Implement classical action of Cast bloq (#1093)

qualtran/bloqs/bookkeeping/cast.py

@@ -25,6 +25,7 @@
     CompositeBloq,
     DecomposeTypeError,
     QDType,
+    QFxp,
     Register,
     Side,
     Signature,
@@ -100,8 +101,13 @@ def add_my_tensors(
         )
 
     def on_classical_vals(self, reg: int) -> Dict[str, 'ClassicalValT']:
-        # TODO: Actually cast the values https://github.com/quantumlib/Qualtran/issues/734
-        return {'reg': reg}
+        if isinstance(self.out_dtype, QFxp):
+            res = reg
+        elif isinstance(self.inp_dtype, QFxp):
+            res = int(reg)
+        else:
+            res = self.out_dtype.from_bits(self.inp_dtype.to_bits(reg))
+        return {'reg': res}
 
     def as_cirq_op(self, qubit_manager, reg: 'CirqQuregT') -> Tuple[None, Dict[str, 'CirqQuregT']]:
         return None, {'reg': reg}

qualtran/bloqs/bookkeeping/cast_test.py

@@ -13,7 +13,7 @@
 #  limitations under the License.
 import subprocess
 
-from qualtran import QFxp, QInt
+from qualtran import QFxp, QInt, QUInt
 from qualtran.bloqs.bookkeeping import Cast
 from qualtran.bloqs.bookkeeping.cast import _cast
 from qualtran.bloqs.for_testing import TestCastToFrom
@@ -40,6 +40,17 @@ def test_cast_classical_sim():
     assert a == 7
     assert b == 9
 
+    c = Cast(QFxp(8, 8), QUInt(8))
+    assert c.call_classically(reg=1.2) == (1,)  # type: ignore
+
+
+def test_cast_unsiged_signed():
+    c = Cast(QUInt(5), QInt(5))
+    assert c.call_classically(reg=31) == (-1,)
+
+    c = Cast(QInt(5), QUInt(5))
+    assert c.call_classically(reg=-1) == (31,)
+
 
 def test_no_circular_import():
     subprocess.check_call(['python', '-c', 'from qualtran.bloqs.bookkeeping import cast'])