QASM parsing for classical controls (quantumlib#4738)

Part 11 of https://tinyurl.com/cirq-feedforward. This is self-contained in the QASM lexer and parser classes.

Author: daxfohl

cirq/contrib/qasm_import/_lexer.py

@@ -30,7 +30,9 @@ def __init__(self):
         'qreg': 'QREG',
         'creg': 'CREG',
         'measure': 'MEASURE',
+        'if': 'IF',
         '->': 'ARROW',
+        '!=': 'NE',
     }
 
     tokens = [
@@ -98,10 +100,18 @@ def t_MEASURE(self, t):
         r"""measure"""
         return t
 
+    def t_IF(self, t):
+        r"""if"""
+        return t
+
     def t_ARROW(self, t):
         """->"""
         return t
 
+    def t_NE(self, t):
+        """!="""
+        return t
+
     def t_ID(self, t):
         r"""[a-zA-Z][a-zA-Z\d_]*"""
         return t

cirq/contrib/qasm_import/_parser.py

@@ -290,15 +290,17 @@ def p_format(self, p):
     # circuit : new_reg circuit
     #         | gate_op circuit
     #         | measurement circuit
+    #         | if circuit
     #         | empty
 
     def p_circuit_reg(self, p):
         """circuit : new_reg circuit"""
         p[0] = self.circuit
 
-    def p_circuit_gate_or_measurement(self, p):
+    def p_circuit_gate_or_measurement_or_if(self, p):
         """circuit :  circuit gate_op
-        |  circuit measurement"""
+        |  circuit measurement
+        |  circuit if"""
         self.circuit.append(p[2])
         p[0] = self.circuit
 
@@ -497,6 +499,13 @@ def p_measurement(self, p):
             ops.MeasurementGate(num_qubits=1, key=creg[i]).on(qreg[i]) for i in range(len(qreg))
         ]
 
+    # if operations
+    # if : IF '(' carg NE NATURAL_NUMBER ')' ID qargs
+
+    def p_if(self, p):
+        """if : IF '(' carg NE NATURAL_NUMBER ')' gate_op"""
+        p[0] = [ops.ClassicallyControlledOperation(conditions=p[3], sub_operation=tuple(p[7])[0])]
+
     def p_error(self, p):
         if p is None:
             raise QasmException('Unexpected end of file')

cirq/contrib/qasm_import/_parser_test.py

@@ -217,6 +217,31 @@ def test_CX_gate():
     assert parsed_qasm.qregs == {'q1': 2, 'q2': 2}
 
 
+def test_classical_control():
+    qasm = """OPENQASM 2.0;
+        qreg q[2];
+        creg m_a[1];
+        measure q[0] -> m_a[0];
+        if (m_a!=0) CX q[0], q[1];
+    """
+    parser = QasmParser()
+
+    q_0 = cirq.NamedQubit('q_0')
+    q_1 = cirq.NamedQubit('q_1')
+    expected_circuit = cirq.Circuit(
+        cirq.measure(q_0, key='m_a_0'),
+        cirq.CNOT(q_0, q_1).with_classical_controls('m_a_0'),
+    )
+
+    parsed_qasm = parser.parse(qasm)
+
+    assert parsed_qasm.supportedFormat
+    assert not parsed_qasm.qelib1Include
+
+    ct.assert_same_circuits(parsed_qasm.circuit, expected_circuit)
+    assert parsed_qasm.qregs == {'q': 2}
+
+
 def test_CX_gate_not_enough_args():
     qasm = """OPENQASM 2.0;
      qreg q[2];