Fix depth for barriers and snapshots (#2748)

* fix depth for barriers and snapshots

* Delete Untitled.ipynb

* fix lint not playing nice with snapshot import

qiskit/circuit/quantumcircuit.py

@@ -624,30 +624,39 @@ class QuantumCircuit:
         # line so that they all stacked at the same depth.
         # Conditional gates act on all cbits in the register
         # they are conditioned on.
-        # We do not consider barriers or snapshots as
+        # We treat barriers or snapshots different as
         # They are transpiler and simulator directives.
         # The max stack height is the circuit depth.
         for instr, qargs, cargs in self.data:
-            if instr.name not in ['barrier', 'snapshot']:
-                levels = []
-                reg_ints = []
-                for ind, reg in enumerate(qargs + cargs):
-                    # Add to the stacks of the qubits and
-                    # cbits used in the gate.
-                    reg_ints.append(reg_map[reg.register.name] + reg.index)
+            levels = []
+            reg_ints = []
+            # If count then add one to stack heights
+            count = True
+            if instr.name in ['barrier', 'snapshot']:
+                count = False
+            for ind, reg in enumerate(qargs + cargs):
+                # Add to the stacks of the qubits and
+                # cbits used in the gate.
+                reg_ints.append(reg_map[reg.register.name] + reg.index)
+                if count:
                     levels.append(op_stack[reg_ints[ind]] + 1)
-                if instr.control:
-                    # Controls operate over all bits in the
-                    # classical register they use.
-                    cint = reg_map[instr.control[0].name]
-                    for off in range(instr.control[0].size):
-                        if cint + off not in reg_ints:
-                            reg_ints.append(cint + off)
-                            levels.append(op_stack[cint + off] + 1)
+                else:
+                    levels.append(op_stack[reg_ints[ind]])
+            # Assuming here that there is no controlled
+            # snapshots or barriers ever.
+            if instr.control:
+                # Controls operate over all bits in the
+                # classical register they use.
+                cint = reg_map[instr.control[0].name]
+                for off in range(instr.control[0].size):
+                    if cint + off not in reg_ints:
+                        reg_ints.append(cint + off)
+                        levels.append(op_stack[cint + off] + 1)
+
+            max_level = max(levels)
+            for ind in reg_ints:
+                op_stack[ind] = max_level
 
-                max_level = max(levels)
-                for ind in reg_ints:
-                    op_stack[ind] = max_level
         return max(op_stack)
 
     def width(self):

test/python/circuit/test_circuit_properties.py

@@ -20,6 +20,8 @@ import unittest
 import numpy as np
 from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit
 from qiskit.test import QiskitTestCase
+# pylint: disable=unused-import
+from qiskit.extensions.simulator import snapshot
 
 
 class TestCircuitProperties(QiskitTestCase):
@@ -227,6 +229,93 @@ class TestCircuitProperties(QiskitTestCase):
         qc.measure(q[3], c[0])
         self.assertEqual(qc.depth(), 5)
 
+    def test_circuit_depth_barriers1(self):
+        """Test circuit depth for barriers #1.
+        """
+        q = QuantumRegister(4, 'q')
+        c = ClassicalRegister(4, 'c')
+        circ = QuantumCircuit(q, c)
+        circ.h(0)
+        circ.cx(0, 1)
+        circ.barrier(q)
+        circ.h(2)
+        circ.cx(2, 3)
+        self.assertEqual(circ.depth(), 4)
+
+    def test_circuit_depth_barriers2(self):
+        """Test circuit depth for barriers #2.
+        """
+        q = QuantumRegister(4, 'q')
+        c = ClassicalRegister(4, 'c')
+        circ = QuantumCircuit(q, c)
+        circ.h(0)
+        circ.barrier(q)
+        circ.cx(0, 1)
+        circ.barrier(q)
+        circ.h(2)
+        circ.barrier(q)
+        circ.cx(2, 3)
+        self.assertEqual(circ.depth(), 4)
+
+    def test_circuit_depth_barriers3(self):
+        """Test circuit depth for barriers #3.
+        """
+        q = QuantumRegister(4, 'q')
+        c = ClassicalRegister(4, 'c')
+        circ = QuantumCircuit(q, c)
+        circ.h(0)
+        circ.barrier(q)
+        circ.cx(0, 1)
+        circ.barrier(q)
+        circ.barrier(q)
+        circ.barrier(q)
+        circ.h(2)
+        circ.barrier(q)
+        circ.cx(2, 3)
+        self.assertEqual(circ.depth(), 4)
+
+    def test_circuit_depth_snap1(self):
+        """Test circuit depth for snapshots #1.
+        """
+        q = QuantumRegister(4, 'q')
+        c = ClassicalRegister(4, 'c')
+        circ = QuantumCircuit(q, c)
+        circ.h(0)
+        circ.cx(0, 1)
+        circ.snapshot('snap')
+        circ.h(2)
+        circ.cx(2, 3)
+        self.assertEqual(circ.depth(), 4)
+
+    def test_circuit_depth_snap2(self):
+        """Test circuit depth for snapshots #2.
+        """
+        q = QuantumRegister(4, 'q')
+        c = ClassicalRegister(4, 'c')
+        circ = QuantumCircuit(q, c)
+        circ.h(0)
+        circ.snapshot('snap0')
+        circ.cx(0, 1)
+        circ.snapshot('snap1')
+        circ.h(2)
+        circ.snapshot('snap2')
+        circ.cx(2, 3)
+        self.assertEqual(circ.depth(), 4)
+
+    def test_circuit_depth_snap3(self):
+        """Test circuit depth for snapshots #3.
+        """
+        q = QuantumRegister(4, 'q')
+        c = ClassicalRegister(4, 'c')
+        circ = QuantumCircuit(q, c)
+        circ.h(0)
+        circ.cx(0, 1)
+        circ.snapshot('snap0')
+        circ.snapshot('snap1')
+        circ.h(2)
+        circ.cx(2, 3)
+        self.assertEqual(circ.depth(), 4)
+
     def test_circuit_size_empty(self):
         """Circuit.size should return 0 for an empty circuit."""
         size = 4