From 54c6cf5c1e1db52ebc0775023b7f64b82b7678b3 Mon Sep 17 00:00:00 2001
From: John van de Wetering <john@vdwetering.name>
Date: Thu, 22 Feb 2024 11:57:58 +0100
Subject: [PATCH] Fixes bug in `to_graph_like`. See #195

---
 pyzx/simplify.py       | 21 ++++++++++++++-------
 tests/test_simplify.py | 14 ++++++++++++++
 2 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/pyzx/simplify.py b/pyzx/simplify.py
index d2225f0a..91c1b227 100644
--- a/pyzx/simplify.py
+++ b/pyzx/simplify.py
@@ -25,7 +25,7 @@
         'pivot_gadget_simp', 'pivot_boundary_simp', 'gadget_simp',
         'lcomp_simp', 'clifford_simp', 'tcount', 'to_gh', 'to_rg',
         'full_reduce', 'teleport_reduce', 'reduce_scalar', 'supplementarity_simp',
-        'to_clifford_normal_form_graph']
+        'to_clifford_normal_form_graph', 'to_graph_like', 'is_graph_like']
 
 from typing import List, Callable, Optional, Union, Generic, Tuple, Dict, Iterator, cast
 
@@ -492,13 +492,20 @@ def to_graph_like(g: BaseGraph[VT,ET]) -> None:
 
         # add dummy spiders for all but one
         for b in boundary_ns[:-1]:
-            z1 = g.add_vertex(ty=VertexType.Z)
-            z2 = g.add_vertex(ty=VertexType.Z)
+            e = g.edge(v,b)
+            if g.edge_type(e) == EdgeType.SIMPLE:
+                z1 = g.add_vertex(ty=VertexType.Z,row=0.3*g.row(v)+0.7*g.row(b),qubit=0.3*g.qubit(v)+0.7*g.qubit(b))
+                z2 = g.add_vertex(ty=VertexType.Z,row=0.7*g.row(v)+0.3*g.row(b),qubit=0.7*g.qubit(v)+0.3*g.qubit(b))
 
-            g.remove_edge(g.edge(v, b))
-            g.add_edge(g.edge(z1, z2), edgetype=EdgeType.HADAMARD)
-            g.add_edge(g.edge(b, z1), edgetype=EdgeType.SIMPLE)
-            g.add_edge(g.edge(z2, v), edgetype=EdgeType.HADAMARD)
+                g.remove_edge(e)
+                g.add_edge(g.edge(z1, z2), edgetype=EdgeType.HADAMARD)
+                g.add_edge(g.edge(b, z1), edgetype=EdgeType.SIMPLE)
+                g.add_edge(g.edge(z2, v), edgetype=EdgeType.HADAMARD)
+            elif g.edge_type(e) == EdgeType.HADAMARD:
+                z = g.add_vertex(ty=VertexType.Z,row=0.5*g.row(v)+0.5*g.row(b),qubit=0.5*g.qubit(v)+0.5*g.qubit(b))
+                g.remove_edge(e)
+                g.add_edge(g.edge(b,z),EdgeType.SIMPLE)
+                g.add_edge(g.edge(z,v),EdgeType.HADAMARD)
 
     assert(is_graph_like(g))
 
diff --git a/tests/test_simplify.py b/tests/test_simplify.py
index 46d1a340..e11c8052 100644
--- a/tests/test_simplify.py
+++ b/tests/test_simplify.py
@@ -103,6 +103,12 @@ def test_teleport_reduce(self):
                 c2 = Circuit.from_graph(teleport_reduce(g))
                 self.assertTrue(c.verify_equality(c2))
 
+    def test_to_graph_like_introduce_boundary_vertices(self):
+        c = qasm(qasm_5)
+        g = c.to_graph()
+        to_graph_like(g)
+        self.assertTrue(compare_tensors(c,g))
+
 qasm_1 = """OPENQASM 2.0;
 include "qelib1.inc";
 qreg q[3];
@@ -155,6 +161,14 @@ def test_teleport_reduce(self):
 cx q[1], q[2];
 """
 
+qasm_5 = """OPENQASM 2.0;
+include "qelib1.inc";
+qreg q[2];
+h q[0];
+h q[1];
+cz q[0],q[1];
+"""
+
 
 if __name__ == '__main__':
     unittest.main()