working on lookahead

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/sat/sat_local_search.h

@@ -219,6 +219,10 @@ namespace sat {
         
         lbool operator()();
 
+        lbool check(unsigned sz, literal const* assumptions) { return l_undef; } // TBD
+
+        void cancel() {} // TBD
+
         local_search_config& config() { return m_config;  }
 
         local_search_config m_config;

src/sat/sat_lookahead.h

@@ -31,6 +31,7 @@ namespace sat {
             unsigned m_max_hlevel; 
             unsigned m_min_cutoff;
             unsigned m_level_cand;
+            float    m_delta_rho;
 
             config() {
                 m_max_hlevel = 50;
@@ -38,15 +39,39 @@ namespace sat {
                 m_max_score = 20.0;
                 m_min_cutoff = 30;
                 m_level_cand = 600;
+                m_delta_rho = (float)0.9995;
             }
         };
 
+        struct prefix {
+            unsigned m_prefix;
+            unsigned m_length;
+            prefix(): m_prefix(0), m_length(0) {}            
+        };
+
+        struct lit_info {
+            float     m_wnb;
+            unsigned  m_double_lookahead;
+            lit_info(): m_wnb(0), m_double_lookahead(0) {}
+        };
+
         struct statistics {
             unsigned m_propagations;
             statistics() { reset(); }
             void reset() { memset(this, 0, sizeof(*this)); }
         };
 
+        enum search_mode {
+            searching,         // normal search
+            lookahead1,        // lookahead mode
+            lookahead2         // double lookahead
+        };
+
+        struct ternary {
+            ternary(literal u, literal v, literal w): m_u(u), m_v(v), m_w(w) {}
+            literal m_u, m_v, m_w;
+        };
+
         config                 m_config;
         double                 m_delta_trigger; 
 
@@ -60,15 +85,25 @@ namespace sat {
         unsigned               m_qhead;         // propagation queue head
         unsigned_vector        m_qhead_lim;
         clause_vector          m_clauses;       // non-binary clauses
+        clause_vector          m_retired_clauses; // clauses that were removed during search
+        svector<ternary>       m_retired_ternary; //
+        unsigned_vector        m_retired_clause_lim; 
         clause_allocator       m_cls_allocator;        
         bool                   m_inconsistent;
         unsigned_vector        m_bstamp;        // literal: timestamp for binary implication
         vector<svector<float> >  m_H;           // literal: fitness score
+        svector<float>*        m_heur;          // current fitness 
         svector<float>         m_rating;        // var:     pre-selection rating
         unsigned               m_bstamp_id;     // unique id for binary implication.
+        unsigned               m_istamp_id;     // unique id for managing double lookaheads
         char_vector            m_assignment;    // literal: assignment 
         vector<watch_list>     m_watches;       // literal: watch structure
+        svector<lit_info>      m_lits;          // literal: attributes.
+        float                  m_weighted_new_binaries; // metric associated with current lookahead1 literal.
+        svector<prefix>        m_prefix;        // var:     prefix where variable participates in propagation
         indexed_uint_set       m_freevars;
+        svector<search_mode>   m_search_modes;  // stack of modes
+        search_mode            m_search_mode;   // mode of search
         statistics             m_stats;
 
         void add_binary(literal l1, literal l2) {
@@ -97,6 +132,15 @@ namespace sat {
                 m_bstamp.fill(0);
             }
         }
+        void inc_istamp() {
+            ++m_istamp_id;
+            if (m_istamp_id == 0) {
+                ++m_istamp_id;
+                for (unsigned i = 0; i < m_lits.size(); ++i) {
+                    m_lits[i].m_double_lookahead = 0;
+                }
+            }
+        }
         void set_bstamp(literal l) {
             m_bstamp[l.index()] = m_bstamp_id;
         }
@@ -159,6 +203,15 @@ namespace sat {
         // pre-selection
         // see also 91 - 102 sat11.w
 
+        void pre_select() {
+            m_lookahead.reset();
+            if (select(scope_lvl())) {
+                get_scc();
+                find_heights();
+                construct_lookahead_table();
+            }
+        }
+
         struct candidate {
             bool_var m_var;
             float    m_rating;
@@ -169,7 +222,7 @@ namespace sat {
         float get_rating(bool_var v) const { return m_rating[v]; }
         float get_rating(literal l) const { return get_rating(l.var()); }
 
