Fix qasm generation/parsing for classical controls

cirq-core/cirq/contrib/qasm_import/_lexer.py

@@ -32,7 +32,7 @@ def __init__(self):
         'measure': 'MEASURE',
         'if': 'IF',
         '->': 'ARROW',
-        '!=': 'NE',
+        '==': 'EQ',
     }
 
     tokens = ['FORMAT_SPEC', 'NUMBER', 'NATURAL_NUMBER', 'QELIBINC', 'ID', 'PI'] + list(
@@ -103,8 +103,8 @@ def t_ARROW(self, t):
         """->"""
         return t
 
-    def t_NE(self, t):
-        """!="""
+    def t_EQ(self, t):
+        """=="""
         return t
 
     def t_ID(self, t):

cirq-core/cirq/contrib/qasm_import/_parser.py

@@ -16,6 +16,7 @@
 from typing import Any, Callable, cast, Dict, Iterable, List, Optional, Sequence, Union
 
 import numpy as np
+import sympy
 from ply import yacc
 
 from cirq import ops, Circuit, NamedQubit, CX
@@ -496,11 +497,19 @@ def p_measurement(self, p):
         ]
 
     # if operations
-    # if : IF '(' carg NE NATURAL_NUMBER ')' ID qargs
+    # if : IF '(' carg EQ 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])]
+        """if : IF '(' carg EQ NATURAL_NUMBER ')' gate_op"""
+        # We have to split the register into bits (since that's what measurement does above),
+        # and create one condition per bit, checking against that part of the binary value.
+        conditions = []
+        for i, key in enumerate(p[3]):
+            v = (p[5] >> i) & 1
+            conditions.append(sympy.Eq(sympy.Symbol(key), v))
+        p[0] = [
+            ops.ClassicallyControlledOperation(conditions=conditions, sub_operation=tuple(p[7])[0])
+        ]
 
     def p_error(self, p):
         if p is None:

cirq-core/cirq/contrib/qasm_import/_parser_test.py

@@ -213,16 +213,17 @@ def test_CX_gate():
 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];
+        creg a[1];
+        measure q[0] -> a[0];
+        if (a==1) 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')
+        cirq.measure(q_0, key='a_0'),
+        cirq.CNOT(q_0, q_1).with_classical_controls(sympy.Eq(sympy.Symbol('a_0'), 1)),
     )
 
     parsed_qasm = parser.parse(qasm)
@@ -233,6 +234,62 @@ def test_classical_control():
     ct.assert_same_circuits(parsed_qasm.circuit, expected_circuit)
     assert parsed_qasm.qregs == {'q': 2}
 
+    # Note this cannot *exactly* round-trip because the way QASM and Cirq handle measurements
+    # into classical registers is different. Cirq parses QASM classical registers into m_a_i for i
+    # in 0..bit_count. Thus the generated key has an extra "_0" at the end.
+    expected_generated_qasm = f"""// Generated from Cirq v{cirq.__version__}
+
+OPENQASM 2.0;
+include "qelib1.inc";
+
+
+// Qubits: [q_0, q_1]
+qreg q[2];
+creg m_a_0[1];
+
+
+measure q[0] -> m_a_0[0];
+if (m_a_0==1) cx q[0],q[1];
+"""
+    assert cirq.qasm(parsed_qasm.circuit) == expected_generated_qasm
+
+
+def test_classical_control_multi_bit():
+    qasm = """OPENQASM 2.0;
+        qreg q[2];
+        creg a[2];
+        measure q[0] -> a[0];
+        measure q[0] -> a[1];
+        if (a==1) CX q[0],q[1];
+    """
+    parser = QasmParser()
+
+    q_0 = cirq.NamedQubit('q_0')
+    q_1 = cirq.NamedQubit('q_1')
+
+    # Since we split the measurement into two, we also need two conditions.
+    # m_a==1 corresponds to m_a[0]==1, m_a[1]==0
+    expected_circuit = cirq.Circuit(
+        cirq.measure(q_0, key='a_0'),
+        cirq.measure(q_0, key='a_1'),
+        cirq.CNOT(q_0, q_1).with_classical_controls(
+            sympy.Eq(sympy.Symbol('a_0'), 1), sympy.Eq(sympy.Symbol('a_1'), 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}
+
+    # Note that this will *not* round-trip, but there's no good way around that due to the
+    # difference in how Cirq and QASM do multi-bit measurements.
+    with pytest.raises(ValueError, match='QASM does not support multiple conditions'):
+        _ = cirq.qasm(parsed_qasm.circuit)
+
 
 def test_CX_gate_not_enough_args():
     qasm = """OPENQASM 2.0;

cirq-core/cirq/ops/classically_controlled_operation.py

@@ -206,5 +206,9 @@ def _control_keys_(self) -> FrozenSet['cirq.MeasurementKey']:
 
