Added the switching term to GOSPA.

[Carlson-J]

Fixes issue described in #897

Adds the switching term to the GOSPA as outlined in #876

[sdhiscocks]

Thanks @Carlson-J. Overall looks good.

My main suggestion, is it'd probably be cleaner to use None and empty dict, rather than specific numbers, to represent unseen/not-associated state. Minor suggestion is making SwitchingLoss "private" (or possibly just part of GOSPA itself, rather than separate class). Diff of suggestion below:

diff --git a/stonesoup/metricgenerator/ospametric.py b/stonesoup/metricgenerator/ospametric.py
index 2ba6ff4e..737427e5 100644
--- a/stonesoup/metricgenerator/ospametric.py
+++ b/stonesoup/metricgenerator/ospametric.py
@@ -11,17 +11,14 @@ from ..types.time import TimeRange
 from ..types.metric import SingleTimeMetric, TimeRangeMetric
 
 
-class SwitchingLoss:
+class _SwitchingLoss:
     """
     Holds state assignment history and computes GOSPA switching term
     See https://www.mathworks.com/help/fusion/ref/trackgospametric-system-object.html#d126e213697
     """
-    def __init__(self, truth_ids, loss_factor, p):
-        self.not_associated = -1
-        self.unseen = -2
-        self.truth_associations = {i: self.unseen for i in truth_ids}
-        self.switching_loss = 0
-        self.has_associations = False
+    def __init__(self, loss_factor, p):
+        self.truth_associations = {}
+        self.switching_loss = None
         self.loss_factor = loss_factor
         self.p = p
         self.switching_penalty = 0.5
@@ -34,25 +31,23 @@ class SwitchingLoss:
         ----------
         truth_associations: dict(truth_track_id: measurement_track_id)
         """
-        self.has_associations = True
         self.switching_loss = 0
 
         for truth_id, meas_id in truth_associations.items():
-            if self.truth_associations[truth_id] == self.unseen and meas_id == self.not_associated:
+            if truth_id not in self.truth_associations and meas_id is None:
                 continue
-            if self.truth_associations[truth_id] == self.unseen:
+            if truth_id not in self.truth_associations:
                 self.truth_associations[truth_id] = meas_id
             elif self.truth_associations[truth_id] != meas_id:
                 self.switching_loss += self.switching_penalty
-                if (meas_id != self.not_associated
-                        and self.truth_associations[truth_id] != self.not_associated):
+                if meas_id is not None and self.truth_associations[truth_id] is not None:
                     self.switching_loss += self.switching_penalty
 
                 self.truth_associations[truth_id] = meas_id
 
     def loss(self):
         """Compute loss based on last association."""
-        if not self.has_associations:
+        if self.switching_loss is None:
             raise RuntimeError("Can't compute switching loss before any association are added.")
         return self.loss_factor * self.switching_loss**(1/self.p)
 
@@ -172,7 +167,7 @@ class GOSPAMetric(MetricGenerator):
             state.timestamp
             for state in chain(measured_states, truth_states)})
 
-        switching_metric = SwitchingLoss(truth_state_ids, self.switching_penalty, self.p)
+        switching_metric = _SwitchingLoss(self.switching_penalty, self.p)
         gospa_metrics = []
         for timestamp in timestamps:
             meas_mask = [state.timestamp == timestamp for state in measured_states]
@@ -185,9 +180,9 @@ class GOSPAMetric(MetricGenerator):
 
             metric, truth_to_measured_assignment = self.compute_gospa_metric(
                     meas_points, truth_points)
-            truth_mapping = {}
-            for i, meas_id in enumerate(truth_to_measured_assignment):
-                truth_mapping[truth_ids[i]] = -1 if meas_id == -1 else meas_ids[meas_id]
+            truth_mapping = {
+                truth_id: meas_ids[meas_id] if meas_id != -1 else None
+                for truth_id, meas_id in zip(truth_ids, truth_to_measured_assignment)}
 
             switching_metric.add_associations(truth_mapping)
             metric.value['switching'] = switching_metric.loss()
diff --git a/stonesoup/metricgenerator/tests/test_ospametric.py b/stonesoup/metricgenerator/tests/test_ospametric.py
index 9d33c11e..58a91421 100644
--- a/stonesoup/metricgenerator/tests/test_ospametric.py
+++ b/stonesoup/metricgenerator/tests/test_ospametric.py
@@ -5,7 +5,7 @@ import numpy as np
 import pytest
 
 from ..manager import MultiManager
-from ..ospametric import GOSPAMetric, OSPAMetric, SwitchingLoss
+from ..ospametric import GOSPAMetric, OSPAMetric, _SwitchingLoss
 from ...types.detection import Detection
 from ...types.groundtruth import GroundTruthPath, GroundTruthState
 from ...types.state import State
@@ -416,23 +416,23 @@ def test_ospa_computemetric_cardinality_error(p, first_value, second_value):
     ([
         {0: 0, 1: 1, 2: 2},
         {0: 0, 1: 1, 2: 2},
-        {0: 0, 1: 1, 2: -1},
+        {0: 0, 1: 1, 2: None},
         {0: 0, 1: 1, 2: 2},
         {0: 1, 1: 0, 2: 2},
         {1: 0, 0: 1, 2: 2},
-        {0: -1, 1: 2, 2: 0},
+        {0: None, 1: 2, 2: 0},
     ],  [0, 0, 0.5, 0.5, 2, 0, 2.5]),
     ([
         {0: 0, 1: 1, 2: 2},
-        {0: -1, 1: -1, 2: -1},
-        {0: 3, 1: -1, 2: -1},
+        {0: None, 1: None, 2: None},
+        {0: 3, 1: None, 2: None},
     ], [0, 1.5, 0.5]),
     ([
-        {0: -1, 1: -1, 2: -1},
+        {0: None, 1: None, 2: None},
         {0: 0, 1: 1, 2: 2},
     ], [0, 0]),
     ([  # The first time we associate with the track it should not count for loss
-        {0: -1, 1: -1, 2: -1},
+        {0: None, 1: None, 2: None},
         {0: 0, 1: 1, 2: 2},
     ], [0, 0]),
     ([  # The first time we associate with the track it should not count for loss
@@ -451,7 +451,7 @@ def test_switching_loss(associations, expected_losses):
     loss_factor = 1
     truth_ids = list(range(3))
 
-    switching_loss = SwitchingLoss(truth_ids, loss_factor, 1)
+    switching_loss = _SwitchingLoss(loss_factor, 1)
 
     with pytest.raises(RuntimeError) as _:
         switching_loss.loss()   # Should raise error if no associations have been added yet.

Changes addressed