diff --git a/src/sat/CMakeLists.txt b/src/sat/CMakeLists.txt
index 649bbb22e..11e290a26 100644
--- a/src/sat/CMakeLists.txt
+++ b/src/sat/CMakeLists.txt
@@ -23,6 +23,7 @@ z3_add_component(sat
     sat_integrity_checker.cpp
     sat_local_search.cpp
     sat_lookahead.cpp
+    sat_lut_finder.cpp
     sat_model_converter.cpp
     sat_mus.cpp
     sat_parallel.cpp
diff --git a/src/sat/sat_aig_simplifier.cpp b/src/sat/sat_aig_simplifier.cpp
index 20d3735b4..6f1613c38 100644
--- a/src/sat/sat_aig_simplifier.cpp
+++ b/src/sat/sat_aig_simplifier.cpp
@@ -18,6 +18,7 @@
 
 #include "sat/sat_aig_simplifier.h"
 #include "sat/sat_xor_finder.h"
+#include "sat/sat_lut_finder.h"
 #include "sat/sat_elim_eqs.h"
 
 namespace sat {
@@ -225,7 +226,23 @@ namespace sat {
         };
         xor_finder xf(s);
         xf.set(on_xor);
-        xf(clauses);       
+        xf(clauses);
+
+#if 0
+        std::function<void(uint64_t, bool_var_vector const&, bool_var)> on_lut = 
+            [&,this](uint64_t l, bool_var_vector const& vars, bool_var v) {
+            m_stats.m_num_luts++;
+        };
+        lut_finder lf(s);
+        lf.set(on_lut);
+        lf(clauses);
+
+
+        statistics st;
+        collect_statistics(st);
+        st.display(std::cout);
+        exit(0);
+#endif
     }
 
     void aig_simplifier::aig2clauses() {
@@ -675,6 +692,7 @@ namespace sat {
         st.update("sat-aig.ands", m_stats.m_num_ands);
         st.update("sat-aig.ites", m_stats.m_num_ites);
         st.update("sat-aig.xors", m_stats.m_num_xors);
+        st.update("sat-aig.luts", m_stats.m_num_luts);
         st.update("sat-aig.dc-reduce", m_stats.m_num_dont_care_reductions);
     }
 
diff --git a/src/sat/sat_aig_simplifier.h b/src/sat/sat_aig_simplifier.h
index 7f523fd2f..9189b08fd 100644
--- a/src/sat/sat_aig_simplifier.h
+++ b/src/sat/sat_aig_simplifier.h
@@ -27,7 +27,7 @@ namespace sat {
     class aig_simplifier {
     public:
         struct stats {
-            unsigned m_num_eqs, m_num_units, m_num_cuts, m_num_xors, m_num_ands, m_num_ites;
+            unsigned m_num_eqs, m_num_units, m_num_cuts, m_num_xors, m_num_ands, m_num_ites, m_num_luts;
             unsigned m_num_calls, m_num_dont_care_reductions, m_num_learned_implies;
             stats() { reset(); }
             void reset() { memset(this, 0, sizeof(*this)); }
diff --git a/src/sat/sat_lut_finder.cpp b/src/sat/sat_lut_finder.cpp
new file mode 100644
index 000000000..d14d2d8fe
--- /dev/null
+++ b/src/sat/sat_lut_finder.cpp
@@ -0,0 +1,295 @@
+/*++
+  Copyright (c) 2020 Microsoft Corporation
+
+  Module Name:
+
+   sat_lut_finder.cpp
+
+  Abstract:
+   
+    lut finder
+
+  Author:
+
+    Nikolaj Bjorner 2020-01-02
+
+  Notes:
+  
+
+  --*/
+
+#include "sat/sat_lut_finder.h"
+#include "sat/sat_solver.h"
+
+namespace sat {
+
+    void lut_finder::operator()(clause_vector& clauses) {
+        m_removed_clauses.reset();
+        unsigned max_size = m_max_lut_size;
+        // we better have enough bits in the combination mask to 
+        // handle clauses up to max_size.
