Remove all references to dead vertices in ZX extraction (#1690)

* Make sure we remove all references when we remove vertices

* Bump tket version

* Remove looping tests

pytket/conanfile.py

@@ -38,7 +38,7 @@ def requirements(self):
         self.requires("pybind11_json/0.2.14")
         self.requires("symengine/0.13.0")
         self.requires("tkassert/0.3.4@tket/stable")
-        self.requires("tket/1.3.48@tket/stable")
+        self.requires("tket/1.3.49@tket/stable")
         self.requires("tklog/0.3.3@tket/stable")
         self.requires("tkrng/0.3.3@tket/stable")
         self.requires("tktokenswap/0.3.9@tket/stable")

tket/conanfile.py

@@ -23,7 +23,7 @@
 
 class TketConan(ConanFile):
     name = "tket"
-    version = "1.3.48"
+    version = "1.3.49"
     package_type = "library"
     license = "Apache 2"
     homepage = "https://github.com/CQCL/tket"

tket/src/Converters/ZXConverters.cpp

@@ -585,7 +585,8 @@ std::pair<ZXDiagram, BoundaryVertMap> circuit_to_zx(const Circuit& circ) {
 
 void clean_frontier(
     ZXDiagram& diag, ZXVertVec& frontier, Circuit& circ,
-    std::map<ZXVert, unsigned>& qubit_map) {
+    std::map<ZXVert, unsigned>& qubit_map,
+    std::map<ZXVert, ZXVert>& input_qubits) {
   std::set<ZXVert> frontier_lookup;
   std::list<std::pair<ZXVert, ZXVert>> czs;
   for (const ZXVert& f : frontier) {
@@ -655,6 +656,8 @@ void clean_frontier(
           (nt0 == ZXType::Output && nt1 == ZXType::Input)) {
         diag.add_wire(ns.at(0), ns.at(1));
         diag.remove_vertex(f);
+        auto input_found = input_qubits.find(f);
+        if (input_found != input_qubits.end()) input_qubits.erase(f);
         removed = true;
       }
     }
@@ -852,10 +855,10 @@ Circuit zx_to_circuit(const ZXDiagram& d) {
   for (const ZXVert& i : ins)
     input_qubits.insert({diag.neighbours(i).at(0), i});
 
-  clean_frontier(diag, frontier, circ, qubit_map);
+  clean_frontier(diag, frontier, circ, qubit_map, input_qubits);
   while (!frontier.empty()) {
     if (remove_all_gadgets(diag, frontier, input_qubits)) {
-      clean_frontier(diag, frontier, circ, qubit_map);
+      clean_frontier(diag, frontier, circ, qubit_map, input_qubits);
     }
     ZXVertSeqSet neighbours = neighbours_of_frontier(diag, frontier);
     boost::bimap<ZXVert, unsigned> correctors, preserve, ys;
@@ -922,7 +925,7 @@ Circuit zx_to_circuit(const ZXDiagram& d) {
       }
     }
 
-    clean_frontier(diag, frontier, circ, qubit_map);
+    clean_frontier(diag, frontier, circ, qubit_map, input_qubits);
   }
 
   qubit_map_t qm;

tket/test/src/ZX/test_ZXSimp.cpp

@@ -15,6 +15,7 @@
 #include <catch2/catch_test_macros.hpp>
 
 #include "tket/Converters/Converters.hpp"
+#include "tket/Predicates/PassLibrary.hpp"
 #include "tket/Transformations/Rebase.hpp"
 #include "tket/ZX/Rewrite.hpp"
 
@@ -180,6 +181,53 @@ SCENARIO("Testing cases for internalising gadgets in MBQC") {
   }
 }
 
+SCENARIO("Testing for vertex reuse bug") {
+  /**
+   * The following circuits were shown to hit errors non-deterministically, with
+   * the error likelihood varying with the device.
+   *
+   * At some point during extraction, a vertex would be removed from the graph,
+   * but some pointers to it lingered. Later, if an input extension occurred,
+   * new vertices would be added which may (depending on how boost was feeling
+   * with memory) reuse the same memory locations, so the old pointers now
+   * worked, meaning lookups in sets/functions would give false positives.
+   */
+  GIVEN("Offender 1") {
+    Circuit circ(3);
+    circ.add_op<unsigned>(OpType::H, {1});
+    circ.add_op<unsigned>(OpType::H, {2});
+    circ.add_op<unsigned>(OpType::CX, {2, 1});
+    circ.add_op<unsigned>(OpType::Rz, 1.1, {1});
+    circ.add_op<unsigned>(OpType::H, {0});
+    circ.add_op<unsigned>(OpType::CX, {2, 1});
+    circ.add_op<unsigned>(OpType::H, {1});
+    circ.add_op<unsigned>(OpType::H, {2});
+    circ.add_op<unsigned>(OpType::Rz, 0.970644, {1});
+    circ.add_op<unsigned>(OpType::Rz, 0.9, {2});
+    circ.add_op<unsigned>(OpType::H, {1});
+    circ.add_op<unsigned>(OpType::H, {2});
+
+    CompilationUnit cu(circ);
+    ZXGraphlikeOptimisation()->apply(cu);
+  }
+  GIVEN("Offender 2") {
+    Circuit circ(6);
+    circ.add_op<unsigned>(OpType::Rx, -0.78, {3});
+    circ.add_op<unsigned>(OpType::Rx, 1.069, {5});
+    circ.add_op<unsigned>(OpType::CX, {0, 3});
+    circ.add_op<unsigned>(OpType::CX, {3, 5});
+    circ.add_op<unsigned>(OpType::CX, {0, 5});
+    circ.add_op<unsigned>(OpType::Rx, 1.069, {3});
+    circ.add_op<unsigned>(OpType::Rx, -5.958, {4});
+    circ.add_op<unsigned>(OpType::CX, {3, 4});
+    circ.add_op<unsigned>(OpType::CX, {3, 5});
+    circ.add_op<unsigned>(OpType::CX, {0, 3});
+
+    CompilationUnit cu(circ);
+    ZXGraphlikeOptimisation()->apply(cu);
+  }
+}
+
 }  // namespace test_ZXSimp
 }  // namespace zx
 }  // namespace tket