Add method to add instructions to a DAGCircuit from an iterator of Pa…

…ckedInstruction (#13032)

* Initial: Add add_from_iter method to DAGCircuit
- Introduce a method that adds a chain of `PackedInstruction` continuously avoiding the re-linking of each bit's output-node until the very end of the iterator.
   - TODO: Add handling of vars
- Add header for a `from_iter` function that will create a `DAGCircuit` based on a chain of `PackedInstruction`.

* Fix: leverage new methods in layers
- Fix incorrect re-insertion of last_node.

* Fix: Keep track of Vars for add_from_iter
- Remove `from_iter`

* Fix: Incorrect modification of last nodes in `add_from_iter`.
- Use `entry` api to either modify or insert a value if missing.

* Fix: Cycling edges in when adding vars.
- A bug that adds duplicate edges to vars has been temporarily fixed. However, the root of this problem hasn't been found yet. A proper fix is pending. For now skip those instances.

* Fix: Remove set collecting all nodes to be connected.
- A set collecting all the new nodes to connect with a new node was preventing additional wires to connect to subsequent nodes.

* Fix: Adapt to #13033

* Refactor: `add_from_iter` is now called `extend` to stick with `Rust` nomenclature.

* Fix docstring

- Caught by @ElePT

Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>

* Fix: Remove duplicate vars check

* Fix: Corrections from code review.
- Use Entry API to modify last nodes in the var.
- Build new_nodes with an allocated vec.
- Add comment explaining the removal of the edge between the output node and its predecessor.

* Fix: Improper use of `Entry API`.
- Use `or_insert_with` instead of `or_insert` to perform actions before inserting a value.

---------

Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>

crates/circuit/src/dag_circuit.rs

@@ -4350,9 +4350,7 @@ def _format(operand):
 
             let mut new_layer = self.copy_empty_like(py, vars_mode)?;
 
-            for (node, _) in op_nodes {
-                new_layer.push_back(py, node.clone())?;
-            }
+            new_layer.extend(py, op_nodes.iter().map(|(inst, _)| (*inst).clone()))?;
 