-        bool_var select(unsigned level) {
+        bool select(unsigned level) {
             init_pre_selection(level);
             unsigned max_num_cand = level == 0 ? m_freevars.size() : m_config.m_level_cand / level;
             max_num_cand = std::max(m_config.m_min_cutoff, max_num_cand);
@@ -179,7 +232,7 @@ namespace sat {
                 sum = init_candidates(level, newbies);
                 if (!m_candidates.empty()) break;
                 if (is_sat()) {
-                    return null_bool_var;
+                    return false;
                 }            
             }
             SASSERT(!m_candidates.empty());
@@ -218,6 +271,7 @@ namespace sat {
                 }
             }
             SASSERT(!m_candidates.empty() && m_candidates.size() <= max_num_cand);
+            return true;
         }
 
         void sift_up(unsigned j) {
@@ -238,6 +292,7 @@ namespace sat {
             m_candidates.reset();
             float sum = 0;
             for (bool_var const* it = m_freevars.begin(), * end = m_freevars.end(); it != end; ++it) {
+                SASSERT(l_undef == value(*it));
                 bool_var x = *it;
                 if (!newbies) {
                     // TBD filter out candidates based on prefix strings or similar method
@@ -277,17 +332,17 @@ namespace sat {
                         h_scores(m_H[i + 1], m_H[(i + 2) % 3]);
                     }
                 }
-                // heur = m_H[1];
+                m_heur = &m_H[1];
             }
             else if (level < max_level) {
                 ensure_H(level);
                 h_scores(m_H[level-1], m_H[level]);
-                // heur = m_H[level];
+                m_heur = &m_H[level];
             }
             else {
                 ensure_H(max_level);
                 h_scores(m_H[max_level-1], m_H[max_level]);
-                // heur = m_H[max_level];
+                m_heur = &m_H[max_level];
             }
         }
 
@@ -415,9 +470,7 @@ namespace sat {
             }
         }
         void add_arc(literal u, literal v) { m_dfs[u.index()].m_next.push_back(v); }
-        bool has_arc(literal v) const { 
+        bool has_arc(literal v) const { return m_dfs[v.index()].m_next.size() > m_dfs[v.index()].m_nextp; } 
-            return m_dfs[v.index()].m_next.size() > m_dfs[v.index()].m_nextp; 
-        } 
         literal pop_arc(literal u) { return m_dfs[u.index()].m_next[m_dfs[u.index()].m_nextp++]; }
         unsigned num_next(literal u) const { return m_dfs[u.index()].m_next.size(); }
         literal get_next(literal u, unsigned i) const { return m_dfs[u.index()].m_next[i]; }
@@ -512,10 +565,6 @@ namespace sat {
             else m_dfs[v.index()].m_min = u; 
         }
 
-        void construct_forest() {
-            find_heights();
-            construct_lookahead_table();
-        }
         void find_heights() {
             literal pp = null_literal;
             set_child(pp, null_literal);
@@ -562,7 +611,7 @@ namespace sat {
         void construct_lookahead_table() {
             literal u = get_child(null_literal), v = null_literal;
             unsigned offset = 0;
-            m_lookahead.reset();
+            SASSERT(m_lookahead.empty());
             while (u != null_literal) {                
                 set_rank(u, m_lookahead.size());
                 set_lookahead(get_vcomp(u));
@@ -592,6 +641,40 @@ namespace sat {
                     tout << m_lookahead[i].m_lit << " : " << m_lookahead[i].m_offset << "\n";);
         }
 
+        // ------------------------------------
+        // clause management
+
+        void attach_clause(clause& c) {
+            if (false && c.size() == 3) { // disable ternary clauses
+                m_watches[(~c[0]).index()].push_back(watched(c[1], c[2]));
+                m_watches[(~c[1]).index()].push_back(watched(c[0], c[2]));
+                m_watches[(~c[2]).index()].push_back(watched(c[0], c[1]));
+            }
+            else {
+                literal block_lit = c[c.size() >> 2];
+                clause_offset cls_off = m_cls_allocator.get_offset(&c);
+                m_watches[(~c[0]).index()].push_back(watched(block_lit, cls_off));
+                m_watches[(~c[1]).index()].push_back(watched(block_lit, cls_off));
+                SASSERT(value(c[0]) == l_undef);
+                SASSERT(value(c[1]) == l_undef);
+            }
+        }
+
+        void detach_clause(clause& c) {
+            clause_offset cls_off = m_cls_allocator.get_offset(&c);
+            m_retired_clauses.push_back(&c);
+            erase_clause_watch(get_wlist(~c[0]), cls_off);
+            erase_clause_watch(get_wlist(~c[1]), cls_off);
+        }
+
+        void detach_ternary(literal l1, literal l2, literal l3) {
+            m_retired_ternary.push_back(ternary(l1, l2, l3));
+            erase_ternary_watch(get_wlist(~l1), l2, l3);
+            erase_ternary_watch(get_wlist(~l2), l1, l3);
+            erase_ternary_watch(get_wlist(~l3), l1, l2);
+        }
+
+        watch_list& get_wlist(literal l) { return m_watches[l.index()]; }
 
