Fix lost nodes in branch and bound

[hlinsen]

Fix lost branch-and-bound nodes when a plunge exits early due to LP solve limits.

• Requeues pending plunge nodes when the LP solve hits CONCURRENT_LIMIT.
• Requeues the current node and pending stack when the LP solve hits ITERATION_LIMIT.
• Ensures pending local stack nodes are returned to the global queue before plunge_with exits early, so the solver does not silently drop unexplored nodes or compute
  an incorrect lower bound.

[coderabbitai]

📝 Walkthrough

Walkthrough

The branch-and-bound solver's plunge_with method now tracks whether the dual solver terminates early due to concurrency or iteration limits. When either limit is reached, the current node is re-inserted onto the local stack and a flag is set. After the main loop, any remaining pending nodes are flushed to the global queue, either due to convergence or because nodes were unconsumed during early termination.

Changes

Pending node requeuing on early termination

Layer / File(s)	Summary
Requeue pending nodes on early termination cpp/src/branch_and_bound/branch_and_bound.cpp	Initialize requeue_pending_nodes flag at method entry. When dual solver hits CONCURRENT_LIMIT or ITERATION_LIMIT, push the current node back to the local stack, set the flag, and break. After the main loop, broaden the flush condition to also re-queue remaining local nodes when the flag is set.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title 'Fix lost nodes in branch and bound' directly relates to the main objective stated in the PR description: re-stacking nodes when encountering iteration and concurrent limits to prevent lost nodes in branch and bound.
Description check	✅ Passed	The PR description accurately describes the changes made to branch_and_bound.cpp, addressing how nodes are requeued when LP solve hits iteration/concurrent limits.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[coderabbitai]

🧹 Nitpick comments (1)

cpp/src/branch_and_bound/branch_and_bound.cpp (1)

1485-1492: Optional cleanup: merge identical limit branches.

Both status branches perform the same operations; collapsing them reduces maintenance drift.

♻️ Proposed simplification

-    } else if (lp_status == dual::status_t::CONCURRENT_LIMIT) {
-      stack.push_front(node_ptr);
-      requeue_pending_nodes = true;
-      break;
-    } else if (lp_status == dual::status_t::ITERATION_LIMIT) {
+    } else if (lp_status == dual::status_t::CONCURRENT_LIMIT ||
+               lp_status == dual::status_t::ITERATION_LIMIT) {
       stack.push_front(node_ptr);
       requeue_pending_nodes = true;
       break;
     }

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@cpp/src/branch_and_bound/branch_and_bound.cpp` around lines 1485 - 1492, The
two consecutive branches checking lp_status == dual::status_t::CONCURRENT_LIMIT
and lp_status == dual::status_t::ITERATION_LIMIT perform identical actions;
collapse them into a single condition to avoid duplication. Replace the two
else-if blocks with a single branch that checks if lp_status is either
dual::status_t::CONCURRENT_LIMIT or dual::status_t::ITERATION_LIMIT, then call
stack.push_front(node_ptr), set requeue_pending_nodes = true, and break;
referencing the existing lp_status variable and the
stack.push_front(node_ptr)/requeue_pending_nodes/break sequence.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In `@cpp/src/branch_and_bound/branch_and_bound.cpp`:
- Around line 1485-1492: The two consecutive branches checking lp_status ==
dual::status_t::CONCURRENT_LIMIT and lp_status ==
dual::status_t::ITERATION_LIMIT perform identical actions; collapse them into a
single condition to avoid duplication. Replace the two else-if blocks with a
single branch that checks if lp_status is either
dual::status_t::CONCURRENT_LIMIT or dual::status_t::ITERATION_LIMIT, then call
stack.push_front(node_ptr), set requeue_pending_nodes = true, and break;
referencing the existing lp_status variable and the
stack.push_front(node_ptr)/requeue_pending_nodes/break sequence.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: a1bddad0-abbf-4409-901c-017c5fabffd6

📥 Commits

Reviewing files that changed from the base of the PR and between 55fb9b9 and 0fed0c7.

📒 Files selected for processing (3)

• cpp/src/branch_and_bound/branch_and_bound.cpp
• cpp/src/mip_heuristics/diversity/lns/rins.cu
• cpp/src/mip_heuristics/diversity/lns/rins.cuh

[nguidotti]

Looks good to me! Thanks for the fix, Hugo.

[github-actions]

🔔 Hi @anandhkb, this pull request has had no activity for 7 days. Please update or let us know if it can be closed. Thank you!

If this is an "epic" issue, then please add the "epic" label to this issue.
If it is a PR and not ready for review, then please convert this to draft.
If you just want to switch off this notification, then use the "skip inactivity reminder" label.

[github-actions]

🔔 Hi @anandhkb, this pull request has had no activity for 7 days. Please update or let us know if it can be closed. Thank you!

If this is an "epic" issue, then please add the "epic" label to this issue.
If it is a PR and not ready for review, then please convert this to draft.
If you just want to switch off this notification, then use the "skip inactivity reminder" label.

[hlinsen]

/merge