(*  Title:      HOL/Tools/sat_solver.ML

    ID:         $Id$

    Author:     Tjark Weber

    Copyright   2004-2006

Interface to external SAT solvers, and (simple) built-in SAT solvers.

*)

signature SAT_SOLVER =

sig

  exception NOT_CONFIGURED

  type assignment = int -> bool option

  type proof      = int list Inttab.table * int

  datatype result = SATISFIABLE of assignment

                  | UNSATISFIABLE of proof option

                  | UNKNOWN

  type solver     = PropLogic.prop_formula -> result

  (* auxiliary functions to create external SAT solvers *)

  val write_dimacs_cnf_file : Path.T -> PropLogic.prop_formula -> unit

  val write_dimacs_sat_file : Path.T -> PropLogic.prop_formula -> unit

  val read_std_result_file  : Path.T -> string * string * string -> result

  val make_external_solver  : string -> (PropLogic.prop_formula -> unit) -> (unit -> result) -> solver

  val read_dimacs_cnf_file : Path.T -> PropLogic.prop_formula

  (* generic solver interface *)

  val solvers       : (string * solver) list ref

  val add_solver    : string * solver -> unit

  val invoke_solver : string -> solver  (* exception Option *)

end;

structure SatSolver : SAT_SOLVER =

struct

  open PropLogic;

(* ------------------------------------------------------------------------- *)

(* should be raised by an external SAT solver to indicate that the solver is *)

(* not configured properly                                                   *)

(* ------------------------------------------------------------------------- *)

  exception NOT_CONFIGURED;

(* ------------------------------------------------------------------------- *)

(* type of partial (satisfying) assignments: 'a i = NONE' means that 'a' is  *)

(*      a satisfying assignment regardless of the value of variable 'i'      *)

(* ------------------------------------------------------------------------- *)

  type assignment = int -> bool option;

(* ------------------------------------------------------------------------- *)

(* a proof of unsatisfiability, to be interpreted as follows: each integer   *)

(*      is a clause ID, each list 'xs' stored under the key 'x' in the table *)

(*      contains the IDs of clauses that must be resolved (in the given      *)

(*      order) to obtain the new clause 'x'.  Each list 'xs' must be         *)

(*      non-empty, and the literal to be resolved upon must always be unique *)

(*      (e.g. "A | ~B" must not be resolved with "~A | B").  Circular        *)

(*      dependencies of clauses are not allowed.  (At least) one of the      *)

(*      clauses in the table must be the empty clause (i.e. contain no       *)

(*      literals); its ID is given by the second component of the proof.     *)

(*      The clauses of the original problem passed to the SAT solver have    *)

(*      consecutive IDs starting with 0.  Clause IDs must be non-negative,   *)

(*      but do not need to be consecutive.                                   *)

(* ------------------------------------------------------------------------- *)

  type proof = int list Inttab.table * int;

(* ------------------------------------------------------------------------- *)

(* return type of SAT solvers: if the result is 'SATISFIABLE', a satisfying  *)

(*      assignment must be returned as well; if the result is                *)

(*      'UNSATISFIABLE', a proof of unsatisfiability may be returned         *)

(* ------------------------------------------------------------------------- *)

  datatype result = SATISFIABLE of assignment

                  | UNSATISFIABLE of proof option

                  | UNKNOWN;

(* ------------------------------------------------------------------------- *)

(* type of SAT solvers: given a propositional formula, a satisfying          *)

(*      assignment may be returned                                           *)

(* ------------------------------------------------------------------------- *)

  type solver = prop_formula -> result;

(* ------------------------------------------------------------------------- *)

(* write_dimacs_cnf_file: serializes a formula 'fm' of propositional logic   *)

(*      to a file in DIMACS CNF format (see "Satisfiability Suggested        *)

(*      Format", May 8 1993, Section 2.1)                                    *)

(* Note: 'fm' must not contain a variable index less than 1.                 *)

(* Note: 'fm' must be given in CNF.                                          *)

(* ------------------------------------------------------------------------- *)

  (* Path.T -> prop_formula -> unit *)

  fun write_dimacs_cnf_file path fm =

  let

    (* prop_formula -> prop_formula *)

    fun cnf_True_False_elim True =

      Or (BoolVar 1, Not (BoolVar 1))

      | cnf_True_False_elim False =

      And (BoolVar 1, Not (BoolVar 1))

      | cnf_True_False_elim fm =

      fm  (* since 'fm' is in CNF, either 'fm'='True'/'False', or 'fm' does not contain 'True'/'False' at all *)