     def _qasm_(self, args: 'cirq.QasmArgs') -> Optional[str]:
         args.validate_version('2.0')
-        all_keys = " && ".join(c.qasm for c in self._conditions)
-        return args.format('if ({0}) {1}', all_keys, protocols.qasm(self._sub_operation, args=args))
+        if len(self._conditions) > 1:
+            raise ValueError('QASM does not support multiple conditions.')
+        subop_qasm = protocols.qasm(self._sub_operation, args=args)
+        if not self._conditions:
+            return subop_qasm
+        return f'if ({self._conditions[0].qasm}) {subop_qasm}'

cirq-core/cirq/ops/classically_controlled_operation_test.py

@@ -198,11 +198,14 @@ def test_diagram_subcircuit_layered():
 
 def test_qasm():
     q0, q1 = cirq.LineQubit.range(2)
-    circuit = cirq.Circuit(cirq.measure(q0, key='a'), cirq.X(q1).with_classical_controls('a'))
+    circuit = cirq.Circuit(
+        cirq.measure(q0, key='a'),
+        cirq.X(q1).with_classical_controls(sympy.Eq(sympy.Symbol('a'), 0)),
+    )
     qasm = cirq.qasm(circuit)
     assert (
         qasm
-        == """// Generated from Cirq v0.15.0.dev
+        == f"""// Generated from Cirq v{cirq.__version__}
 
 OPENQASM 2.0;
 include "qelib1.inc";
@@ -214,11 +217,49 @@ def test_qasm():
 
 
 measure q[0] -> m_a[0];
-if (m_a!=0) x q[1];
+if (m_a==0) x q[1];
 """
     )
 
 
+def test_qasm_no_conditions():
+    q0, q1 = cirq.LineQubit.range(2)
+    circuit = cirq.Circuit(
+        cirq.measure(q0, key='a'), cirq.ClassicallyControlledOperation(cirq.X(q1), [])
+    )
+    qasm = cirq.qasm(circuit)
+    assert (
+        qasm
+        == f"""// Generated from Cirq v{cirq.__version__}
+
+OPENQASM 2.0;
+include "qelib1.inc";
+
+
+// Qubits: [q(0), q(1)]
+qreg q[2];
+creg m_a[1];
+
+
+measure q[0] -> m_a[0];
+x q[1];
+"""
+    )
+
+
+def test_qasm_multiple_conditions():
+    q0, q1 = cirq.LineQubit.range(2)
+    circuit = cirq.Circuit(
+        cirq.measure(q0, key='a'),
+        cirq.measure(q0, key='b'),
+        cirq.X(q1).with_classical_controls(
+            sympy.Eq(sympy.Symbol('a'), 0), sympy.Eq(sympy.Symbol('b'), 0)
+        ),
+    )
+    with pytest.raises(ValueError, match='QASM does not support multiple conditions'):
+        _ = cirq.qasm(circuit)
+
+
 @pytest.mark.parametrize('sim', ALL_SIMULATORS)
 def test_key_unset(sim):
     q0, q1 = cirq.LineQubit.range(2)

cirq-core/cirq/value/condition.py

@@ -113,9 +113,7 @@ def _from_json_dict_(cls, key, **kwargs):
 
     @property
     def qasm(self):
-        if self.index != -1:
-            raise NotImplementedError('Only most recent measurement at key can be used for QASM.')
-        return f'm_{self.key}!=0'
+        raise ValueError('QASM is defined only for SympyConditions of type key == constant.')
 
 
 @dataclasses.dataclass(frozen=True)
@@ -162,4 +160,8 @@ def _from_json_dict_(cls, expr, **kwargs):
 
     @property
     def qasm(self):
-        raise NotImplementedError()
+        if isinstance(self.expr, sympy.Equality):
+            if isinstance(self.expr.lhs, sympy.Symbol) and isinstance(self.expr.rhs, sympy.Integer):
+                # Measurements get prepended with "m_", so the condition needs to be too.
+                return f'm_{self.expr.lhs}=={self.expr.rhs}'
+        raise ValueError('QASM is defined only for SympyConditions of type key == constant.')

cirq-core/cirq/value/condition_test.py

@@ -67,7 +67,8 @@ def resolve(records):
 
 
 def test_key_condition_qasm():
-    assert cirq.KeyCondition(cirq.MeasurementKey('a')).qasm == 'm_a!=0'
+    with pytest.raises(ValueError, match='QASM is defined only for SympyConditions'):
+        _ = cirq.KeyCondition(cirq.MeasurementKey('a')).qasm
 
 
 def test_sympy_condition_with_keys():
@@ -111,5 +112,9 @@ def resolve(records):
 
 
 def test_sympy_condition_qasm():
-    with pytest.raises(NotImplementedError):
-        _ = init_sympy_condition.qasm
+    # Measurements get prepended with "m_", so the condition needs to be too.
+    assert cirq.SympyCondition(sympy.Eq(sympy.Symbol('a'), 2)).qasm == 'm_a==2'
+    with pytest.raises(
+        ValueError, match='QASM is defined only for SympyConditions of type key == constant'
+    ):
+        _ = cirq.SympyCondition(sympy.Symbol('a') != 2).qasm