[Oxidize BasisTranslator]: Move basis_search and `BasisSearchVisito…

…r` to rust. (#12811)

* Add: Basis search function
- Add rust counterpart for `basis_search`.
- Consolidated the `BasisSearchVisitor` into the function due to differences in rust behavior.

* Fix: Wrong return value for `basis_search`

* Fix: Remove `IndexMap` and duplicate declarations.

* Fix: Adapt to #12730

* Remove: unused imports

* Docs: Edit docstring for rust native `basis_search`

* Fix: Use owned Strings.
- Due to the nature of `hashbrown` we must use owned Strings instead of `&str`.

* Add: mutable graph view that the `BasisTranslator` can access in Rust.
- Remove import of `random` in `basis_translator`.

* Fix: Review comments
- Rename `EquivalenceLibrary`'s `mut_graph` method to `graph_mut` to keep consistent with rust naming conventions.
- Use `&HashSet<String>` instead of `HashSet<&str>` to avoid extra conversion.
- Use `u32::MAX` as num_qubits for dummy node.
- Use for loop instead of foreachj to add edges to dummy node.
- Add comment explaining usage of flatten in `initialize_num_gates_remain_for_rule`.
- Remove stale comments.

* Update crates/accelerate/src/basis/basis_translator/basis_search.rs

---------

Co-authored-by: Matthew Treinish <[email protected]>

crates/accelerate/src/basis/basis_translator/basis_search.rs

@@ -0,0 +1,217 @@
+// This code is part of Qiskit.
+//
+// (C) Copyright IBM 2024
+//
+// 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 http://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 std::cell::RefCell;
+
+use hashbrown::{HashMap, HashSet};
+use pyo3::prelude::*;
+
+use crate::equivalence::{EdgeData, Equivalence, EquivalenceLibrary, Key, NodeData};
+use qiskit_circuit::operations::Operation;
+use rustworkx_core::petgraph::stable_graph::{EdgeReference, NodeIndex, StableDiGraph};
+use rustworkx_core::petgraph::visit::Control;
+use rustworkx_core::traversal::{dijkstra_search, DijkstraEvent};
+
+use super::compose_transforms::{BasisTransformIn, GateIdentifier};
+
+/// Search for a set of transformations from source_basis to target_basis.
+/// Args:
+///     equiv_lib (EquivalenceLibrary): Source of valid translations
+///     source_basis (Set[Tuple[gate_name: str, gate_num_qubits: int]]): Starting basis.
+///     target_basis (Set[gate_name: str]): Target basis.
+///
+/// Returns:
+///     Optional[List[Tuple[gate, equiv_params, equiv_circuit]]]: List of (gate,
+///         equiv_params, equiv_circuit) tuples tuples which, if applied in order
+///         will map from source_basis to target_basis. Returns None if no path
+///         was found.
+#[pyfunction]
+#[pyo3(name = "basis_search")]
+pub(crate) fn py_basis_search(
+    py: Python,
+    equiv_lib: &mut EquivalenceLibrary,
+    source_basis: HashSet<GateIdentifier>,
+    target_basis: HashSet<String>,
+) -> PyObject {
+    basis_search(equiv_lib, &source_basis, &target_basis).into_py(py)
+}
+
+type BasisTransforms = Vec<(GateIdentifier, BasisTransformIn)>;
+/// Search for a set of transformations from source_basis to target_basis.
+///
+/// Performs a Dijkstra search algorithm on the `EquivalenceLibrary`'s core graph
+/// to rate and classify different possible equivalent circuits to the provided gates.
+///
+/// This is done by connecting all the nodes represented in the `target_basis` to a dummy
+/// node, and then traversing the graph until all the nodes described in the `source
+/// basis` are reached.
+pub(crate) fn basis_search(
+    equiv_lib: &mut EquivalenceLibrary,
+    source_basis: &HashSet<GateIdentifier>,
+    target_basis: &HashSet<String>,
+) -> Option<BasisTransforms> {
+    // Build the visitor attributes:
+    let mut num_gates_remaining_for_rule: HashMap<usize, usize> = HashMap::default();
+    let predecessors: RefCell<HashMap<GateIdentifier, Equivalence>> =
+        RefCell::new(HashMap::default());
+    let opt_cost_map: RefCell<HashMap<GateIdentifier, u32>> = RefCell::new(HashMap::default());
+    let mut basis_transforms: Vec<(GateIdentifier, BasisTransformIn)> = vec![];
+
+    // Initialize visitor attributes:
+    initialize_num_gates_remain_for_rule(equiv_lib.graph(), &mut num_gates_remaining_for_rule);
+
+    let mut source_basis_remain: HashSet<Key> = source_basis
+        .iter()
+        .filter_map(|(gate_name, gate_num_qubits)| {
+            if !target_basis.contains(gate_name) {
+                Some(Key {
+                    name: gate_name.to_string(),
+                    num_qubits: *gate_num_qubits,
+                })
+            } else {
+                None
+            }
+        })
+        .collect();
+
+    // If source_basis is empty, no work needs to be done.
+    if source_basis_remain.is_empty() {
+        return Some(vec![]);
+    }
+
+    // This is only necessary since gates in target basis are currently reported by
+    // their names and we need to have in addition the number of qubits they act on.
+    let target_basis_keys: Vec<Key> = equiv_lib