         // ------------------------------------
         // initialization
@@ -608,6 +691,10 @@ namespace sat {
             m_rating.push_back(0);
             m_dfs.push_back(dfs_info());
             m_dfs.push_back(dfs_info());
+            m_lits.push_back(lit_info());
+            m_lits.push_back(lit_info());
+            m_prefix.push_back(prefix());
+            m_freevars.insert(v);
         }
 
         void init() {
@@ -642,8 +729,9 @@ namespace sat {
             clause_vector::const_iterator end = s.m_clauses.end();
             for (; it != end; ++it) {
                 clause& c = *(*it);
-                m_clauses.push_back(m_cls_allocator.mk_clause(c.size(), c.begin(), false));
+                clause* c1 = m_cls_allocator.mk_clause(c.size(), c.begin(), false);
-                // TBD: add watch
+                m_clauses.push_back(c1);
+                attach_clause(c);
             }
 
             // copy units
@@ -655,17 +743,37 @@ namespace sat {
             }
         }
 
-        void push(literal lit) { 
+        // ------------------------------------
+        // search
+        
+        void push(literal lit, search_mode mode) { 
             m_binary_trail_lim.push_back(m_binary_trail.size());
             m_units_lim.push_back(m_units.size());
             m_trail_lim.push_back(m_trail.size());
+            m_retired_clause_lim.push_back(m_retired_clauses.size());
             m_qhead_lim.push_back(m_qhead);
             m_trail.push_back(lit);
+            m_search_modes.push_back(m_search_mode);
+            m_search_mode = mode;
             assign(lit);
             propagate();
         }
 
         void pop() { 
+            m_inconsistent = false;
+
+            // search mode
+            m_search_mode = m_search_modes.back();
+            m_search_modes.pop_back();
+
+            // unretire clauses
+            unsigned rsz = m_retired_clause_lim.back();
+            for (unsigned i = rsz; i < m_retired_clauses.size(); ++i) {
+                attach_clause(*m_retired_clauses[i]);
+            }
+            m_retired_clauses.resize(rsz);
+            m_retired_clause_lim.pop_back();
+
             // remove local binary clauses
             unsigned old_sz = m_binary_trail_lim.back();
             m_binary_trail_lim.pop_back();
@@ -673,24 +781,31 @@ namespace sat {
                 del_binary(m_binary_trail[i]);
             }
 
+            // undo assignments
+            for (unsigned i = m_trail.size(); i > m_trail_lim.size(); ) {
+                --i;
+                literal l = m_trail[i];
+                m_freevars.insert(l.var());
+                m_assignment[l.index()] = l_undef;
+                m_assignment[(~l).index()] = l_undef;
+            }
+            m_trail.shrink(m_trail_lim.size()); // reset assignment.
+            m_trail_lim.pop_back();
+
             // add implied binary clauses
             unsigned new_unit_sz = m_units_lim.back();
             for (unsigned i = new_unit_sz; i < m_units.size(); ++i) {
-                add_binary(~m_trail.back(), m_units[i]);
+                try_add_binary(~m_trail.back(), m_units[i]);
             }
             m_units.shrink(new_unit_sz);
             m_units_lim.pop_back();
-            m_trail.shrink(m_trail_lim.size()); // reset assignment.
+            
-            m_trail_lim.pop_back();
+            // reset propagation queue
             m_qhead_lim.pop_back();
             m_qhead = m_qhead_lim.back();
-
-            m_inconsistent = false;
         }        
 
