Bitarray postselect (backport #12693)

qiskit/primitives/containers/bit_array.py

@@ -470,6 +470,97 @@ def slice_shots(self, indices: int | Sequence[int]) -> "BitArray":
         arr = arr[..., indices, :]
         return BitArray(arr, self.num_bits)
 
+    def postselect(
+        self,
+        indices: Sequence[int] | int,
+        selection: Sequence[bool | int] | bool | int,
+    ) -> BitArray:
+        """Post-select this bit array based on sliced equality with a given bitstring.
+
+        .. note::
+            If this bit array contains any shape axes, it is first flattened into a long list of shots
+            before applying post-selection. This is done because :class:`~BitArray` cannot handle
+            ragged numbers of shots across axes.
+
+        Args:
+            indices: A list of the indices of the cbits on which to postselect.
+                If this bit array was produced by a sampler, then an index ``i`` corresponds to the
+                :class:`~.ClassicalRegister` location ``creg[i]`` (as in :meth:`~slice_bits`).
+                Negative indices are allowed.
+
+            selection: A list of binary values (will be cast to ``bool``) of length matching
+                ``indices``, with ``indices[i]`` corresponding to ``selection[i]``. Shots will be
+                discarded unless all cbits specified by ``indices`` have the values given by
+                ``selection``.
+
+        Returns:
+            A new bit array with ``shape=(), num_bits=data.num_bits, num_shots<=data.num_shots``.
+
+        Raises:
+            IndexError: If ``max(indices)`` is greater than or equal to :attr:`num_bits`.
+            IndexError: If ``min(indices)`` is less than negative :attr:`num_bits`.
+            ValueError: If the lengths of ``selection`` and ``indices`` do not match.
+        """
+        if isinstance(indices, int):
+            indices = (indices,)
+        if isinstance(selection, (bool, int)):
+            selection = (selection,)
+        selection = np.asarray(selection, dtype=bool)
+
+        num_indices = len(indices)
+
+        if len(selection) != num_indices:
+            raise ValueError("Lengths of indices and selection do not match.")
+
+        num_bytes = self._array.shape[-1]
+        indices = np.asarray(indices)
+
+        if num_indices > 0:
+            if indices.max() >= self.num_bits:
+                raise IndexError(
+                    f"index {int(indices.max())} out of bounds for the number of bits {self.num_bits}."
+                )
+            if indices.min() < -self.num_bits:
+                raise IndexError(
+                    f"index {int(indices.min())} out of bounds for the number of bits {self.num_bits}."
+                )
+
+        flattened = self.reshape((), self.size * self.num_shots)
+
+        # If no conditions, keep all data, but flatten as promised:
+        if num_indices == 0:
+            return flattened
+
+        # Make negative bit indices positive:
+        indices %= self.num_bits
+
+        # Handle special-case of contradictory conditions:
+        if np.intersect1d(indices[selection], indices[np.logical_not(selection)]).size > 0:
+            return BitArray(np.empty((0, num_bytes), dtype=np.uint8), num_bits=self.num_bits)
+
+        # Recall that creg[0] is the LSb:
+        byte_significance, bit_significance = np.divmod(indices, 8)
+        # least-significant byte is at last position:
+        byte_idx = (num_bytes - 1) - byte_significance
+        # least-significant bit is at position 0:
+        bit_offset = bit_significance.astype(np.uint8)
+
+        # Get bitpacked representation of `indices` (bitmask):
+        bitmask = np.zeros(num_bytes, dtype=np.uint8)
+        np.bitwise_or.at(bitmask, byte_idx, np.uint8(1) << bit_offset)
+
+        # Get bitpacked representation of `selection` (desired bitstring):
+        selection_bytes = np.zeros(num_bytes, dtype=np.uint8)
+        ## This assumes no contradictions present, since those were already checked for:
+        np.bitwise_or.at(
+            selection_bytes, byte_idx, np.asarray(selection, dtype=np.uint8) << bit_offset
+        )
+
+        return BitArray(
+            flattened._array[((flattened._array & bitmask) == selection_bytes).all(axis=-1)],
+            num_bits=self.num_bits,
+        )
+
     def expectation_values(self, observables: ObservablesArrayLike) -> NDArray[np.float64]:
         """Compute the expectation values of the provided observables, broadcasted against
         this bit array.

