use Counter multiset for edge_set

pyzx/graph/diff.py

@@ -51,17 +51,15 @@ def calculate_diff(self, g1: BaseGraph[VT,ET], g2: BaseGraph[VT,ET]) -> None:
 		new_verts = g2.vertex_set()
 		self.removed_verts = list(old_verts - new_verts)
 		self.new_verts = list(new_verts - old_verts)
+		old_edges = g1.edge_set()
+		new_edges = g2.edge_set()
 		self.new_edges = []
 		self.removed_edges = []
 
-		g1_edges = list(g1.edges())
-		new_edges = [i for i in g2.edges() if not i in g1_edges or g1_edges.remove(i)]
-		for e in (new_edges):
+		for e in (new_edges - old_edges):
 			self.new_edges.append((g2.edge_st(e), g2.edge_type(e)))
 
-		g2_edges = list(g2.edges())
-		removed_edges = [i for i in g1.edges() if not i in g2_edges or g2_edges.remove(i)]
-		for e in (removed_edges):
+		for e in (old_edges - new_edges):
 			s,t = g1.edge_st(e)
 			if s in self.removed_verts or t in self.removed_verts: continue
 			self.removed_edges.append(e)
@@ -86,8 +84,8 @@ def calculate_diff(self, g1: BaseGraph[VT,ET], g2: BaseGraph[VT,ET]) -> None:
 				pos2 = g2.qubit(v), g2.row(v)
 				self.changed_pos[v] = pos2
 
-		for e in g2.edges():
-			if e in g1.edges():
+		for e in new_edges:
+			if e in old_edges:
 				if g1.edge_type(e) != g2.edge_type(e):
 					self.changed_edge_types[e] = g2.edge_type(e)
 			else:

pyzx/graph/multigraph.py

@@ -14,6 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from collections import Counter
 from fractions import Fraction
 from typing import Tuple, Dict, Set, Any
 
@@ -304,8 +305,10 @@ def edges(self, s=None, t=None):
 
     def edge(self, s, t):
         return (s,t) if s < t else (t,s)
+
     def edge_set(self):
-        return set(self.edges())
+        return Counter(self.edges())
+
     def edge_st(self, edge):
         return (edge[0], edge[1])