+        // max_size = 5 -> 32 bits
+        // max_size = 6 -> 64 bits
+        SASSERT(sizeof(m_combination)*8 >= (1ull << static_cast<uint64_t>(max_size)));
+        init_clause_filter();
+        for (unsigned i = 0; i <= 6; ++i) init_mask(i);
+        m_var_position.resize(s.num_vars());
+        for (clause* cp : clauses) {
+            cp->unmark_used();
+        }
+        for (; max_size > 2; --max_size) {
+            for (clause* cp : clauses) {
+                clause& c = *cp;
+                if (c.size() == max_size && !c.was_removed() && !c.is_learned() && !c.was_used()) {
+                    check_lut(c);
+                }
+            }
+        }
+        m_clause_filters.clear();
+        
+        for (clause* cp : clauses) cp->unmark_used();
+        for (clause* cp : m_removed_clauses) cp->mark_used();
+        std::function<bool(clause*)> not_used = [](clause* cp) { return !cp->was_used(); };
+        clauses.filter_update(not_used);
+    }
+
+    void lut_finder::check_lut(clause& c) {
+        SASSERT(c.size() > 2);
+        unsigned filter = get_clause_filter(c);
+        s.init_visited();
+        unsigned mask = 0, i = 0;        
+        m_vars.reset();
+        for (literal l : c) {
+            m_vars.push_back(l.var());
+            m_var_position[l.var()] = i;
+            s.mark_visited(l.var());
+            mask |= (l.sign() << (i++)); 
+        }
+        m_clauses_to_remove.reset();
+        m_clauses_to_remove.push_back(&c);
+        m_clause.resize(c.size());
+        m_combination = 0;
+        m_num_combinations = 0;
+        set_combination(mask);
+        c.mark_used();
+        for (literal l : c) {
+            for (auto const& cf : m_clause_filters[l.var()]) {                
+                if ((filter == (filter | cf.m_filter)) && 
+                    !cf.m_clause->was_used() &&
+                    extract_lut(*cf.m_clause)) {
+                    add_lut();
+                    return;
+                }
+            }
+            // TBD: replace by BIG
+            // loop over binary clauses in watch list
+            for (watched const & w : s.get_wlist(l)) {
+                if (w.is_binary_clause() && s.is_visited(w.get_literal().var()) && w.get_literal().index() < l.index()) {
+                    if (extract_lut(~l, w.get_literal())) {
+                        add_lut();
+                        return;
+                    }
+                }
+            }
+            l.neg();
+            for (watched const & w : s.get_wlist(l)) {
+                if (w.is_binary_clause() && s.is_visited(w.get_literal().var()) && w.get_literal().index() < l.index()) {
+                    if (extract_lut(~l, w.get_literal())) {
+                        add_lut();
+                        return;
+                    }
+                }
+            }
+        }
+    }
+
+    void lut_finder::add_lut() {
+        DEBUG_CODE(for (clause* cp : m_clauses_to_remove) VERIFY(cp->was_used()););
+        m_removed_clauses.append(m_clauses_to_remove);
+        bool_var v;
+        uint64_t lut = convert_combination(m_vars, v);
+        m_on_lut(lut, m_vars, v);
+    }
+
+    bool lut_finder::extract_lut(literal l1, literal l2) {
+        SASSERT(s.is_visited(l1.var()));
+        SASSERT(s.is_visited(l2.var()));
+        m_missing.reset();
+        unsigned mask = 0;
+        for (unsigned i = 0; i < m_vars.size(); ++i) {
+            if (m_vars[i] == l1.var()) {
+                mask |= (l1.sign() << i);
+            }
+            else if (m_vars[i] == l2.var()) {
+                mask |= (l2.sign() << i);
+            }
+            else {
+                m_missing.push_back(i);
+            }
+        }
+        return update_combinations(mask);
+    }
+
+    bool lut_finder::extract_lut(clause& c2) {
+        for (literal l : c2) {            
+            if (!s.is_visited(l.var())) return false;
+        }
+        if (c2.size() == m_vars.size()) {
+            m_clauses_to_remove.push_back(&c2);
+            c2.mark_used();
+        }
+        // insert missing
+        unsigned mask = 0;
