From a3f4d58b000f8ef91c5db61f74adf05329686ba3 Mon Sep 17 00:00:00 2001
From: Nikolaj Bjorner <nbjorner@microsoft.com>
Date: Tue, 18 Apr 2017 16:58:56 -0700
Subject: [PATCH] use lookahead for simplification

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>
---
 src/sat/sat_config.cpp       |   4 +-
 src/sat/sat_config.h         |   1 +
 src/sat/sat_local_search.cpp |  42 ++++++-----
 src/sat/sat_local_search.h   |   1 +
 src/sat/sat_lookahead.h      | 135 +++++++++++++++++++++++++++++------
 src/sat/sat_params.pyg       |   1 +
 src/sat/sat_scc.cpp          |   4 +-
 src/sat/sat_simplifier.cpp   |   8 +++
 src/sat/sat_simplifier.h     |   6 +-
 src/sat/sat_solver.cpp       |  23 ++++++
 src/sat/sat_solver.h         |   3 +
 11 files changed, 183 insertions(+), 45 deletions(-)

diff --git a/src/sat/sat_config.cpp b/src/sat/sat_config.cpp
index 4b019c2b7..42c185ee1 100644
--- a/src/sat/sat_config.cpp
+++ b/src/sat/sat_config.cpp
@@ -37,6 +37,7 @@ namespace sat {
         m_psm_glue("psm_glue") {
         m_num_threads = 1;
         m_local_search = 0;
+        m_lookahead_search = false;
         updt_params(p); 
     }
 
@@ -81,7 +82,8 @@ namespace sat {
         m_max_conflicts   = p.max_conflicts();
         m_num_threads     = p.threads();
         m_local_search    = p.local_search();
-        
+        m_lookahead_search = p.lookahead_search();
+
         // These parameters are not exposed
         m_simplify_mult1  = _p.get_uint("simplify_mult1", 300);
         m_simplify_mult2  = _p.get_double("simplify_mult2", 1.5);
diff --git a/src/sat/sat_config.h b/src/sat/sat_config.h
index 313b4ec49..8c10983d2 100644
--- a/src/sat/sat_config.h
+++ b/src/sat/sat_config.h
@@ -59,6 +59,7 @@ namespace sat {
         unsigned           m_max_conflicts;
         unsigned           m_num_threads;
         unsigned           m_local_search;
+        bool               m_lookahead_search;
 
         unsigned           m_simplify_mult1;
         double             m_simplify_mult2;
diff --git a/src/sat/sat_local_search.cpp b/src/sat/sat_local_search.cpp
index c5e7fcf54..5372011ed 100644
--- a/src/sat/sat_local_search.cpp
+++ b/src/sat/sat_local_search.cpp
@@ -71,9 +71,8 @@ namespace sat {
     void local_search::init_cur_solution() {
         for (unsigned v = 0; v < num_vars(); ++v) {
             // use bias with a small probability
-            if (m_rand() % 100 < 3) {
-                //m_vars[v].m_value = ((unsigned)(m_rand() % 100) < m_vars[v].m_bias);
-                m_vars[v].m_value = (50 < m_vars[v].m_bias);
+            if (m_rand() % 100 < 2) {
+                m_vars[v].m_value = ((unsigned)(m_rand() % 100) < m_vars[v].m_bias);
             }
         }
     }
@@ -138,20 +137,24 @@ namespace sat {
     }
 
