PBC transformations for C (#15756)

* Expose PBC operations to C

* Fix docs & swap ZX order to be consistent with Pauli

* Expose LitinskiTrafo to C

* Add ConvertToPauliRotations to C

* Extra sausage for win math

* Fix compat with lates main

* Shelly's comments

* review comments

- additional test if litinski is noop
- update test names
- remove some idle code

Author: Cryoris

crates/cext/src/transpiler/passes/convert_to_pauli_rotations.rs

@@ -0,0 +1,45 @@
+// This code is part of Qiskit.
+//
+// (C) Copyright IBM 2026
+//
+// This code is licensed under the Apache License, Version 2.0. You may
+// obtain a copy of this license in the LICENSE.txt file in the root directory
+// of this source tree or at https://www.apache.org/licenses/LICENSE-2.0.
+//
+// Any modifications or derivative works of this code must retain this
+// copyright notice, and modified files need to carry a notice indicating
+// that they have been altered from the originals.
+
+use qiskit_circuit::{circuit_data::CircuitData, dag_circuit::DAGCircuit};
+use qiskit_transpiler::passes::py_convert_to_pauli_rotations;
+
+use crate::pointers::mut_ptr_as_ref;
+
+/// @ingroup QkTranspilerPassesStandalone
+/// Run the ``ConvertToPauliRotations`` pass in-place on a circuit.
+///
+/// This pass converts all standard gates (with less than 4 qubits) in the circuit into a sequence
+/// of ``QkPauliProductRotation`` gates and measurements into ``QkPauliProductMeasurement``
+/// instructions. Note that this pass panics if the circuit contains non-standard gates.
+/// The suggested workflow is to first transpile into a standard basis, keeping rotation gates
+/// (such as ``QkGate_RXX`` and others) intact where possible, and then call this pass.
+///
+/// @param circuit A pointer to the circuit on which to run the pass.
+///
+/// # Safety
+///
+/// Behavior is undefined if ``circuit`` is not valid, non-null pointers to a ``QkCircuit``.
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn qk_transpiler_pass_standalone_convert_to_pauli_rotations(
+    circuit: *mut CircuitData,
+) {
+    // SAFETY: The user guarantees the pointer is safe to read.
+    let circuit = unsafe { mut_ptr_as_ref(circuit) };
+    let dag = match DAGCircuit::from_circuit_data(circuit, false, None, None, None, None) {
+        Ok(dag) => dag,
+        Err(_) => panic!("Internal Circuit -> DAG conversion failed"),
+    };
+    let out = py_convert_to_pauli_rotations(&dag).expect("Failed running PBC conversion.");
+    // If a DAG is returned, the circuit has been modified. Else just leave it as is.
+    *circuit = CircuitData::from_dag_ref(&out).expect("Internal DAG -> Circuit conversion failed");
+}

crates/cext/src/transpiler/passes/litinski_transformation.rs

@@ -0,0 +1,51 @@
+// This code is part of Qiskit.
+//
+// (C) Copyright IBM 2026
+//
+// This code is licensed under the Apache License, Version 2.0. You may
+// obtain a copy of this license in the LICENSE.txt file in the root directory
+// of this source tree or at https://www.apache.org/licenses/LICENSE-2.0.
+//
+// Any modifications or derivative works of this code must retain this
+// copyright notice, and modified files need to carry a notice indicating
+// that they have been altered from the originals.
+
+use qiskit_circuit::{circuit_data::CircuitData, dag_circuit::DAGCircuit};
+use qiskit_transpiler::passes::run_litinski_transformation;
+
+use crate::pointers::mut_ptr_as_ref;
+
+/// @ingroup QkTranspilerPassesStandalone
+/// Run the ``LitinskiTransformation`` pass in-place on a circuit.
+///
+/// This pass commutes all Clifford gates to the end of the circuit, converting Pauli rotation
+/// gates into ``QkPauliProductRotation`` gates and measurements into ``QkPauliProductMeasurement``
+/// instructions. Note that this pass currently only supports circuits that have ``QkGate_T``,
+/// ``QkGate_Tdg`` or ``QkGate_RZ`` gates as non-Cliffords and panics otherwise.  The suggested
+/// workflow is to first transpile into a Clifford+RZ basis and then call this pass.
+///
+/// @param circuit A pointer to the circuit on which to run the pass.
+/// @param fix_clifford If ``true``, leave the Clifford gates at the end of the circuit. If
+///   ``false`` they are omitted.
+///
+/// # Safety
+///
+/// Behavior is undefined if ``circuit`` is not valid, non-null pointers to a ``QkCircuit``.
+#[unsafe(no_mangle)]
+pub unsafe extern "C" fn qk_transpiler_pass_standalone_litinski_transformation(
+    circuit: *mut CircuitData,
+    fix_clifford: bool,
+) {
+    // SAFETY: The user guarantees the pointer is safe to read.
+    let circuit = unsafe { mut_ptr_as_ref(circuit) };
+    let dag = DAGCircuit::from_circuit_data(circuit, false, None, None, None, None)
+        .expect("Internal Circuit -> DAG conversion failed");
+
+    let maybe_out = run_litinski_transformation(&dag, fix_clifford, false, true)
+        .expect("Failed running Litinski transformation");
+    // If a DAG is returned, the circuit has been modified. Else just leave it as is.
+    if let Some(out) = maybe_out {
+        *circuit =
+            CircuitData::from_dag_ref(&out).expect("Internal DAG -> Circuit conversion failed")
+    };
+}

crates/cext/src/transpiler/passes/mod.rs

@@ -13,9 +13,11 @@
 pub mod basis_translator;
 pub mod commutative_cancellation;
 pub mod consolidate_blocks;
+pub mod convert_to_pauli_rotations;
 pub mod elide_permutations;
 pub mod gate_direction;
 pub mod inverse_cancellation;
+pub mod litinski_transformation;
 pub mod optimize_1q_sequences;
 pub mod remove_diagonal_gates_before_measure;
 pub mod remove_identity_equiv;

crates/transpiler/src/passes/convert_to_pauli_rotations.rs

@@ -480,7 +480,7 @@ fn generate_pauli_product_rotation_gate(paulis: &[BitTerm], angle: Param) -> Pau
 /// the pass.
 #[pyfunction]
 #[pyo3(name = "convert_to_pauli_rotations")]
-pub fn py_convert_to_pauli_rotations(dag: &mut DAGCircuit) -> PyResult<DAGCircuit> {
+pub fn py_convert_to_pauli_rotations(dag: &DAGCircuit) -> PyResult<DAGCircuit> {
     let mut new_dag = dag.copy_empty_like(VarsMode::Alike, BlocksMode::Drop)?;
 
     // Iterate over nodes in the DAG and collect nodes

releasenotes/notes/pbc-transformations-c-7dc8230831d133c3.yaml

@@ -0,0 +1,9 @@
+---
+features_c:
+  - Added :c:func:`qk_transpiler_pass_standalone_litinski_transformation` to apply a Litinski
+    transformation to a ``QkCircuit`` in the C API. This is the equivalent of Python's
+    :class:`.LitinskiTransformation` pass.
+  - Added :c:func:`qk_transpiler_pass_standalone_convert_to_pauli_rotations` to convert a 
+    ``QkCircuit`` made up of standard gates and measurements into Pauli-based computation format 
+    using ``QkPauliProductRotation`` and ``QkPauliProductMeasurement`` instructions.
+    This is the equivalent of Python's :class:`.ConvertToPauliRotations` pass.