             let new_layer_op_nodes = new_layer.op_nodes(false).filter_map(|node_index| {
                 match new_layer.dag.node_weight(node_index) {
@@ -6366,6 +6364,143 @@ impl DAGCircuit {
             Err(DAGCircuitError::new_err("Specified node is not an op node"))
         }
     }
+
+    /// Extends the DAG with valid instances of [PackedInstruction]
+    pub fn extend<I>(&mut self, py: Python, iter: I) -> PyResult<Vec<NodeIndex>>
+    where
+        I: IntoIterator<Item = PackedInstruction>,
+    {
+        // Create HashSets to keep track of each bit/var's last node
+        let mut qubit_last_nodes: HashMap<Qubit, NodeIndex> = HashMap::default();
+        let mut clbit_last_nodes: HashMap<Clbit, NodeIndex> = HashMap::default();
+        // TODO: Refactor once Vars are in rust
+        // Dict [ Var: (int, VarWeight)]
+        let vars_last_nodes: Bound<PyDict> = PyDict::new_bound(py);
+
+        // Consume into iterator to obtain size hint
+        let iter = iter.into_iter();
+        // Store new nodes to return
+        let mut new_nodes = Vec::with_capacity(iter.size_hint().1.unwrap_or_default());
+        for instr in iter {
+            let op_name = instr.op.name();
+            let (all_cbits, vars): (Vec<Clbit>, Option<Vec<PyObject>>) = {
+                if self.may_have_additional_wires(py, &instr) {
+                    let mut clbits: HashSet<Clbit> =
+                        HashSet::from_iter(self.cargs_interner.get(instr.clbits).iter().copied());
+                    let (additional_clbits, additional_vars) =
+                        self.additional_wires(py, instr.op.view(), instr.condition())?;
+                    for clbit in additional_clbits {
+                        clbits.insert(clbit);
+                    }
+                    (clbits.into_iter().collect(), Some(additional_vars))
+                } else {
+                    (self.cargs_interner.get(instr.clbits).to_vec(), None)
+                }
+            };
+
+            // Increment the operation count
+            self.increment_op(op_name);
+
+            // Get the correct qubit indices
+            let qubits_id = instr.qubits;
+
+            // Insert op-node to graph.
+            let new_node = self.dag.add_node(NodeType::Operation(instr));
+            new_nodes.push(new_node);
+
+            // Check all the qubits in this instruction.
+            for qubit in self.qargs_interner.get(qubits_id) {
+                // Retrieve each qubit's last node
+                let qubit_last_node = *qubit_last_nodes.entry(*qubit).or_insert_with(|| {
+                    // If the qubit is not in the last nodes collection, the edge between the output node and its predecessor.
+                    // Then, store the predecessor's NodeIndex in the last nodes collection.
+                    let output_node = self.qubit_io_map[qubit.0 as usize][1];
+                    let (edge_id, predecessor_node) = self
+                        .dag
+                        .edges_directed(output_node, Incoming)
+                        .next()
+                        .map(|edge| (edge.id(), edge.source()))
+                        .unwrap();
+                    self.dag.remove_edge(edge_id);
+                    predecessor_node
+                });
+                qubit_last_nodes
+                    .entry(*qubit)
+                    .and_modify(|val| *val = new_node);
+                self.dag
+                    .add_edge(qubit_last_node, new_node, Wire::Qubit(*qubit));
+            }
+
+            // Check all the clbits in this instruction.
+            for clbit in all_cbits {
+                let clbit_last_node = *clbit_last_nodes.entry(clbit).or_insert_with(|| {
+                    // If the qubit is not in the last nodes collection, the edge between the output node and its predecessor.
+                    // Then, store the predecessor's NodeIndex in the last nodes collection.
+                    let output_node = self.clbit_io_map[clbit.0 as usize][1];
+                    let (edge_id, predecessor_node) = self
+                        .dag
+                        .edges_directed(output_node, Incoming)
+                        .next()
+                        .map(|edge| (edge.id(), edge.source()))
+                        .unwrap();
+                    self.dag.remove_edge(edge_id);
+                    predecessor_node
+                });
+                clbit_last_nodes
+                    .entry(clbit)
+                    .and_modify(|val| *val = new_node);
+                self.dag
+                    .add_edge(clbit_last_node, new_node, Wire::Clbit(clbit));
+            }
+
+            // If available, check all the vars in this instruction
+            for var in vars.iter().flatten() {
+                let var_last_node = if let Some(result) = vars_last_nodes.get_item(var)? {
+                    let node: usize = result.extract()?;
+                    vars_last_nodes.del_item(var)?;
+                    NodeIndex::new(node)
+                } else {
+                    // If the var is not in the last nodes collection, the edge between the output node and its predecessor.
+                    // Then, store the predecessor's NodeIndex in the last nodes collection.
+                    let output_node = self.var_output_map.get(py, var).unwrap();
+                    let (edge_id, predecessor_node) = self
+                        .dag
+                        .edges_directed(output_node, Incoming)
+                        .next()
+                        .map(|edge| (edge.id(), edge.source()))
+                        .unwrap();
+                    self.dag.remove_edge(edge_id);
+                    predecessor_node
+                };
+
+                vars_last_nodes.set_item(var, new_node.index())?;
+                self.dag
+                    .add_edge(var_last_node, new_node, Wire::Var(var.clone_ref(py)));
+            }
+        }
+
+        // Add the output_nodes back to qargs
+        for (qubit, node) in qubit_last_nodes {
+            let output_node = self.qubit_io_map[qubit.0 as usize][1];
+            self.dag.add_edge(node, output_node, Wire::Qubit(qubit));
+        }
+
+        // Add the output_nodes back to cargs
+        for (clbit, node) in clbit_last_nodes {
+            let output_node = self.clbit_io_map[clbit.0 as usize][1];
+            self.dag.add_edge(node, output_node, Wire::Clbit(clbit));
+        }
+
+        // Add the output_nodes back to vars
+        for item in vars_last_nodes.items() {
+            let (var, node): (PyObject, usize) = item.extract()?;
+            let output_node = self.var_output_map.get(py, &var).unwrap();
+            self.dag
+                .add_edge(NodeIndex::new(node), output_node, Wire::Var(var));
+        }
+
+        Ok(new_nodes)
+    }
 }
 
 /// Add to global phase. Global phase can only be Float or ParameterExpression so this