move _decoding_mwpf to new _decoding folder

glue/sample/src/sinter/_decoding/_decoding_mwpf.py

@@ -15,7 +15,7 @@ def mwpf_import_error() -> ImportError:
     return ImportError(
         "The decoder 'MWPF' isn't installed\n"
         "To fix this, install the python package 'MWPF' into your environment.\n"
-        "For example, if you are using pip, run `pip install MWPF~=0.1.1`.\n"
+        "For example, if you are using pip, run `pip install MWPF~=0.1.5`.\n"
     )
 
 
@@ -75,12 +75,18 @@ def compile_decoder_for_dem(
         # For example, `SolverSerialUnionFind` is the most basic solver without any plugin: it only
         # grows the clusters until the first valid solution appears; some more optimized solvers uses
         # one or more plugins to further optimize the solution, which requires longer decoding time.
+        cluster_node_limit: int = 50,  # The maximum number of nodes in a cluster.
     ) -> CompiledDecoder:
         solver, fault_masks = detector_error_model_to_mwpf_solver_and_fault_masks(
-            dem, decoder_cls=decoder_cls
+            dem,
+            decoder_cls=decoder_cls,
+            cluster_node_limit=cluster_node_limit,
         )
         return MwpfCompiledDecoder(
-            solver, fault_masks, dem.num_detectors, dem.num_observables
+            solver,
+            fault_masks,
+            dem.num_detectors,
+            dem.num_observables,
         )
 
     def decode_via_files(
@@ -220,26 +226,31 @@ def _helper(m: stim.DetectorErrorModel, reps: int):
 def deduplicate_hyperedges(
     hyperedges: List[Tuple[List[int], float, int]]
 ) -> List[Tuple[List[int], float, int]]:
-    indices: dict[frozenset[int], int] = dict()
+    indices: dict[frozenset[int], Tuple[int, float]] = dict()
     result: List[Tuple[List[int], float, int]] = []
     for dets, weight, mask in hyperedges:
         dets_set = frozenset(dets)
         if dets_set in indices:
-            idx = indices[dets_set]
+            idx, min_weight = indices[dets_set]
             p1 = 1 / (1 + math.exp(weight))
             p2 = 1 / (1 + math.exp(result[idx][1]))
             p = p1 * (1 - p2) + p2 * (1 - p1)
-            # not sure why would this fail? two hyperedges with different masks?
-            # assert mask == result[idx][2], (result[idx], (dets, weight, mask))
-            result[idx] = (dets, math.log((1 - p) / p), result[idx][2])
+            # choosing the mask from the most likely error
+            new_mask = result[idx][2]
+            if weight < min_weight:
+                indices[dets_set] = (idx, weight)
+                new_mask = mask
+            result[idx] = (dets, math.log((1 - p) / p), new_mask)
         else:
-            indices[dets_set] = len(result)
+            indices[dets_set] = (len(result), weight)
             result.append((dets, weight, mask))
     return result
 
 
 def detector_error_model_to_mwpf_solver_and_fault_masks(
-    model: stim.DetectorErrorModel, decoder_cls: Any = None
+    model: stim.DetectorErrorModel,
+    decoder_cls: Any = None,
+    cluster_node_limit: int = 50,
 ) -> Tuple[Optional["mwpf.SolverSerialJointSingleHair"], np.ndarray]:
     """Convert a stim error model into a NetworkX graph."""
 
@@ -261,7 +272,7 @@ def handle_error(p: float, dets: List[int], frame_changes: List[int]):
             # Accept it and keep going, though of course decoding will probably perform terribly.
             return
         if p > 0.5:
-            # mwpf doesn't support negative edge weights.
+            # mwpf doesn't support negative edge weights (yet, will be supported in the next version).
             # approximate them as weight 0.
             p = 0.5
         weight = math.log((1 - p) / p)
@@ -280,7 +291,7 @@ def handle_detector_coords(detector: int, coords: np.ndarray):
     # mwpf package panic on duplicate edges, thus we need to handle them here
     hyperedges = deduplicate_hyperedges(hyperedges)
 
-    # fix the input by connecting an edge to all isolated vertices
+    # fix the input by connecting an edge to all isolated vertices; will be supported in the next version
     for idx in range(num_detectors):
         if not is_detector_connected[idx]:
             hyperedges.append(([idx], 0, 0))
@@ -301,7 +312,7 @@ def handle_detector_coords(detector: int, coords: np.ndarray):
         decoder_cls = mwpf.SolverSerialJointSingleHair
     return (
         (
-            decoder_cls(initializer)
+            decoder_cls(initializer, config={"cluster_node_limit": cluster_node_limit})
             if num_detectors > 0 and len(rescaled_edges) > 0
             else None
         ),