-        unsigned diff() const { return m_units.size() - m_units_lim.back(); }
+        float mix_diff(float l, float r) const { return l + r + (1 << 10) * l * r; }
-
-        unsigned mix_diff(unsigned l, unsigned r) const { return l + r + (1 << 10) * l * r; }
 
         clause const& get_clause(watch_list::iterator it) const {
             clause_offset cls_off = it->get_clause_offset();
@@ -715,6 +830,7 @@ namespace sat {
                     UNREACHABLE();
                     break;
                 case watched::TERNARY: {
+                    UNREACHABLE(); // we avoid adding ternary clauses for now.
                     literal l1 = it->get_literal1();
                     literal l2 = it->get_literal2();
                     lbool val1 = value(l1);
@@ -731,7 +847,17 @@ namespace sat {
                         set_conflict();
                     }
                     else if (val1 == l_undef && val2 == l_undef) {
-                        // TBD: the clause has become binary.
+                        switch (m_search_mode) {
+                        case searching:
+                            detach_ternary(l, l1, l2);
+                            try_add_binary(l1, l2);
+                            break;
+                        case lookahead1:
+                            m_weighted_new_binaries += (*m_heur)[l1.index()] * (*m_heur)[l2.index()];
+                            break;
+                        case lookahead2:
+                            break;
+                        }
                     }
                     *it2 = *it;
                     it2++;
@@ -750,29 +876,37 @@ namespace sat {
                     }
                     literal * l_it  = c.begin() + 2;
                     literal * l_end = c.end();
-                    unsigned found = 0;
+                    bool found = false;
-                    for (; l_it != l_end && found < 2; ++l_it) {
+                    for (; l_it != l_end && !found; ++l_it) {
                         if (value(*l_it) != l_false) {
-                            ++found;
+                            found = true;
-                            if (found == 2) {
+                            c[1]  = *l_it;
-                                break;
+                            *l_it = ~l;
-                            }
+                            m_watches[(~c[1]).index()].push_back(watched(c[0], cls_off));
-                            else {
-                                c[1]  = *l_it;
-                                *l_it = ~l;
-                                m_watches[(~c[1]).index()].push_back(watched(c[0], cls_off));
-                            }
                         }
                     }
-                    if (found == 1) {
+                    if (found) {
-                        // TBD: clause has become binary
+                        found = false;
+                        for (; l_it != l_end && !found; ++l_it) {
+                            found = value(*l_it) != l_false;
+                        }
+                        // normal clause was converted to a binary clause.
+                        if (!found && value(c[1]) == l_undef && value(c[0]) == l_undef) {
+                            switch (m_search_mode) {
+                            case searching:
+                                detach_clause(c);
+                                try_add_binary(c[0], c[1]);
+                                break;
+                            case lookahead1:
+                                m_weighted_new_binaries += (*m_heur)[c[0].index()]* (*m_heur)[c[1].index()];
+                                break;
+                            case lookahead2:
+                                break;
+                            }
+                        }
                         break; 
                     }
-                    if (found > 1) {
+                    if (value(c[0]) == l_false) {
-                        // not a binary clause
-                        break;
-                    }
-                    else if (value(c[0]) == l_false) {
                         set_conflict();
                     }
                     else {
@@ -829,41 +963,57 @@ namespace sat {
             return l;
         }
 
+        // TBD: 
+        // Handle scope properly for nested implications.
+        // Suppose u -> v, and u -> w and we process v first, then the 
+        // consequences of v should remain when processing u.
+        // March and sat11.w solve this by introducing timestamps on truth values.
+        // regular push/pop doesn't really work here: we basically need a traversal of the
+        // lookahead tree and push/pop according to that (or adapt timestamps)
+        // 
         bool choose1(literal& l) {
-            
+            pre_select();
             l = null_literal;
             if (m_lookahead.empty()) {
                 return true;
             }
-            unsigned h = 0, count = 1;
+            float h = 0;
-            for (unsigned i = 0; i < m_lookahead.size(); ++i) {
+            unsigned count = 1;
+            for (unsigned i = 0; !inconsistent() && i < m_lookahead.size(); ++i) {
                 literal lit = m_lookahead[i].m_lit;
+                if (value(lit) != l_undef) {
+                    continue;
+                }
+                SASSERT(value(lit) == l_undef);
+                SASSERT(!inconsistent());
 
