add solver pool abstraction for Spacer

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

src/solver/solver_pool.h

@@ -0,0 +1,69 @@
+/**
+Copyright (c) 2017 Microsoft Corporation
+
+Module Name:
+
+    solver_pool.h
+
+Abstract:
+
+   Maintain a pool of solvers
+
+Author:
+
+    Nikolaj Bjorner
+    Arie Gurfinkel
+
+Notes:
+
+    This is a revision of spacer_virtual_solver by Arie Gurfinkel
+
+--*/
+#ifndef SOLVER_POOL_H_
+#define SOLVER_POOL_H_
+
+#include "solver/solver.h"
+#include "util/stopwatch.h"
+
+class pool_solver;
+
+class solver_pool {
+
+    friend class pool_solver;
+    
+    struct stats {
+        unsigned m_num_checks;
+        unsigned m_num_sat_checks;
+        unsigned m_num_undef_checks;
+        stats() { reset(); }
+        void reset() { memset(this, 0, sizeof(*this)); }
+    };
+
+    ref<solver>        m_base_solver;
+    unsigned           m_num_solvers_per_pool;
+    unsigned           m_num_solvers_in_last_pool;
+    sref_vector<solver> m_solvers;
+    stats              m_stats;
+
+    stopwatch m_check_watch;
+    stopwatch m_check_sat_watch;
+    stopwatch m_check_undef_watch;
+    stopwatch m_proof_watch;
+
+    void refresh(solver* s);
+
+    ptr_vector<solver> get_base_solvers() const;
+  
+public:
+    solver_pool(solver* base_solver, unsigned num_solvers_per_pool);
+
+    void collect_statistics(statistics &st) const;
+    void reset_statistics();
+
+    solver* mk_solver();
+
+    void reset_solver(solver* s);
+};
+
+
+#endif