Fix single-bit-condition equality in QuantumCircuit and DAGCircuit (#…

…8930) (#8943)

* Fix single-bit-condition equality in QuantumCircuit and DAGCircuit

Previously, the `condition` of a given operation would not be converted
into bit indices in order do the semantic-equality checking that the
rest of the qargs and cargs get in a circuit.  This caused loose-bit
comparisons to produce false negatives.

* Improve temporary variable names

Co-authored-by: Matthew Treinish <[email protected]>

Co-authored-by: Matthew Treinish <[email protected]>
(cherry picked from commit 3bebd0f)

Co-authored-by: Jake Lishman <[email protected]>

qiskit/dagcircuit/dagnode.py

@@ -16,6 +16,17 @@
 
 import warnings
 
+from qiskit.circuit import Clbit
+
+
+def _condition_as_indices(operation, bit_indices):
+    cond = getattr(operation, "condition", None)
+    if cond is None:
+        return None
+    bits, value = cond
+    indices = [bit_indices[bits]] if isinstance(bits, Clbit) else [bit_indices[x] for x in bits]
+    return indices, value
+
 
 class DAGNode:
     """Parent class for DAGOpNode, DAGInNode, and DAGOutNode."""
@@ -77,16 +88,19 @@ def semantic_eq(node1, node2, bit_indices1=None, bit_indices2=None):
             if node1.op.name == node2.op.name and node1.name in {"barrier", "swap"}:
                 return set(node1_qargs) == set(node2_qargs)
 
-            if node1_qargs == node2_qargs:
-                if node1_cargs == node2_cargs:
-                    if getattr(node1.op, "condition", None) == getattr(node2.op, "condition", None):
-                        if node1.op == node2.op:
-                            return True
-        elif (isinstance(node1, DAGInNode) and isinstance(node2, DAGInNode)) or (
+            return (
+                node1_qargs == node2_qargs
+                and node1_cargs == node2_cargs
+                and (
+                    _condition_as_indices(node1.op, bit_indices1)
+                    == _condition_as_indices(node2.op, bit_indices2)
+                )
+                and node1.op == node2.op
+            )
+        if (isinstance(node1, DAGInNode) and isinstance(node2, DAGInNode)) or (
             isinstance(node1, DAGOutNode) and isinstance(node2, DAGOutNode)
         ):
-            if bit_indices1.get(node1.wire, None) == bit_indices2.get(node2.wire, None):
-                return True
+            return bit_indices1.get(node1.wire, None) == bit_indices2.get(node2.wire, None)
 
         return False
 

releasenotes/notes/fix-circuit-condition-compare-d8d85e5ca47c1416.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    The equality checkers for :class:`.QuantumCircuit` and :class:`.DAGCircuit`
+    (with objects of the same type) will now correctly handle conditions on single
+    bits.  Previously, these would produce false negatives for equality, as the
+    bits would use "exact" equality checks instead of the "semantic" checks the rest
+    of the properties of circuit instructions get.

test/python/circuit/test_circuit_operations.py

@@ -1162,6 +1162,36 @@ def test_compare_two_different_circuits(self):
 
         self.assertFalse(qc1 == qc2)
 
+    def test_compare_circuits_with_single_bit_conditions(self):
+        """Test that circuits with single-bit conditions can be compared correctly."""
+        qreg = QuantumRegister(1, name="q")
+        creg = ClassicalRegister(1, name="c")
+        qc1 = QuantumCircuit(qreg, creg, [Clbit()])
+        qc1.x(0).c_if(qc1.cregs[0], 1)
+        qc1.x(0).c_if(qc1.clbits[-1], True)
+        qc2 = QuantumCircuit(qreg, creg, [Clbit()])
+        qc2.x(0).c_if(qc2.cregs[0], 1)
+        qc2.x(0).c_if(qc2.clbits[-1], True)
+        self.assertEqual(qc1, qc2)
+
+        # Order of operations transposed.
+        qc1 = QuantumCircuit(qreg, creg, [Clbit()])
+        qc1.x(0).c_if(qc1.cregs[0], 1)
+        qc1.x(0).c_if(qc1.clbits[-1], True)
+        qc2 = QuantumCircuit(qreg, creg, [Clbit()])
+        qc2.x(0).c_if(qc2.clbits[-1], True)
+        qc2.x(0).c_if(qc2.cregs[0], 1)
+        self.assertNotEqual(qc1, qc2)
+
+        # Single-bit condition values not the same.
+        qc1 = QuantumCircuit(qreg, creg, [Clbit()])
+        qc1.x(0).c_if(qc1.cregs[0], 1)
+        qc1.x(0).c_if(qc1.clbits[-1], True)
+        qc2 = QuantumCircuit(qreg, creg, [Clbit()])
+        qc2.x(0).c_if(qc2.cregs[0], 1)
+        qc2.x(0).c_if(qc2.clbits[-1], False)
+        self.assertNotEqual(qc1, qc2)
+
     def test_compare_a_circuit_with_none(self):
         """Test to compare that a circuit is different to None."""
         qc1 = QuantumCircuit(2, 2)

test/python/dagcircuit/test_dagcircuit.py

@@ -1462,6 +1462,36 @@ def test_node_params_equal_unequal(self):
         self.assertEqual(dag1, dag2)
         self.assertNotEqual(dag2, dag3)
 
+    def test_semantic_conditions(self):
+        """Test that the semantic equality is applied to the bits in conditions as well."""
+        qreg = QuantumRegister(1, name="q")
+        creg = ClassicalRegister(1, name="c")
+        qc1 = QuantumCircuit(qreg, creg, [Clbit()])
+        qc1.x(0).c_if(qc1.cregs[0], 1)
+        qc1.x(0).c_if(qc1.clbits[-1], True)
+        qc2 = QuantumCircuit(qreg, creg, [Clbit()])
+        qc2.x(0).c_if(qc2.cregs[0], 1)
+        qc2.x(0).c_if(qc2.clbits[-1], True)
+        self.assertEqual(circuit_to_dag(qc1), circuit_to_dag(qc2))
+
+        # Order of operations transposed.
+        qc1 = QuantumCircuit(qreg, creg, [Clbit()])
+        qc1.x(0).c_if(qc1.cregs[0], 1)
+        qc1.x(0).c_if(qc1.clbits[-1], True)
+        qc2 = QuantumCircuit(qreg, creg, [Clbit()])
+        qc2.x(0).c_if(qc2.clbits[-1], True)
+        qc2.x(0).c_if(qc2.cregs[0], 1)
+        self.assertNotEqual(circuit_to_dag(qc1), circuit_to_dag(qc2))
+
+        # Single-bit condition values not the same.
+        qc1 = QuantumCircuit(qreg, creg, [Clbit()])
+        qc1.x(0).c_if(qc1.cregs[0], 1)
+        qc1.x(0).c_if(qc1.clbits[-1], True)
+        qc2 = QuantumCircuit(qreg, creg, [Clbit()])
+        qc2.x(0).c_if(qc2.cregs[0], 1)
+        qc2.x(0).c_if(qc2.clbits[-1], False)
+        self.assertNotEqual(circuit_to_dag(qc1), circuit_to_dag(qc2))
+
 
 class TestDagSubstitute(QiskitTestCase):
     """Test substituting a dag node with a sub-dag"""