+        m_missing.reset();
+        SASSERT(c2.size() <= m_vars.size());
+        for (unsigned i = 0; i < m_vars.size(); ++i) {
+            m_clause[i] = null_literal;
+        }
+        for (literal l : c2) {
+            unsigned pos = m_var_position[l.var()];
+            m_clause[pos] = l;
+        }
+        for (unsigned j = 0; j < m_vars.size(); ++j) {
+            literal lit = m_clause[j];
+            if (lit == null_literal) {
+                m_missing.push_back(j);
+            }
+            else {
+                mask |= (m_clause[j].sign() << j);
+            }
+        }                    
+        return update_combinations(mask);
+    }
+
+    bool lut_finder::update_combinations(unsigned mask) {
+        unsigned num_missing = m_missing.size();
+        for (unsigned k = 0; k < (1ul << num_missing); ++k) {
+            unsigned mask2 = mask;
+            for (unsigned i = 0; i < num_missing; ++i) {
+                if ((k & (1 << i)) != 0) {
+                    mask2 |= 1ul << m_missing[i];
+                }
+            }
+            set_combination(mask2);
+        }
+        return lut_is_defined(m_vars.size());
+    }
+
+    bool lut_finder::lut_is_defined(unsigned sz) {
+        if (m_num_combinations < (1ull << (sz/2))) 
+            return false;
+        for (unsigned i = 0; i < sz; ++i) {
+            if (lut_is_defined(i, sz)) 
+                return true;
+        }
+        return false;
+    }
+
+    /**
+     * \brief create the masks
+     * i = 0: 101010101010101
+     * i = 1: 1100110011001100
+     * i = 2: 1111000011110000
+     * i = 3: 111111110000000011111111
+     */
+
+    void lut_finder::init_mask(unsigned i) {
+        SASSERT(i <= 6);
+        uint64_t m = 0;
+        if (i == 6) {
+            m = ~((uint64_t)0);
+        }
+        else {
+            m = (1ull << (1u << i)) - 1;   // i = 0: m = 1
+            unsigned w = 1u << (i + 1);    // i = 0: w = 2
+            while (w < 64) {
+                m |= (m << w);             // i = 0: m = 1 + 4
+                w *= 2;
+            }
+        }
+        m_masks[i] = m;
+    }
+
+    /**
+     * \brief check if all output combinations for variable i are defined.
+     */
+    bool lut_finder::lut_is_defined(unsigned i, unsigned sz) {
+        uint64_t c = m_combination | (m_combination >> (1ull << (uint64_t)i));
+        uint64_t m = m_masks[i];
+        if (sz < 6) m &= ((1ull << sz) - 1);
+        return (c & m) == m;
+    }
+
+    /**
+     * find variable where it is defined
+     * convert bit-mask to truth table for that variable.
+     * remove variable from vars, 
+     * return truth table.
+     */
+
+    uint64_t lut_finder::convert_combination(bool_var_vector& vars, bool_var& v) {
+        SASSERT(lut_is_defined(vars.size()));
+        unsigned i = 0, j = 0;
+        for (; i < vars.size(); ++i) {
+            if (lut_is_defined(i, vars.size())) {
+                break;
+            }
+        }
+        SASSERT(i < vars.size());
+        v = vars[i];
+        vars.erase(v);
+        uint64_t r = 0;
+        unsigned stride_sz = (1u << i);
+        unsigned num_strides = (1u << vars.size()) / (stride_sz * 2);
+        
+        switch (i) {
+        case 0:
+            for (unsigned j = 0; j < (1u << vars.size()); ++j) {
+                if (0 == (m_combination & (1ull << 2*j))) {
+                    r |= (1ull << j);
+                }
+            }
+            break;
+        case 1:
+            // (0, 2) (1, 3), (4, 6), (5, 7)
+            for (unsigned j = 0; j < (1u << vars.size()); ++j) {
+                
+            }
+            // TBD
+            break;
+        default:
+            // TBD
+            break;
+        }
+        return r;
+    }
+
+    void lut_finder::init_clause_filter() {
+        m_clause_filters.reset();
+        m_clause_filters.resize(s.num_vars());
+        init_clause_filter(s.m_clauses);
+        init_clause_filter(s.m_learned);
+    }
+
+    void lut_finder::init_clause_filter(clause_vector& clauses) {
+        for (clause* cp : clauses) {