     void local_search::reinit() {
-        // the following methods does NOT converge for pseudo-boolean
-        // can try other way to define "worse" and "better"
-        // the current best noise is below 1000
-#if 0
-        if (m_best_unsat_rate > m_last_best_unsat_rate) {
-            // worse
-            m_noise -= m_noise * 2 * m_noise_delta;
-            m_best_unsat_rate *= 1000.0;
+
+        if (!m_is_pb) {
+            //
+            // the following methods does NOT converge for pseudo-boolean
+            // can try other way to define "worse" and "better"
+            // the current best noise is below 1000
+            //
+            if (m_best_unsat_rate > m_last_best_unsat_rate) {
+                // worse
+                m_noise -= m_noise * 2 * m_noise_delta;
+                m_best_unsat_rate *= 1000.0;
+            }
+            else {
+                // better
+                m_noise += (10000 - m_noise) * m_noise_delta;
+            }
         }
-        else {
-            // better
-            m_noise += (10000 - m_noise) * m_noise_delta;
-        }
-#endif
+
         for (unsigned i = 0; i < m_constraints.size(); ++i) {
             constraint& c = m_constraints[i];
             c.m_slack = c.m_k;
@@ -264,7 +267,7 @@ namespace sat {
         unsigned id = m_constraints.size();
         m_constraints.push_back(constraint(k));
         for (unsigned i = 0; i < sz; ++i) {
-            m_vars.reserve(c[i].var() + 1);
+            m_vars.reserve(c[i].var() + 1);            
             literal t(~c[i]);            
             m_vars[t.var()].m_watch[is_pos(t)].push_back(pbcoeff(id, coeffs[i]));
             m_constraints.back().push(t); // add coefficient to constraint?
@@ -279,6 +282,7 @@ namespace sat {
     }
 
     void local_search::import(solver& s, bool _init) {        
+        m_is_pb = false;
         m_vars.reset();
         m_constraints.reset();
 
@@ -349,6 +353,7 @@ namespace sat {
                     // =  ~c.lit() or (~c.lits() <= n - k)
                     // =  k*c.lit() + ~c.lits() <= n 
                     // 
+                    m_is_pb = true;
                     lits.reset();
                     coeffs.reset();
                     for (unsigned j = 0; j < n; ++j) lits.push_back(c[j]), coeffs.push_back(1);
@@ -616,8 +621,7 @@ namespace sat {
         // verify_unsat_stack();
     }
 
-    void local_search::flip_gsat(bool_var flipvar)
-    {
+    void local_search::flip_gsat(bool_var flipvar) {
         // already changed truth value!!!!
         m_vars[flipvar].m_value = !cur_solution(flipvar);
 
diff --git a/src/sat/sat_local_search.h b/src/sat/sat_local_search.h
index 918f5328d..edce6cc9c 100644
--- a/src/sat/sat_local_search.h
+++ b/src/sat/sat_local_search.h
@@ -144,6 +144,7 @@ namespace sat {
         literal_vector m_assumptions;
 
         unsigned m_num_non_binary_clauses;
+        bool     m_is_pb; 
 
         inline bool is_pos(literal t) const { return !t.sign(); }
         inline bool is_true(bool_var v) const { return cur_solution(v); }
diff --git a/src/sat/sat_lookahead.h b/src/sat/sat_lookahead.h
index c4f6a4bba..6bc528220 100644
--- a/src/sat/sat_lookahead.h
+++ b/src/sat/sat_lookahead.h
@@ -20,6 +20,8 @@ Notes:
 #ifndef _SAT_LOOKAHEAD_H_
 #define _SAT_LOOKAHEAD_H_
 
+#include "sat_elim_eqs.h"
+
 namespace sat {
 
     struct pp_prefix {
@@ -34,6 +36,9 @@ namespace sat {
         for (unsigned i = 0; i <= d; ++i) {
             if (0 != (p.m_prefix & (1ull << i))) out << "1"; else out << "0";
         }
+        if (d < p.m_depth) {
+            out << " d:" << p.m_depth;
+        }
         return out;
     }
 