+        .keys()
+        .filter(|&key| target_basis.contains(key.name.as_str()))
+        .cloned()
+        .collect();
+
+    // Dummy node is inserted in the graph. Which is where the search will start
+    let dummy: NodeIndex = equiv_lib.graph_mut().add_node(NodeData {
+        equivs: vec![],
+        key: Key {
+            name: "key".to_string(),
+            num_qubits: u32::MAX,
+        },
+    });
+
+    // Extract indices for the target_basis gates, to avoid borrowing from graph.
+    let target_basis_indices: Vec<NodeIndex> = target_basis_keys
+        .iter()
+        .map(|key| equiv_lib.node_index(key))
+        .collect();
+
+    // Connect each edge in the target_basis to the dummy node.
+    for node in target_basis_indices {
+        equiv_lib.graph_mut().add_edge(dummy, node, None);
+    }
+
+    // Edge cost function for Visitor
+    let edge_weight = |edge: EdgeReference<Option<EdgeData>>| -> Result<u32, ()> {
+        if edge.weight().is_none() {
+            return Ok(1);
+        }
+        let edge_data = edge.weight().as_ref().unwrap();
+        let mut cost_tot = 0;
+        let borrowed_cost = opt_cost_map.borrow();
+        for instruction in edge_data.rule.circuit.0.iter() {
+            let instruction_op = instruction.op.view();
+            cost_tot += borrowed_cost[&(
+                instruction_op.name().to_string(),
+                instruction_op.num_qubits(),
+            )];
+        }
+        Ok(cost_tot
+            - borrowed_cost[&(
+                edge_data.source.name.to_string(),
+                edge_data.source.num_qubits,
+            )])
+    };
+
+    let basis_transforms = match dijkstra_search(
+        &equiv_lib.graph(),
+        [dummy],
+        edge_weight,
+        |event: DijkstraEvent<NodeIndex, &Option<EdgeData>, u32>| {
+            match event {
+                DijkstraEvent::Discover(n, score) => {
+                    let gate_key = &equiv_lib.graph()[n].key;
+                    let gate = (gate_key.name.to_string(), gate_key.num_qubits);
+                    source_basis_remain.remove(gate_key);
+                    let mut borrowed_cost_map = opt_cost_map.borrow_mut();
+                    if let Some(entry) = borrowed_cost_map.get_mut(&gate) {
+                        *entry = score;
+                    } else {
+                        borrowed_cost_map.insert(gate.clone(), score);
+                    }
+                    if let Some(rule) = predecessors.borrow().get(&gate) {
+                        basis_transforms.push((
+                            (gate_key.name.to_string(), gate_key.num_qubits),
+                            (rule.params.clone(), rule.circuit.clone()),
+                        ));
+                    }
+
+                    if source_basis_remain.is_empty() {
+                        basis_transforms.reverse();
+                        return Control::Break(());
+                    }
+                }
+                DijkstraEvent::EdgeRelaxed(_, target, Some(edata)) => {
+                    let gate = &equiv_lib.graph()[target].key;
+                    predecessors
+                        .borrow_mut()
+                        .entry((gate.name.to_string(), gate.num_qubits))
+                        .and_modify(|value| *value = edata.rule.clone())
+                        .or_insert(edata.rule.clone());
+                }
+                DijkstraEvent::ExamineEdge(_, target, Some(edata)) => {
+                    num_gates_remaining_for_rule
+                        .entry(edata.index)
+                        .and_modify(|val| *val -= 1)
+                        .or_insert(0);
+                    let target = &equiv_lib.graph()[target].key;
+
+                    // If there are gates in this `rule` that we have not yet generated, we can't apply
+                    // this `rule`. if `target` is already in basis, it's not beneficial to use this rule.
+                    if num_gates_remaining_for_rule[&edata.index] > 0
+                        || target_basis_keys.contains(target)
+                    {
+                        return Control::Prune;
+                    }
+                }
+                _ => {}
+            };
+            Control::Continue
+        },
+    ) {
+        Ok(Control::Break(_)) => Some(basis_transforms),
+        _ => None,
+    };
+    equiv_lib.graph_mut().remove_node(dummy);
+    basis_transforms
+}
+
+fn initialize_num_gates_remain_for_rule(
+    graph: &StableDiGraph<NodeData, Option<EdgeData>>,
+    source: &mut HashMap<usize, usize>,
+) {
+    let mut save_index = usize::MAX;
+    // When iterating over the edges, ignore any none-valued ones by calling `flatten`
+    for edge_data in graph.edge_weights().flatten() {
+        if save_index == edge_data.index {
+            continue;
+        }
+        source.insert(edge_data.index, edge_data.num_gates);
+        save_index = edge_data.index;
+    }
+}

