Simplify backbone/parallel code from PR #9343

- search_tree.h: move #pragma once before includes (correct ordering)
- search_tree.h: replace nested ternary in display() with switch statement
- search_tree.h: move find_nonclosed_nodes_with_literal_rec to private section
- smt_parallel.cpp: simplify check_backbone return to single expression
- smt_parallel.cpp: use vector constructor instead of resize for threads
- smt_parallel.h: move comment inside update_max_thread_conflicts body

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/smt/smt_parallel.cpp

@@ -601,14 +601,7 @@ namespace smt {
 
         LOG_BB_WORKER(1, " RESULT: " << r << " FOR CANDIDATE: " << mk_bounded_pp(bb_candidate, m, 3) << "\n");
 
-        if (r == l_false) {
+        return r == l_false && ctx->unsat_core().size() == 1;
-            auto core = ctx->unsat_core();
-            if (core.size() == 1) {
-                return true;
-            }                
-        }
-
-        return false;
     }
 
     void parallel::worker::share_units() {
@@ -1241,8 +1234,7 @@ namespace smt {
         m_batch_manager.initialize(num_global_bb_threads);
         
         // Launch threads
-        vector<std::thread> threads;
+        vector<std::thread> threads(total_threads);
-        threads.resize(total_threads);
         unsigned thread_idx = 0;
         for (auto* w : m_workers) 
             threads[thread_idx++] = std::thread([&, w]() { w->run(); });                

src/smt/smt_parallel.h

@@ -234,8 +234,9 @@ namespace smt {
             void share_units();
 
             void update_max_thread_conflicts() {
+                // allow for backoff scheme of conflicts within the thread for cube timeouts.
                 m_config.m_threads_max_conflicts = (unsigned)(m_config.m_max_conflict_mul * m_config.m_threads_max_conflicts);
-            } // allow for backoff scheme of conflicts within the thread for cube timeouts.
+            }
 
             void simplify();
             bb_candidates find_backbone_candidates(unsigned k = 10);

src/util/search_tree.h

@@ -32,9 +32,9 @@ Author:
 
 --*/
 
+#pragma once
 #include "util/util.h"
 #include "util/vector.h"
-#pragma once
 
 namespace search_tree {
 
@@ -100,7 +100,11 @@ namespace search_tree {
             for (unsigned i = 0; i < indent; ++i)
                 out << " ";
             Config::display_literal(out, m_literal);
-            out << (get_status() == status::open ? " (o)" : get_status() == status::closed ? " (c)" : " (a)");
+            switch (get_status()) {
+            case status::open:   out << " (o)"; break;
+            case status::closed: out << " (c)"; break;
+            default:             out << " (a)"; break;
+            }
             out << "\n";
             if (m_left)
                 m_left->display(out, indent + 2);
@@ -428,6 +432,15 @@ namespace search_tree {
             return res;
         }
 
+        void find_nonclosed_nodes_with_literal_rec(node<Config>* n, literal const& lit, ptr_vector<node<Config>>& out) {
+            if (!n)
+                return;
+            if (!Config::literal_is_null(n->get_literal()) && n->get_literal() == lit && n->get_status() != status::closed)
+                out.push_back(n);
+            find_nonclosed_nodes_with_literal_rec(n->left(), lit, out);
+            find_nonclosed_nodes_with_literal_rec(n->right(), lit, out);
+        }
+
     public:
         tree(literal const &null_literal) : m_null_literal(null_literal) {
             reset();
@@ -528,17 +541,6 @@ namespace search_tree {
             find_nonclosed_nodes_with_literal_rec(m_root.get(), lit, out);
         }
 
-        void find_nonclosed_nodes_with_literal_rec(node<Config>* n, literal const& lit, ptr_vector<node<Config>>& out) {
-            if (!n)
-                return;
-
-            if (!Config::literal_is_null(n->get_literal()) && n->get_literal() == lit && n->get_status() != status::closed)
-                out.push_back(n);
-
-            find_nonclosed_nodes_with_literal_rec(n->left(), lit, out);
-            find_nonclosed_nodes_with_literal_rec(n->right(), lit, out);
-        }
-
         void dec_active_workers(node<Config>* n) {
             if (!n)
                 return;