@@ -309,6 +314,7 @@ namespace sat {
                         assign(u);        
                         return false;
                     }
+                    IF_VERBOSE(3, verbose_stream() << "tc1: " << u << " " << w << "\n";);
                     add_binary(u, w);
                 }
             }
@@ -367,7 +373,8 @@ namespace sat {
 
         bool select(unsigned level) {
             init_pre_selection(level);
-            unsigned max_num_cand = level == 0 ? m_freevars.size() : m_config.m_level_cand / level;
+            unsigned level_cand = std::max(m_config.m_level_cand, m_freevars.size() / 50);
+            unsigned max_num_cand = level == 0 ? m_freevars.size() : level_cand / level; 
             max_num_cand = std::max(m_config.m_min_cutoff, max_num_cand);
 
             float sum = 0;
@@ -1010,7 +1017,8 @@ namespace sat {
             m_lits.push_back(lit_info());
             m_rating.push_back(0);
             m_vprefix.push_back(prefix());
-            m_freevars.insert(v);
+            if (!s.was_eliminated(v)) 
+                m_freevars.insert(v);
         }
 
         void init() {
@@ -1040,11 +1048,31 @@ namespace sat {
                 }
             }
 
+            copy_clauses(s.m_clauses);
+            copy_clauses(s.m_learned);
+
+            // copy units
+            unsigned trail_sz = s.init_trail_size();
+            for (unsigned i = 0; i < trail_sz; ++i) {
+                literal l = s.m_trail[i];
+				if (!s.was_eliminated(l.var()))
+				{
+					if (s.m_config.m_drat) m_drat.add(l, false);
+					assign(l);
+				}
+            }
+            propagate();
+            m_qhead = m_trail.size();
+            TRACE("sat", s.display(tout); display(tout););
+        }
+
+        void copy_clauses(clause_vector const& clauses) {
             // copy clauses
-            clause_vector::const_iterator it =  s.m_clauses.begin();
-            clause_vector::const_iterator end = s.m_clauses.end();
-            for (; it != end; ++it) {
+            clause_vector::const_iterator it =  clauses.begin();
+            clause_vector::const_iterator end = clauses.end();
+            for (; it != end; ++it) {                
                 clause& c = *(*it);
+                if (c.was_removed()) continue;
                 clause* c1 = m_cls_allocator.mk_clause(c.size(), c.begin(), false);
                 m_clauses.push_back(c1);
                 attach_clause(*c1);
@@ -1053,17 +1081,6 @@ namespace sat {
                 }
                 if (s.m_config.m_drat) m_drat.add(c, false);
             }
-
-            // copy units
-            unsigned trail_sz = s.init_trail_size();
-            for (unsigned i = 0; i < trail_sz; ++i) {
-                literal l = s.m_trail[i];
-                if (s.m_config.m_drat) m_drat.add(l, false);
-                assign(l);
-            }
-            propagate();
-            m_qhead = m_trail.size();
-            TRACE("sat", s.display(tout); display(tout););
         }
 
         // ------------------------------------
@@ -1393,19 +1410,24 @@ namespace sat {
             TRACE("sat", display_lookahead(tout); );
             unsigned base = 2;
             bool change = true;
+            bool first = true;
             while (change && !inconsistent()) {
                 change = false;
                 for (unsigned i = 0; !inconsistent() && i < m_lookahead.size(); ++i) {
+                    s.checkpoint();
                     literal lit = m_lookahead[i].m_lit;
                     if (is_fixed_at(lit, c_fixed_truth)) continue;
                     unsigned level = base + m_lookahead[i].m_offset;
                     if (m_stamp[lit.var()] >= level) {
                         continue;
                     }
+                    if (scope_lvl() == 1) {
+                        IF_VERBOSE(3, verbose_stream() << scope_lvl() << " " << lit << " binary: " << m_binary_trail.size() << " trail: " << m_trail_lim.back() << "\n";);
+                    }
                     TRACE("sat", tout << "lookahead: " << lit << " @ " << m_lookahead[i].m_offset << "\n";);
                     reset_wnb(lit);
                     push_lookahead1(lit, level);
-                    do_double(lit, base);
+                    if (!first) do_double(lit, base);
                     bool unsat = inconsistent();                    
                     pop_lookahead1(lit); 
                     if (unsat) {
@@ -1424,7 +1446,13 @@ namespace sat {
                 if (c_fixed_truth - 2 * m_lookahead.size() < base) {
                     break;
                 }
-                base += 2 * m_lookahead.size();
+                if (first && !change) {
+                    first = false;
+                    change = true;
+                }
+                reset_wnb();
+                init_wnb();
+                // base += 2 * m_lookahead.size();
             }
             reset_wnb();
             TRACE("sat", display_lookahead(tout); );
@@ -1487,6 +1515,7 @@ namespace sat {
         }
 
