Fix random seed initialization between runs in SabreSwap (backport #9127

) (#9132)

* Fix random seed initialization between runs in SabreSwap (#9127)

* Fix random seed initialization between runs in SabreSwap

This commit fixes an issue in the case no random seed is provided by the
user when initializing an instance of SabreSwap. The pass was previously
potentially reusing the random seed between runs even if no seed was
specified. This was an artifact of storing the initial seed as an
instance variable. Instead this commit just relies on the the Rust RNG
to initialize from entropy if no seed is specified.

* Remove unused numpy import

(cherry picked from commit dfbc738)

* Restore numpy import

On the main branch the SabreSwap python code has been refactored so it
no longer is using numpy besides the previous use for the RNG. Removing
the numpy RNG usage in this PR meant also removing the numpy import as
nothing else was using it anymore. However, the version of SabreSwap in
stable/0.22 does not have this refactor and numpy is still used to
construct intermediate objects which are passed to rhe rust code. This
commit restores the import to fix the backported PR so it works with the
version of SabreSwap in stable/0.22

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

qiskit/transpiler/passes/routing/sabre_swap.py

@@ -151,12 +151,7 @@ def __init__(self, coupling_map, heuristic="basic", seed=None, fake_run=False, t
             self._neighbor_table = NeighborTable(retworkx.adjacency_matrix(self.coupling_map.graph))
 
         self.heuristic = heuristic
-
-        if seed is None:
-            ii32 = np.iinfo(np.int32)
-            self.seed = np.random.default_rng(None).integers(0, ii32.max, dtype=int)
-        else:
-            self.seed = seed
+        self.seed = seed
         if trials is None:
             self.trials = CPU_COUNT
         else:

releasenotes/notes/fix-sabre-swap-random-seed-dcf3dace63042791.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixed an issue with the :class:`~.SabreSwap` pass which would cause the
+    output of multiple runs of the pass without the ``seed`` argument specified
+    to reuse the same random number generator seed between runs instead of
+    using different seeds. This previously caused identical results to be
+    returned between runs even when no ``seed`` was specified.

src/sabre_swap/mod.rs

@@ -158,14 +158,17 @@ pub fn build_swap_map(
     neighbor_table: &NeighborTable,
     distance_matrix: PyReadonlyArray2<f64>,
     heuristic: &Heuristic,
-    seed: u64,
+    seed: Option<u64>,
     layout: &mut NLayout,
     num_trials: usize,
 ) -> (SwapMap, PyObject) {
     let run_in_parallel = getenv_use_multiple_threads();
     let dist = distance_matrix.as_array();
     let coupling_graph: DiGraph<(), ()> = cmap_from_neighor_table(neighbor_table);
-    let outer_rng = Pcg64Mcg::seed_from_u64(seed);
+    let outer_rng = match seed {
+        Some(seed) => Pcg64Mcg::seed_from_u64(seed),
+        None => Pcg64Mcg::from_entropy(),
+    };
     let seed_vec: Vec<u64> = outer_rng
         .sample_iter(&rand::distributions::Standard)
         .take(num_trials)