Force decimals in scientific notation for OpenQASM 2.0 (#6802)

* Force decimals in scientific notation for OpenQASM 2.0

- OpenQASM 2.0 grammar requires decimal points in scientific
notation.  For instance, 1e-10 is not allowed according to a
strict interpretation of the grammar.
- This PR forces the formatter to use a decimal when printing
out scientific notation.

cirq-core/cirq/protocols/qasm.py

@@ -55,14 +55,23 @@ def __init__(
         self.qubit_id_map = {} if qubit_id_map is None else qubit_id_map
         self.meas_key_id_map = {} if meas_key_id_map is None else meas_key_id_map
 
+    def _format_number(self, value) -> str:
+        """OpenQASM 2.0 does not support '1e-5' and wants '1.0e-5'"""
+        s = f'{value}'
+        if 'e' in s and not '.' in s:
+            return s.replace('e', '.0e')
+        return s
+
     def format_field(self, value: Any, spec: str) -> str:
         """Method of string.Formatter that specifies the output of format()."""
         if isinstance(value, (float, int)):
             if isinstance(value, float):
                 value = round(value, self.precision)
             if spec == 'half_turns':
-                value = f'pi*{value}' if value != 0 else '0'
+                value = f'pi*{self._format_number(value)}' if value != 0 else '0'
                 spec = ''
+            else:
+                value = self._format_number(value)
         elif isinstance(value, ops.Qid):
             value = self.qubit_id_map[value]
         elif isinstance(value, str) and spec == 'meas':

cirq-core/cirq/protocols/qasm_test.py

@@ -52,3 +52,15 @@ def test_qasm():
 
     assert cirq.qasm(ExpectsArgs(), args=cirq.QasmArgs()) == 'text'
     assert cirq.qasm(ExpectsArgsQubits(), args=cirq.QasmArgs(), qubits=()) == 'text'
+
+
+def test_qasm_args_formatting():
+    args = cirq.QasmArgs()
+    assert args.format_field(0.01, '') == '0.01'
+    assert args.format_field(0.01, 'half_turns') == 'pi*0.01'
+    assert args.format_field(0.00001, '') == '1.0e-05'
+    assert args.format_field(0.00001, 'half_turns') == 'pi*1.0e-05'
+    assert args.format_field(1e-10, 'half_turns') == 'pi*1.0e-10'
+    args = cirq.QasmArgs(precision=6)
+    assert args.format_field(0.00001234, '') == '1.2e-05'
+    assert args.format_field(0.00001234, 'half_turns') == 'pi*1.2e-05'