         bool check_autarky(literal l, unsigned level) {
+            return false;
             // no propagations are allowed to reduce clauses.
             clause_vector::const_iterator it  = m_full_watches[l.index()].begin();
             clause_vector::const_iterator end = m_full_watches[l.index()].end();
@@ -1568,7 +1597,7 @@ namespace sat {
         }
 
         void do_double(literal l, unsigned& base) {
-            if (!inconsistent() && scope_lvl() > 0 && dl_enabled(l)) {                
+            if (!inconsistent() && scope_lvl() > 1 && dl_enabled(l)) {                
                 if (get_wnb(l) > m_delta_trigger) {
                     if (dl_no_overflow(base)) {
                         ++m_stats.m_double_lookahead_rounds;
@@ -1588,6 +1617,7 @@ namespace sat {
             SASSERT(dl_no_overflow(base));
             unsigned dl_truth = base + 2 * m_lookahead.size() * (m_config.m_dl_max_iterations + 1);            
             scoped_level _sl(*this, dl_truth);
+            IF_VERBOSE(2, verbose_stream() << "double: " << l << "\n";);
             init_wnb();
             assign(l);
             propagate();
@@ -1769,9 +1799,6 @@ namespace sat {
             m_drat(s),
             m_level(2),
             m_prefix(0) {
-                m_search_mode = lookahead_mode::searching;
-                scoped_level _sl(*this, c_fixed_truth);
-                init();
         }
 
         ~lookahead() {
@@ -1779,9 +1806,73 @@ namespace sat {
         }
         
         lbool check() {
+            {
+                m_search_mode = lookahead_mode::searching;
+                scoped_level _sl(*this, c_fixed_truth);
+                init();
+            }
             return search();
         }
 