releasenotes/notes/bitarray-postselect-659b8f7801ccaa60.yaml

@@ -0,0 +1,11 @@
+---
+features_primitives:
+  - |
+    Added a new method :meth:`.BitArray.postselect` that returns all shots containing specified bit values.
+    Example usage::
+
+      from qiskit.primitives.containers import BitArray
+
+      ba = BitArray.from_counts({'110': 2, '100': 4, '000': 3})
+      print(ba.postselect([0,2], [0,1]).get_counts())
+      # {'110': 2, '100': 4}

test/python/primitives/containers/test_bit_array.py

@@ -719,3 +719,82 @@ def test_expectation_values(self):
                 _ = ba.expectation_values("Z")
             with self.assertRaisesRegex(ValueError, "is not diagonal"):
                 _ = ba.expectation_values("X" * ba.num_bits)
+
+    def test_postselection(self):
+        """Test the postselection method."""
+
+        flat_data = np.array(
+            [
+                [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+                [0, 0, 0, 0, 0, 1, 1, 1, 1, 1],
+                [0, 1, 0, 1, 0, 1, 0, 1, 0, 1],
+            ],
+            dtype=bool,
+        )
+
+        shaped_data = np.array(
+            [
+                [
+                    [
+                        [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
+                        [0, 0, 0, 0, 0, 1, 1, 1, 1, 1],
+                        [0, 1, 0, 1, 0, 1, 0, 1, 0, 1],
+                    ],
+                    [
+                        [1, 0, 1, 0, 1, 0, 1, 0, 1, 0],
+                        [1, 1, 1, 1, 1, 0, 0, 0, 0, 0],
+                        [1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
+                    ],
+                ]
+            ],
+            dtype=bool,
+        )
+
+        for dataname, bool_array in zip(["flat", "shaped"], [flat_data, shaped_data]):
+
+            bit_array = BitArray.from_bool_array(bool_array, order="little")
+            # indices value of i <-> creg[i] <-> bool_array[..., i]
+
+            num_bits = bool_array.shape[-1]
+            bool_array = bool_array.reshape(-1, num_bits)
+
+            test_cases = [
+                ("basic", [0, 1], [0, 0]),
+                ("multibyte", [0, 9], [0, 1]),
+                ("repeated", [5, 5, 5], [0, 0, 0]),
+                ("contradict", [5, 5, 5], [1, 0, 0]),
+                ("unsorted", [5, 0, 9, 3], [1, 0, 1, 0]),
+                ("negative", [-5, 1, -2, -10], [1, 0, 1, 0]),
+                ("negcontradict", [4, -6], [1, 0]),
+                ("trivial", [], []),
+                ("bareindex", 6, 0),
+            ]
+
+            for name, indices, selection in test_cases:
+                with self.subTest("_".join([dataname, name])):
+                    postselected_bools = np.unpackbits(
+                        bit_array.postselect(indices, selection).array[:, ::-1],
+                        count=num_bits,
+                        axis=-1,
+                        bitorder="little",
+                    ).astype(bool)
+                    if isinstance(indices, int):
+                        indices = (indices,)
+                    if isinstance(selection, bool):
+                        selection = (selection,)
+                    answer = bool_array[np.all(bool_array[:, indices] == selection, axis=-1)]
+                    if name in ["contradict", "negcontradict"]:
+                        self.assertEqual(len(answer), 0)
+                    else:
+                        self.assertGreater(len(answer), 0)
+                    np.testing.assert_equal(postselected_bools, answer)
+
+            error_cases = [
+                ("aboverange", [0, 6, 10], [True, True, False], IndexError),
+                ("belowrange", [0, 6, -11], [True, True, False], IndexError),
+                ("mismatch", [0, 1, 2], [False, False], ValueError),
+            ]
+            for name, indices, selection, error in error_cases:
+                with self.subTest(dataname + "_" + name):
+                    with self.assertRaises(error):
+                        bit_array.postselect(indices, selection)