Fix panic in dump reordering after swap (#67)

This fixes an issue where state dump could crash if run after a swap of qubit ids. Specifically, a qubit id swap followed by a release of one of the qubits can put the id map into a state where not every desired id for the swap is occupied by an existing qubit. As a result, updating the map for the swapped key should be a best effort and not a panic if the key is not present.  Also adds a unit test to cover this specific case.

backend/src/simulator.rs

@@ -82,13 +82,11 @@ impl QuantumSim {
         sorted_keys.iter().enumerate().for_each(|(index, &key)| {
             if index != self.id_map[&key] {
                 self.swap_qubit_state(self.id_map[&key], index);
-                let swapped_key = *self
-                    .id_map
-                    .iter()
-                    .find(|(_, &value)| value == index)
-                    .unwrap()
-                    .0;
-                *(self.id_map.get_mut(&swapped_key).unwrap()) = self.id_map[&key];
+                if let Some((&swapped_key, _)) =
+                    self.id_map.iter().find(|(_, &value)| value == index)
+                {
+                    *(self.id_map.get_mut(&swapped_key).unwrap()) = self.id_map[&key];
+                }
                 *(self.id_map.get_mut(&key).unwrap()) = index;
             }
         });
@@ -177,13 +175,11 @@ impl QuantumSim {
         sorted_keys.iter().enumerate().for_each(|(index, &key)| {
             if index != self.id_map[&key] {
                 self.swap_qubit_state(self.id_map[&key], index);
-                let swapped_key = *self
-                    .id_map
-                    .iter()
-                    .find(|(_, &value)| value == index)
-                    .unwrap()
-                    .0;
-                *(self.id_map.get_mut(&swapped_key).unwrap()) = self.id_map[&key];
+                if let Some((&swapped_key, _)) =
+                    self.id_map.iter().find(|(_, &value)| value == index)
+                {
+                    *(self.id_map.get_mut(&swapped_key).unwrap()) = self.id_map[&key];
+                }
                 *(self.id_map.get_mut(&key).unwrap()) = index;
             }
         });
@@ -1270,6 +1266,17 @@ mod tests {
         assert_eq!(val1, val2);
     }
 
+    /// Verify that dump after swap on released qubits doesn't crash.
+    #[test]
+    fn test_swap_dump() {
+        let mut sim = QuantumSim::new();
+        let q = sim.allocate();
+        let inner_q = sim.allocate();
+        sim.swap_qubit_ids(q, inner_q);
+        sim.release(inner_q);
+        sim.dump();
+    }
+
     /// Utility for testing operation equivalence.
     fn assert_operation_equal_referenced<F1, F2>(mut op: F1, mut reference: F2, count: usize)
     where