+        void simplify() {
+            SASSERT(m_prefix == 0);
+            SASSERT(m_watches.empty());
+            m_search_mode = lookahead_mode::searching;
+            scoped_level _sl(*this, c_fixed_truth);
+            init();                
+            if (inconsistent()) return;
+            inc_istamp();            
+            literal l = choose();
+            if (inconsistent()) return;
+            SASSERT(m_trail_lim.empty());
+            unsigned num_units = 0;
+            for (unsigned i = 0; i < m_trail.size(); ++i) {
+                literal lit = m_trail[i];
+                if (s.value(lit) == l_undef && !s.was_eliminated(lit.var())) {
+                    s.m_simplifier.propagate_unit(lit);                    
+                    ++num_units;
+                }
+            }
+            IF_VERBOSE(1, verbose_stream() << "units found: " << num_units << "\n";);
+            
+            s.m_simplifier.subsume();
+            m_lookahead.reset();
+        }
+
+        void scc() {
+            SASSERT(m_prefix == 0);
+            SASSERT(m_watches.empty());
+            m_search_mode = lookahead_mode::searching;
+            scoped_level _sl(*this, c_fixed_truth);
+            init();                
+            if (inconsistent()) return;
+            inc_istamp();
+            m_lookahead.reset();
+            if (select(0)) {
+                // extract equivalences
+                get_scc();
+                if (inconsistent()) return;
+                literal_vector roots;
+                bool_var_vector to_elim;
+                for (unsigned i = 0; i < s.num_vars(); ++i) {
+                    roots.push_back(literal(i, false));
+                }
+                for (unsigned i = 0; i < m_candidates.size(); ++i) {
+                    bool_var v = m_candidates[i].m_var;
+                    literal lit = literal(v, false);
+                    literal p = get_parent(lit);
+                    if (p != null_literal && p.var() != v && !s.is_external(v) && !s.was_eliminated(v) && !s.was_eliminated(p.var())) {
+                        to_elim.push_back(v);
+                        roots[v] = p;
+                    }
+                }
+                IF_VERBOSE(1, verbose_stream() << "eliminate " << to_elim.size() << " variables\n";);
+                elim_eqs elim(s);
+                elim(roots, to_elim);
+            }
+            m_lookahead.reset();
+        }
+
         std::ostream& display(std::ostream& out) const {
             out << "Prefix: " << pp_prefix(m_prefix, m_trail_lim.size()) << "\n";
             out << "Level: " << m_level << "\n";
diff --git a/src/sat/sat_params.pyg b/src/sat/sat_params.pyg
index 045fd803a..ffc699d02 100644
--- a/src/sat/sat_params.pyg
+++ b/src/sat/sat_params.pyg
@@ -29,4 +29,5 @@ def_module_params('sat',
                           ('cardinality.solver', BOOL, False, 'use cardinality solver'),
 	                  ('xor.solver', BOOL, False, 'use xor solver'),
                           ('local_search', UINT, 0, 'number of local search threads to find satisfiable solution'),
+                          ('lookahead_search', BOOL, False, 'use lookahead solver')
                        ))
diff --git a/src/sat/sat_scc.cpp b/src/sat/sat_scc.cpp
index ffbdb31c6..3dfc42f6a 100644
--- a/src/sat/sat_scc.cpp
+++ b/src/sat/sat_scc.cpp
@@ -76,7 +76,9 @@ namespace sat {
         lowlink.resize(num_lits, UINT_MAX);
         in_s.resize(num_lits, false);
         literal_vector roots;
-        roots.resize(m_solver.num_vars(), null_literal);
+		for (unsigned i = 0; i < m_solver.num_vars(); ++i) {
+			roots.push_back(literal(i, false));
+		}
         unsigned next_index = 0;
         svector<frame>   frames;
         bool_var_vector  to_elim;
diff --git a/src/sat/sat_simplifier.cpp b/src/sat/sat_simplifier.cpp
index fe019427f..8cbedb86b 100644
--- a/src/sat/sat_simplifier.cpp
+++ b/src/sat/sat_simplifier.cpp
@@ -21,6 +21,7 @@ Revision History:
 #include"sat_simplifier.h"
 #include"sat_simplifier_params.hpp"
 #include"sat_solver.h"
+#include"sat_lookahead.h"
 #include"stopwatch.h"
 #include"trace.h"
 
@@ -204,6 +205,11 @@ namespace sat {
         }
         while (!m_sub_todo.empty());
 
+        if (!learned) {
+            // perform lookahead simplification
+            lookahead(s).simplify();
+        }
+
         bool vars_eliminated = m_num_elim_vars > old_num_elim_vars;
 
         if (m_need_cleanup) {
@@ -219,9 +225,11 @@ namespace sat {
                 cleanup_clauses(s.m_learned, true, true, learned_in_use_lists);
             }
         }
+
         CASSERT("sat_solver", s.check_invariant());
         TRACE("after_simplifier", s.display(tout); tout << "model_converter:\n"; s.m_mc.display(tout););
         finalize();
+
     }
 
     /**
diff --git a/src/sat/sat_simplifier.h b/src/sat/sat_simplifier.h
index 9ee239083..47648cc10 100644
--- a/src/sat/sat_simplifier.h
+++ b/src/sat/sat_simplifier.h
@@ -130,13 +130,11 @@ namespace sat {
 
         bool cleanup_clause(clause & c, bool in_use_list);
         bool cleanup_clause(literal_vector & c);
-        void propagate_unit(literal l);
         void elim_lit(clause & c, literal l);
         void elim_dup_bins();
         bool subsume_with_binaries();
         void mark_as_not_learned_core(watch_list & wlist, literal l2);
         void mark_as_not_learned(literal l1, literal l2);
-        void subsume();
 
