From: Sylvester Kaczmarek
 <16242628+sylvesterkaczmarek@users.noreply.github.com>
Date: Sat, 19 Sep 2026 09:52:04 +0100
Subject: [PATCH] Extract single-particle likelihoods explicitly in Linker

---
 ssapy/linker.py                          |  2 +-
 tests/test_linker_particle_likelihood.py | 61 ++++++++++++++++++++++++
 2 files changed, 62 insertions(+), 1 deletion(-)
 create mode 100644 tests/test_linker_particle_likelihood.py

diff --git a/ssapy/linker.py b/ssapy/linker.py
index d5ea4f2a..7856b32f 100644
--- a/ssapy/linker.py
+++ b/ssapy/linker.py
@@ -184,7 +184,7 @@ def sample_orbit_selectors_from_data_conditional(self, track_ndx, verbose=True):
         for j in range(n):
             # FIXME: Add prior on orbit model parameters here?
             theta = self.iods[j].draw_orbit()
-            lnL[j] = lnlike(theta)
+            lnL[j] = np.asarray(lnlike(theta)).item()
 
         if verbose:
             print("lnL:", lnL)
diff --git a/tests/test_linker_particle_likelihood.py b/tests/test_linker_particle_likelihood.py
new file mode 100644
index 00000000..1ba53c7d
--- /dev/null
+++ b/tests/test_linker_particle_likelihood.py
@@ -0,0 +1,61 @@
+import numpy as np
+import pytest
+
+from ssapy.linker import Linker
+from ssapy.particles import Particles
+
+
+class _Probability:
+    epoch = 0.0
+
+    def __init__(self, likelihoods):
+        self.likelihoods = dict(zip([7.0e6, 8.0e6], likelihoods))
+
+    def lnprior(self, orbit):
+        return 0.0
+
+    def lnlike(self, orbit):
+        return self.likelihoods[orbit.r[0]]
+
+
+@pytest.mark.filterwarnings(
+    "error:Conversion of an array with ndim > 0 to a scalar is deprecated:DeprecationWarning"
+)
+@pytest.mark.parametrize("track_index", [0, 1])
+@pytest.mark.parametrize("likelihoods", [(-1.0, -2.0), (-1000.0, -1001.0)])
+def test_selector_uses_single_likelihood_from_real_particles(
+    monkeypatch, track_index, likelihoods
+):
+    probability = _Probability(likelihoods)
+    populations = [
+        Particles(
+            np.array([[radius, 0.0, 0.0, 0.0, 7500.0, 0.0]]),
+            probability,
+        )
+        for radius in [7.0e6, 8.0e6]
+    ]
+    linker = Linker(populations)
+    linker.p_orbit[1] = [0.25, 0.75]
+    captured = {}
+
+    def multinomial(n, pvals):
+        captured["n"] = n
+        captured["pvals"] = pvals.copy()
+        selected = np.zeros(len(pvals), dtype=int)
+        selected[-1] = 1
+        return selected
+
+    monkeypatch.setattr(np.random, "multinomial", multinomial)
+    selected = linker.sample_orbit_selectors_from_data_conditional(
+        track_index, verbose=False
+    )
+
+    count = track_index + 1
+    if max(likelihoods[:count]) < -500.0:
+        expected = np.full(count, 1.0 / count)
+    else:
+        expected = np.exp(likelihoods[:count]) * linker.p_orbit[track_index]
+        expected /= expected.sum()
+    assert captured["n"] == 1
+    np.testing.assert_allclose(captured["pvals"], expected)
+    np.testing.assert_array_equal(selected, np.eye(count, dtype=int)[-1])