Do not allow creating registers with bitsize 0 (#6298)

* Do not allow creating registers with bitsize 0

* Fix mypy errors

Author: tanujkhattar

Diff for: cirq-ft/cirq_ft/algos/and_gate.py

@@ -141,7 +141,11 @@ def _decompose_via_tree(
     def decompose_from_registers(
         self, *, context: cirq.DecompositionContext, **quregs: NDArray[cirq.Qid]
     ) -> cirq.OP_TREE:
-        control, ancilla, target = quregs['control'], quregs['ancilla'], quregs['target']
+        control, ancilla, target = (
+            quregs['control'],
+            quregs.get('ancilla', np.array([])),
+            quregs['target'],
+        )
         if len(self.cv) == 2:
             yield self._decompose_single_and(
                 self.cv[0], self.cv[1], control[0], control[1], *target

Diff for: cirq-ft/cirq_ft/algos/apply_gate_to_lth_target.py

@@ -77,7 +77,7 @@ def selection_registers(self) -> Tuple[infra.SelectionRegister, ...]:
 
     @cached_property
     def target_registers(self) -> Tuple[infra.Register, ...]:
-        total_iteration_size = np.product(
+        total_iteration_size = np.prod(
             tuple(reg.iteration_length for reg in self.selection_registers)
         )
         return (infra.Register('target', int(total_iteration_size)),)

Diff for: cirq-ft/cirq_ft/algos/prepare_uniform_superposition.py

@@ -69,7 +69,7 @@ def decompose_from_registers(
         context: cirq.DecompositionContext,
         **quregs: NDArray[cirq.Qid],  # type:ignore[type-var]
     ) -> cirq.OP_TREE:
-        controls, target = quregs['controls'], quregs['target']
+        controls, target = quregs.get('controls', ()), quregs['target']
         # Find K and L as per https://arxiv.org/abs/1805.03662 Fig 12.
         n, k = self.n, 0
         while n > 1 and n % 2 == 0:

Diff for: cirq-ft/cirq_ft/algos/qrom.py

@@ -111,7 +111,9 @@ def selection_registers(self) -> Tuple[infra.SelectionRegister, ...]:
 
     @cached_property
     def target_registers(self) -> Tuple[infra.Register, ...]:
-        return tuple(infra.Register(f'target{i}', l) for i, l in enumerate(self.target_bitsizes))
+        return tuple(
+            infra.Register(f'target{i}', l) for i, l in enumerate(self.target_bitsizes) if l
+        )
 
     def __repr__(self) -> str:
         data_repr = f"({','.join(cirq._compat.proper_repr(d) for d in self.data)})"
@@ -129,7 +131,7 @@ def _load_nth_data(
         **target_regs: NDArray[cirq.Qid],  # type: ignore[type-var]
     ) -> cirq.OP_TREE:
         for i, d in enumerate(self.data):
-            target = target_regs[f'target{i}']
+            target = target_regs.get(f'target{i}', ())
             for q, bit in zip(target, f'{int(d[selection_idx]):0{len(target)}b}'):
                 if int(bit):
                     yield gate(q)

Diff for: cirq-ft/cirq_ft/algos/selected_majorana_fermion.py

@@ -77,7 +77,7 @@ def selection_registers(self) -> Tuple[infra.SelectionRegister, ...]:
 
     @cached_property
     def target_registers(self) -> Tuple[infra.Register, ...]:
-        total_iteration_size = np.product(
+        total_iteration_size = np.prod(
             tuple(reg.iteration_length for reg in self.selection_registers)
         )
         return (infra.Register('target', int(total_iteration_size)),)

Diff for: cirq-ft/cirq_ft/algos/state_preparation.py

@@ -167,7 +167,7 @@ def decompose_from_registers(
         **quregs: NDArray[cirq.Qid],  # type:ignore[type-var]
     ) -> cirq.OP_TREE:
         selection, less_than_equal = quregs['selection'], quregs['less_than_equal']
-        sigma_mu, alt, keep = quregs['sigma_mu'], quregs['alt'], quregs['keep']
+        sigma_mu, alt, keep = quregs.get('sigma_mu', ()), quregs['alt'], quregs.get('keep', ())
         N = self.selection_registers[0].iteration_length
         yield prepare_uniform_superposition.PrepareUniformSuperposition(N).on(*selection)
         yield cirq.H.on_each(*sigma_mu)

Diff for: cirq-ft/cirq_ft/infra/gate_with_registers.py

@@ -32,11 +32,16 @@ class Register:
     """
 
     name: str
-    bitsize: int
+    bitsize: int = attr.field()
     shape: Tuple[int, ...] = attr.field(
         converter=lambda v: (v,) if isinstance(v, int) else tuple(v), default=()
     )
 
+    @bitsize.validator
+    def bitsize_validator(self, attribute, value):
+        if value <= 0:
+            raise ValueError(f"Bitsize for {self=} must be a positive integer. Found {value}.")
+
     def all_idxs(self) -> Iterable[Tuple[int, ...]]:
         """Iterate over all possible indices of a multidimensional register."""
         yield from itertools.product(*[range(sh) for sh in self.shape])
@@ -46,7 +51,7 @@ def total_bits(self) -> int:
 
         This is the product of each of the dimensions in `shape`.
         """
-        return self.bitsize * int(np.product(self.shape))
+        return self.bitsize * int(np.prod(self.shape))
 
     def __repr__(self):
         return f'cirq_ft.Register(name="{self.name}", bitsize={self.bitsize}, shape={self.shape})'
@@ -137,7 +142,7 @@ def __repr__(self):
 
     @classmethod
     def build(cls, **registers: int) -> 'Registers':
-        return cls(Register(name=k, bitsize=v) for k, v in registers.items())
+        return cls(Register(name=k, bitsize=v) for k, v in registers.items() if v > 0)
 
     @overload
     def __getitem__(self, key: int) -> Register:

Diff for: cirq-ft/cirq_ft/infra/gate_with_registers_test.py

@@ -25,6 +25,9 @@ def test_register():
     assert r.bitsize == 5
     assert r.shape == (1, 2)
 
+    with pytest.raises(ValueError, match="must be a positive integer"):
+        _ = cirq_ft.Register("zero bitsize register", bitsize=0)
+
 
 def test_registers():
     r1 = cirq_ft.Register("r1", 5)
@@ -96,7 +99,7 @@ def test_selection_registers_indexing(n, N, m, M):
             assert np.ravel_multi_index((x, y), (N, M)) == x * M + y
             assert np.unravel_index(x * M + y, (N, M)) == (x, y)
 
-    assert np.product(tuple(reg.iteration_length for reg in regs)) == N * M
+    assert np.prod(tuple(reg.iteration_length for reg in regs)) == N * M
 
 
 def test_selection_registers_consistent():