crates/accelerate/src/basis/basis_translator/mod.rs

@@ -12,10 +12,12 @@
 
 use pyo3::prelude::*;
 
+pub mod basis_search;
 mod compose_transforms;
 
 #[pymodule]
 pub fn basis_translator(m: &Bound<PyModule>) -> PyResult<()> {
+    m.add_wrapped(wrap_pyfunction!(basis_search::py_basis_search))?;
     m.add_wrapped(wrap_pyfunction!(compose_transforms::py_compose_transforms))?;
     Ok(())
 }

crates/accelerate/src/equivalence.rs

@@ -697,6 +697,11 @@ impl EquivalenceLibrary {
     pub fn graph(&self) -> &GraphType {
         &self.graph
     }
+
+    /// Expose a mutable view of the inner graph.
+    pub(crate) fn graph_mut(&mut self) -> &mut GraphType {
+        &mut self.graph
+    }
 }
 
 fn raise_if_param_mismatch(

qiskit/__init__.py

@@ -56,6 +56,8 @@
 sys.modules["qiskit._accelerate.basis"] = _accelerate.basis
 sys.modules["qiskit._accelerate.basis.basis_translator"] = _accelerate.basis.basis_translator
 sys.modules["qiskit._accelerate.converters"] = _accelerate.converters
+sys.modules["qiskit._accelerate.basis"] = _accelerate.basis
+sys.modules["qiskit._accelerate.basis.basis_translator"] = _accelerate.basis.basis_translator
 sys.modules["qiskit._accelerate.convert_2q_block_matrix"] = _accelerate.convert_2q_block_matrix
 sys.modules["qiskit._accelerate.dense_layout"] = _accelerate.dense_layout
 sys.modules["qiskit._accelerate.equivalence"] = _accelerate.equivalence