+            clause& c = *cp;
+            if (c.size() <= m_max_lut_size && s.all_distinct(c)) {
+                clause_filter cf(get_clause_filter(c), cp);
+                for (literal l : c) {
+                    m_clause_filters[l.var()].push_back(cf);
+                }            
+            }
+        }
+    }
+
+    unsigned lut_finder::get_clause_filter(clause const& c) {
+        unsigned filter = 0;
+        for (literal l : c) {
+            filter |= 1 << ((l.var() % 32));
+        }
+        return filter;
+    }
+
+
+}
diff --git a/src/sat/sat_lut_finder.h b/src/sat/sat_lut_finder.h
new file mode 100644
index 000000000..8af848136
--- /dev/null
+++ b/src/sat/sat_lut_finder.h
@@ -0,0 +1,83 @@
+/*++
+  Copyright (c) 2020 Microsoft Corporation
+
+  Module Name:
+
+   sat_lut_finder.h
+
+  Abstract:
+   
+    lut finder
+
+  Author:
+
+    Nikolaj Bjorner 2020-02-03
+
+  Notes:
+  
+    Find LUT with small input fan-ins
+
+  --*/
+
+#pragma once
+
+#include "util/params.h"
+#include "util/statistics.h"
+#include "sat/sat_clause.h"
+#include "sat/sat_types.h"
+#include "sat/sat_solver.h"
+
+namespace sat {
+
+    class lut_finder {
+        solver& s;
+        struct clause_filter {
+            unsigned m_filter;
+            clause*  m_clause;            
+            clause_filter(unsigned f, clause* cp):
+                m_filter(f), m_clause(cp) {}
+        };
+        typedef svector<bool> bool_vector;
+        unsigned                m_max_lut_size;
+        vector<svector<clause_filter>>   m_clause_filters;  // index of clauses.
+        uint64_t                m_combination;              // bit-mask of parities that have been found
+        unsigned                m_num_combinations;
+        clause_vector           m_clauses_to_remove;        // remove clauses that become luts
+        unsigned_vector         m_var_position;             // position of var in main clause
+        bool_var_vector         m_vars;                     // reference to variables being tested for LUT
+        literal_vector          m_clause;                   // reference clause with literals sorted according to main clause
+        unsigned_vector         m_missing;                  // set of indices not occurring in clause.
+        uint64_t                m_masks[7];
+        clause_vector           m_removed_clauses;
+        std::function<void (uint64_t, svector<bool_var> const& vars, bool_var v)> m_on_lut;
+
+        inline void set_combination(unsigned mask) { 
+            if (!get_combination(mask)) { 
+                m_combination |= (1ull << mask); 
+                m_num_combinations++;
+            }
+        }
+        inline bool get_combination(unsigned mask) const { return (m_combination & (1ull << mask)) != 0; }
+        bool lut_is_defined(unsigned sz);
+        bool lut_is_defined(unsigned i, unsigned sz);
+        uint64_t convert_combination(bool_var_vector& vars, bool_var& v);
+        void check_lut(clause& c);
+        void add_lut();
+        bool extract_lut(literal l1, literal l2);
+        bool extract_lut(clause& c2);
+        bool update_combinations(unsigned mask);
+        void init_mask(unsigned i);
+        void init_clause_filter();
+        void init_clause_filter(clause_vector& clauses);
+        unsigned get_clause_filter(clause const& c);
+
+    public:
+        lut_finder(solver& s) : s(s), m_max_lut_size(5) { }
+        ~lut_finder() {}        
+
+        void set(std::function<void (uint64_t, bool_var_vector const&, bool_var)>& f) { m_on_lut = f; }
+
+        unsigned max_lut_size() const { return m_max_lut_size; }
+        void operator()(clause_vector& clauses);        
+    };
+}
diff --git a/src/sat/sat_solver.h b/src/sat/sat_solver.h
index 8aff22f2d..9996e886c 100644
--- a/src/sat/sat_solver.h
+++ b/src/sat/sat_solver.h
@@ -219,6 +219,7 @@ namespace sat {
         friend class scoped_detach;
         friend class xor_finder;
         friend class aig_finder;
+        friend class lut_finder;
     public:
         solver(params_ref const & p, reslimit& l);
         ~solver() override;