         void cleanup_watches();
         void cleanup_clauses(clause_vector & cs, bool learned, bool vars_eliminated, bool in_use_lists);
@@ -191,6 +189,10 @@ namespace sat {
 
         void collect_statistics(statistics & st) const;
         void reset_statistics();
+
+        void propagate_unit(literal l);
+        void subsume();
+
     };
 };
 
diff --git a/src/sat/sat_solver.cpp b/src/sat/sat_solver.cpp
index d25c8bdee..5ebd661ee 100644
--- a/src/sat/sat_solver.cpp
+++ b/src/sat/sat_solver.cpp
@@ -22,6 +22,7 @@ Revision History:
 #include"trace.h"
 #include"max_cliques.h"
 #include"scoped_ptr_vector.h"
+#include"sat_lookahead.h"
 
 // define to update glue during propagation
 #define UPDATE_GLUE
@@ -783,6 +784,9 @@ namespace sat {
         pop_to_base_level();
         IF_VERBOSE(2, verbose_stream() << "(sat.sat-solver)\n";);
         SASSERT(at_base_lvl());
+        if (m_config.m_lookahead_search && num_lits == 0) {
+            return lookahead_search();
+        }
         if ((m_config.m_num_threads > 1 || m_config.m_local_search > 0) && !m_par) {
             return check_par(num_lits, lits);
         }
@@ -855,6 +859,20 @@ namespace sat {
         ERROR_EX
     };
 
+    lbool solver::lookahead_search() {
+        lookahead lh(*this);
+        lbool r = l_undef;
+        try {
+            r = lh.check();
+            m_model = lh.get_model();
+        }
+        catch (z3_exception&) {
+            lh.collect_statistics(m_lookahead_stats);
+            throw;
+        }
+        lh.collect_statistics(m_lookahead_stats);
+        return r;
+    }
 
     lbool solver::check_par(unsigned num_lits, literal const* lits) {
         scoped_ptr_vector<local_search> ls;
@@ -1295,6 +1313,8 @@ namespace sat {
             CASSERT("sat_simplify_bug", check_invariant());
         }
 
+        lookahead(*this).scc();
+
         sort_watch_lits();
         CASSERT("sat_simplify_bug", check_invariant());
 
@@ -2762,6 +2782,7 @@ namespace sat {
         m_asymm_branch.collect_statistics(st);
         m_probing.collect_statistics(st);
         if (m_ext) m_ext->collect_statistics(st);
+        st.copy(m_lookahead_stats);
     }
 
     void solver::reset_statistics() {
@@ -2770,6 +2791,7 @@ namespace sat {
         m_simplifier.reset_statistics();
         m_asymm_branch.reset_statistics();
         m_probing.reset_statistics();
+        m_lookahead_stats.reset();
     }
 
     // -----------------------
@@ -3605,6 +3627,7 @@ namespace sat {
             if (m_solver.m_num_frozen > 0)
                 out << " :frozen " << m_solver.m_num_frozen;
         }
+        out << " :units " << m_solver.init_trail_size();
         out << " :gc-clause " << m_solver.m_stats.m_gc_clause;
         out << mem_stat();
     }
diff --git a/src/sat/sat_solver.h b/src/sat/sat_solver.h
index c35a0296c..1bf393696 100644
--- a/src/sat/sat_solver.h
+++ b/src/sat/sat_solver.h
@@ -141,6 +141,8 @@ namespace sat {
         unsigned                m_par_num_vars;
         bool                    m_par_syncing_clauses;
 
+        statistics              m_lookahead_stats;
+
         void del_clauses(clause * const * begin, clause * const * end);
 
         friend class integrity_checker;
@@ -346,6 +348,7 @@ namespace sat {
         void sort_watch_lits();
         void exchange_par();
         lbool check_par(unsigned num_lits, literal const* lits);
+        lbool lookahead_search();
 
         // -----------------------
         //