-                push(lit);
+                reset_wnb(lit);
-                if (do_double()) double_look();
+                push(lit, lookahead1);                
+                do_double(lit);
                 if (inconsistent()) {
                     pop();
                     assign(~lit);
-                    if (do_double()) double_look();
+                    propagate();
-                    if (inconsistent()) return true;
                     continue;
                 }
-                unsigned diff1 = diff();
+                update_wnb(lit);
+                float diff1 = m_weighted_new_binaries;
                 pop();
                 
-                push(~lit);
+                reset_wnb(~lit);
-                if (do_double()) double_look();
+                push(~lit, lookahead1);
-                bool unsat2 = inconsistent();
+                do_double(~lit);
-                unsigned diff2 = diff();
+                if (inconsistent()) {
-                pop();
+                    pop();
-
-                if (unsat2) {
                     assign(lit);
+                    propagate();
                     continue;
                 }
+                update_wnb(~lit);
+                float diff2 = m_weighted_new_binaries;
+                pop();
 
-                unsigned mixd = mix_diff(diff1, diff2);
+                float mixd = mix_diff(diff1, diff2);
-
 
                 if (mixd > h || (mixd == h && s.m_rand(count) == 0)) {
                     CTRACE("sat", l != null_literal, tout << lit << " diff1: " << diff1 << " diff2: " << diff2 << "\n";);
@@ -872,7 +1022,30 @@ namespace sat {
                     l = diff1 < diff2 ? lit : ~lit;
                 }
             }
-            return l != null_literal;
+            return l != null_literal || inconsistent();
+        }
+
+        void set_wnb(literal l, float f) { m_lits[l.index()].m_wnb = f; }
+        void inc_wnb(literal l, float f) { m_lits[l.index()].m_wnb += f; }
+        float get_wnb(literal l) const { return m_lits[l.index()].m_wnb; }
+        bool dl_enabled(literal l) const { return m_lits[l.index()].m_double_lookahead != m_istamp_id; }
+        void dl_disable(literal l) { m_lits[l.index()].m_double_lookahead = m_istamp_id; }
+            
+        void reset_wnb(literal l) {
+            m_weighted_new_binaries = 0;
+
+            // inherit propagation effect from parent.
+            literal p = get_parent(l);
+            set_wnb(l, p == null_literal ? 0 : get_wnb(p));
+        }
+
+        void update_wnb(literal l) {
+            if (m_weighted_new_binaries == 0) {
+                // TBD autarky
+            }
+            else {
+                inc_wnb(l, m_weighted_new_binaries);
+            }
         }
 
         void double_look() {
@@ -881,7 +1054,7 @@ namespace sat {
                 literal lit = m_lookahead[i].m_lit;
                 if (value(lit) != l_undef) continue;
 
-                push(lit);
+                push(lit, lookahead2);
                 unsat = inconsistent();
                 pop();
                 if (unsat) {
@@ -890,7 +1063,7 @@ namespace sat {
                     continue;
                 }
 
-                push(~lit);
+                push(~lit, lookahead2);
                 unsat = inconsistent();
                 pop();
                 if (unsat) {
@@ -898,7 +1071,6 @@ namespace sat {
                     assign(lit);
                 }
             }
-            update_delta_trigger();
         }
 
         bool is_fixed(literal l) const { return value(l) != l_undef; }
@@ -906,6 +1078,7 @@ namespace sat {
         bool is_true(literal l) const { return value(l) == l_true; }
         void set_conflict() { m_inconsistent = true; }
         lbool value(literal l) const { return static_cast<lbool>(m_assignment[l.index()]); }
+        lbool value(bool_var v) const { return value(literal(v, false)); }
         unsigned scope_lvl() const { return m_trail_lim.size(); }
 
         void assign(literal l) {
@@ -919,14 +1092,13 @@ namespace sat {
                 m_assignment[l.index()] = l.sign() ? l_false : l_true;
                 m_assignment[(~l).index()] = l.sign() ? l_false : l_true;
                 m_trail.push_back(l);
+                m_freevars.remove(l.var());
                 break;
             }
         }
 
-        void set_inconsistent() { m_inconsistent = true; }
         bool inconsistent() { return m_inconsistent; }
 
-
         void select_variables(literal_vector& P) {
             for (unsigned i = 0; i < s.num_vars(); ++i) {
                 if (value(literal(i,false)) == l_undef) {
@@ -935,19 +1107,16 @@ namespace sat {
             }
         }
 
