Merge pull request #40 from oscarhiggott/fix-syndrome-on-boundary

Zero out syndrome in the support of UserGraph boundary nodes. Fixes #41

Author: oscarhiggott

src/pymatching/sparse_blossom/driver/mwpm_decoding.cc

@@ -86,7 +86,9 @@ void process_timeline_until_completion(pm::Mwpm& mwpm, const std::vector<uint64_
         for (auto& detection : detection_events) {
             if (detection >= mwpm.flooder.graph.nodes.size())
                 throw std::invalid_argument("Detection event index too large");
-            mwpm.create_detection_event(&mwpm.flooder.graph.nodes[detection]);
+            if (detection + 1 > mwpm.flooder.graph.is_user_graph_boundary_node.size() ||
+                !mwpm.flooder.graph.is_user_graph_boundary_node[detection])
+                mwpm.create_detection_event(&mwpm.flooder.graph.nodes[detection]);
         }
 
     } else {
@@ -102,7 +104,9 @@ void process_timeline_until_completion(pm::Mwpm& mwpm, const std::vector<uint64_
                     "Detection event index `" + std::to_string(detection) +
                     "` is larger than any detector node index in the graph.");
             if (!mwpm.flooder.graph.nodes[detection].radius_of_arrival) {
-                mwpm.create_detection_event(&mwpm.flooder.graph.nodes[detection]);
+                if (detection + 1 > mwpm.flooder.graph.is_user_graph_boundary_node.size() ||
+                    !mwpm.flooder.graph.is_user_graph_boundary_node[detection])
+                    mwpm.create_detection_event(&mwpm.flooder.graph.nodes[detection]);
             } else {
                 // Unmark node
                 mwpm.flooder.graph.nodes[detection].radius_of_arrival = 0;

src/pymatching/sparse_blossom/driver/user_graph.cc

@@ -258,6 +258,12 @@ pm::MatchingGraph pm::UserGraph::to_matching_graph(pm::weight_int num_distinct_w
                 matching_graph.add_boundary_edge(u, weight, observables);
         });
     matching_graph.normalising_constant = normalising_constant;
+    if (boundary_nodes.size() > 0) {
+        matching_graph.is_user_graph_boundary_node.clear();
+        matching_graph.is_user_graph_boundary_node.resize(nodes.size(), false);
+        for (auto& i : boundary_nodes)
+            matching_graph.is_user_graph_boundary_node[i] = true;
+    }
     return matching_graph;
 }
 

src/pymatching/sparse_blossom/driver/user_graph.test.cc

@@ -13,6 +13,7 @@
 // limitations under the License.
 
 #include "pymatching/sparse_blossom/driver/user_graph.h"
+#include "pymatching/sparse_blossom/driver/mwpm_decoding.h"
 
 #include <cmath>
 #include <gtest/gtest.h>
@@ -143,3 +144,25 @@ TEST(UserGraph, NodesAlongShortestPath) {
         ASSERT_EQ(nodes, nodes_expected);
     }
 }
+
+TEST(UserGraph, DecodeUserGraphDetectionEventOnBoundaryNode) {
+    {
+        pm::UserGraph graph;
+        graph.add_or_merge_edge(0, 1, {0}, 1.0, -1);
+        graph.add_or_merge_edge(1, 2, {1}, 1.0, -1);
+        graph.set_boundary({2});
+        auto& mwpm = graph.get_mwpm();
+        pm::ExtendedMatchingResult res(mwpm.flooder.graph.num_observables);
+        pm::decode_detection_events(mwpm, {2}, res.obs_crossed.data(), res.weight);
+    }
+
+    {
+        pm::UserGraph graph;
+        graph.add_or_merge_edge(0, 1, {0}, -1.0, -1);
+        graph.add_or_merge_edge(1, 2, {1}, 1.0, -1);
+        graph.set_boundary({2});
+        auto& mwpm = graph.get_mwpm();
+        pm::ExtendedMatchingResult res(mwpm.flooder.graph.num_observables);
+        pm::decode_detection_events(mwpm, {2}, res.obs_crossed.data(), res.weight);
+    }
+}

src/pymatching/sparse_blossom/flooder/graph.cc

@@ -104,6 +104,7 @@ MatchingGraph::MatchingGraph(MatchingGraph&& graph) noexcept
     : nodes(std::move(graph.nodes)),
       negative_weight_detection_events_set(std::move(graph.negative_weight_detection_events_set)),
       negative_weight_observables_set(std::move(graph.negative_weight_observables_set)),
+      is_user_graph_boundary_node(std::move(graph.is_user_graph_boundary_node)),
       negative_weight_sum(graph.negative_weight_sum),
       num_nodes(graph.num_nodes),
       num_observables(graph.num_observables),

src/pymatching/sparse_blossom/flooder/graph.h

@@ -38,6 +38,11 @@ class MatchingGraph {
     std::set<size_t> negative_weight_observables_set;
     /// The sum of the negative edge weights. This number is negative, rather than the absolute value.
     pm::total_weight_int negative_weight_sum;
+    /// is_user_graph_boundary_node is only filled if MatchingGraph is constructed from a UserGraph
+    /// is_user_graph_boundary_node[i] is true if i is a boudary node in the UserGraph
+    /// This vector is used to check that a detection event is not added to a node marked as a boundary node
+    /// in the UserGraph (as such an event cannot be matched, and will raise an error).
+    std::vector<bool> is_user_graph_boundary_node;
     size_t num_nodes;
     size_t num_observables;
     /// This is the normalising constant that the edge weights were multiplied by when converting from floats to

tests/matching/decode_test.py

@@ -205,3 +205,16 @@ def test_decode_wrong_syndrome_type_raises_type_error():
         m = Matching()
         m.add_edge(0, 1)
         m.decode([0, "A"])
+
+
+def test_syndrome_on_boundary_nodes():
+    m = Matching()
+    m.add_edge(0, 1, fault_ids={0})
+    m.add_edge(1, 2, fault_ids={1})
+    m.add_edge(2, 3, fault_ids={2})
+    m.add_edge(3, 4, fault_ids={3})
+    m.set_boundary_nodes({3, 4})
+    m.decode([0, 0, 0, 1, 0])
+    m.decode([0, 0, 0, 0, 1])
+    m.decode([0, 0, 0, 1, 1])
+    m.decode([1, 0, 1, 0, 1])