Merge pull request #45 from noajshu/codex/copy-get_dets_logicals-to-decomposer

Deduplicate detector and logical targets modulo 2 in decomposer

Author: noajshu

src/py/decomposer.py

@@ -1,6 +1,17 @@
 import stim
 
 
+def get_dets_logicals(error: stim.DemInstruction):
+  dets = set()
+  logicals = set()
+  for t in error.targets_copy():
+    if t.is_logical_observable_id():
+      logicals = logicals.symmetric_difference({t.val})
+    elif t.is_relative_detector_id():
+      dets = dets.symmetric_difference({t.val})
+  return dets, logicals
+
+
 def _decompose_by_detector_partition(dem, partition_func):
   detector_coords = {}
 
@@ -19,17 +30,13 @@ def _decompose_by_detector_partition(dem, partition_func):
     # Make a new instruction where the detectors are decomposed.
     targets_by_basis = [[], []]
     observables = []
-    for d in instruction.targets_copy():
-      if d.is_separator():
-        # Ignore the existing decomposition, if present.
-        continue
-      if d.is_relative_detector_id():
-        coord = detector_coords[d.val]
-        targets_by_basis[partition_func(coord)].append(d)
-      else:
-        # Logical observables are placed in component 0.
-        assert d.is_logical_observable_id()
-        observables.append(d)
+    dets, logicals = get_dets_logicals(instruction)
+    for det in sorted(dets):
+      coord = detector_coords[det]
+      targets_by_basis[partition_func(coord)].append(stim.target_relative_detector_id(det))
+    for obs in sorted(logicals):
+      # Logical observables are placed in component 0.
+      observables.append(stim.target_logical_observable_id(obs))
 
     all_targets = []
     for targets in targets_by_basis:

src/py/decomposer_test.py

@@ -60,3 +60,20 @@ def test_do_decomposition_stim_surface_code_requires_xy_coords():
 
   with pytest.raises(AssertionError, match='requires detector x/y coordinates'):
     do_decomposition_stim_surface_code(dem)
+
+
+def test_do_decomposition_last_coordinate_index_dedupes_mod_2_targets():
+  dem = stim.DetectorErrorModel('''
+      detector(0, 0, 0, 0) D0
+      detector(1, 0, 0, 1) D1
+      error(0.125) D0 D0 D1 D1 L0 L0
+  ''')
+
+  actual = do_decomposition_last_coordinate_index(dem)
+
+  expected = stim.DetectorErrorModel('''
+      detector(0, 0, 0, 0) D0
+      detector(1, 0, 0, 1) D1
+      error(0.125)
+  ''')
+  assert actual == expected