-        bool do_double() {
+        void do_double(literal l) {
-            return !inconsistent() && diff() > m_delta_trigger;
+            if (!inconsistent() && scope_lvl() > 0 && dl_enabled(l)) {
-        }
+                if (get_wnb(l) > m_delta_trigger) {
-
+                    double_look();
-        void update_delta_trigger() {
+                    m_delta_trigger = get_wnb(l);
-            if (inconsistent()) {
+                    dl_disable(l);
-                m_delta_trigger -= (1 - m_config.m_dl_success) / m_config.m_dl_success; 
+                }
-            }
+                else {
-            else {
+                    m_delta_trigger *= m_config.m_delta_rho;
-                m_delta_trigger += 1;
+                }
-            }
-            if (m_delta_trigger >= s.num_vars()) {
-                // reset it.
             }
         }
 
@@ -961,8 +1130,9 @@ namespace sat {
 
         lbool search() {
             literal_vector trail;
-
+            m_search_mode = searching;
             while (true) {
+                inc_istamp();
                 s.checkpoint();
                 literal l = choose();
                 if (inconsistent()) {
@@ -973,7 +1143,7 @@ namespace sat {
                     return l_true;
                 }
                 TRACE("sat", tout << "choose: " << l << " " << trail << "\n";);
-                push(l);
+                push(l, searching);
                 trail.push_back(l);
             }
         }

src/sat/sat_solver.cpp

@@ -782,7 +782,7 @@ namespace sat {
         pop_to_base_level();
         IF_VERBOSE(2, verbose_stream() << "(sat.sat-solver)\n";);
         SASSERT(at_base_lvl());
-        if (m_config.m_num_threads > 1 && !m_par) {
+        if ((m_config.m_num_threads > 1 || m_local_search) && !m_par) {
             return check_par(num_lits, lits);
         }
         flet<bool> _searching(m_searching, true);
@@ -854,9 +854,17 @@ namespace sat {
         ERROR_EX
     };
 
+    local_search& solver::init_local_search() {
+        if (!m_local_search) {
+            m_local_search = alloc(local_search, *this);
+        }
+        return *m_local_search.get();
+    }
+
+
     lbool solver::check_par(unsigned num_lits, literal const* lits) {
         int num_threads = static_cast<int>(m_config.m_num_threads);
-        int num_extra_solvers = num_threads - 1;
+        int num_extra_solvers = num_threads - 1 + (m_local_search ? 1 : 0);
         sat::parallel par(*this);
         par.reserve(num_threads, 1 << 12);
         par.init_solvers(*this, num_extra_solvers);
@@ -870,7 +878,10 @@ namespace sat {
         for (int i = 0; i < num_threads; ++i) {
             try {                
                 lbool r = l_undef;
-                if (i < num_extra_solvers) {
+                if (m_local_search && i + 1 == num_extra_solvers) {
+                    r = m_local_search->check(num_lits, lits);
+                }
+                else if (i < num_extra_solvers) {
                     r = par.get_solver(i).check(num_lits, lits);
                 }
                 else {
@@ -886,6 +897,9 @@ namespace sat {
                     }
                 }
                 if (first) {
+                    if (m_local_search) {
+                        m_local_search->cancel();
+                    }
                     for (int j = 0; j < num_extra_solvers; ++j) {
                         if (i != j) {
                             par.cancel_solver(j);

src/sat/sat_solver.h

@@ -35,6 +35,7 @@ Revision History:
 #include"sat_mus.h"
 #include"sat_drat.h"
 #include"sat_parallel.h"
+#include"sat_local_search.h"
 #include"params.h"
 #include"statistics.h"
 #include"stopwatch.h"
@@ -89,6 +90,7 @@ namespace sat {
         probing                 m_probing;
         mus                     m_mus;           // MUS for minimal core extraction
         drat                    m_drat;          // DRAT for generating proofs
+        scoped_ptr<local_search> m_local_search;
         bool                    m_inconsistent;
         bool                    m_searching;
         // A conflict is usually a single justification. That is, a justification
@@ -460,6 +462,9 @@ namespace sat {
 
         lbool get_consequences(literal_vector const& assms, bool_var_vector const& vars, vector<literal_vector>& conseq);
 
+        // initialize and retrieve local search.
+        local_search& init_local_search();
+
     private:
 
         typedef hashtable<unsigned, u_hash, u_eq> index_set;