    (* prop_formula -> int *)

    fun cnf_number_of_clauses (And (fm1,fm2)) =

      (cnf_number_of_clauses fm1) + (cnf_number_of_clauses fm2)

      | cnf_number_of_clauses _ =

      1

    (* prop_formula -> string list *)

    fun cnf_string fm =

    let

      (* prop_formula -> string list -> string list *)

      fun cnf_string_acc True acc =

        error "formula is not in CNF"

        | cnf_string_acc False acc =

        error "formula is not in CNF"

        | cnf_string_acc (BoolVar i) acc =

        (i>=1 orelse error "formula contains a variable index less than 1";

        string_of_int i :: acc)

        | cnf_string_acc (Not (BoolVar i)) acc =

        "-" :: cnf_string_acc (BoolVar i) acc

        | cnf_string_acc (Not _) acc =

        error "formula is not in CNF"

        | cnf_string_acc (Or (fm1,fm2)) acc =

        cnf_string_acc fm1 (" " :: cnf_string_acc fm2 acc)

        | cnf_string_acc (And (fm1,fm2)) acc =

        cnf_string_acc fm1 (" 0\n" :: cnf_string_acc fm2 acc)

    in

      cnf_string_acc fm []

    end

    val fm'               = cnf_True_False_elim fm

    val number_of_vars    = maxidx fm'

    val number_of_clauses = cnf_number_of_clauses fm'

  in

    File.write path

      ("c This file was generated by SatSolver.write_dimacs_cnf_file\n" ^

       "p cnf " ^ string_of_int number_of_vars ^ " " ^ string_of_int number_of_clauses ^ "\n");

    File.append_list path

      (cnf_string fm');

    File.append path

      " 0\n"

  end;

(* ------------------------------------------------------------------------- *)

(* write_dimacs_sat_file: serializes a formula 'fm' of propositional logic   *)

(*      to a file in DIMACS SAT format (see "Satisfiability Suggested        *)

(*      Format", May 8 1993, Section 2.2)                                    *)

(* Note: 'fm' must not contain a variable index less than 1.                 *)

(* ------------------------------------------------------------------------- *)

  (* Path.T -> prop_formula -> unit *)

  fun write_dimacs_sat_file path fm =

  let

    (* prop_formula -> string list *)

    fun sat_string fm =

    let

      (* prop_formula -> string list -> string list *)

      fun sat_string_acc True acc =

        "*()" :: acc

        | sat_string_acc False acc =

        "+()" :: acc

        | sat_string_acc (BoolVar i) acc =

        (i>=1 orelse error "formula contains a variable index less than 1";

        string_of_int i :: acc)

        | sat_string_acc (Not (BoolVar i)) acc =

        "-" :: sat_string_acc (BoolVar i) acc

        | sat_string_acc (Not fm) acc =

        "-(" :: sat_string_acc fm (")" :: acc)

        | sat_string_acc (Or (fm1,fm2)) acc =

        "+(" :: sat_string_acc_or fm1 (" " :: sat_string_acc_or fm2 (")" :: acc))

        | sat_string_acc (And (fm1,fm2)) acc =

        "*(" :: sat_string_acc_and fm1 (" " :: sat_string_acc_and fm2 (")" :: acc))

      (* optimization to make use of n-ary disjunction/conjunction *)

      (* prop_formula -> string list -> string list *)

      and sat_string_acc_or (Or (fm1,fm2)) acc =

        sat_string_acc_or fm1 (" " :: sat_string_acc_or fm2 acc)

        | sat_string_acc_or fm acc =

        sat_string_acc fm acc

      (* prop_formula -> string list -> string list *)

      and sat_string_acc_and (And (fm1,fm2)) acc =

        sat_string_acc_and fm1 (" " :: sat_string_acc_and fm2 acc)

        | sat_string_acc_and fm acc =

        sat_string_acc fm acc

    in

      sat_string_acc fm []

    end

    val number_of_vars = Int.max (maxidx fm, 1)

  in

    File.write path

      ("c This file was generated by SatSolver.write_dimacs_sat_file\n" ^

       "p sat " ^ string_of_int number_of_vars ^ "\n" ^

       "(");

    File.append_list path

      (sat_string fm);

    File.append path

      ")\n"

  end;

(* ------------------------------------------------------------------------- *)

(* read_std_result_file: scans a SAT solver's output file for a satisfying   *)

(*      variable assignment.  Returns the assignment, or 'UNSATISFIABLE' if  *)

(*      the file contains 'unsatisfiable', or 'UNKNOWN' if the file contains *)

(*      neither 'satisfiable' nor 'unsatisfiable'.  Empty lines are ignored. *)

(*      The assignment must be given in one or more lines immediately after  *)

(*      the line that contains 'satisfiable'.  These lines must begin with   *)

(*      'assignment_prefix'.  Variables must be separated by " ".  Non-      *)

(*      integer strings are ignored.  If variable i is contained in the      *)

(*      assignment, then i is interpreted as 'true'.  If ~i is contained in  *)

(*      the assignment, then i is interpreted as 'false'.  Otherwise the     *)

(*      value of i is taken to be unspecified.                               *)

(* ------------------------------------------------------------------------- *)

  (* Path.T -> string * string * string -> result *)

  fun read_std_result_file path (satisfiable, assignment_prefix, unsatisfiable) =

  let

    (* string -> int list *)

    fun int_list_from_string s =

      List.mapPartial Int.fromString (space_explode " " s)

    (* int list -> assignment *)

    fun assignment_from_list [] i =

      NONE  (* the SAT solver didn't provide a value for this variable *)

      | assignment_from_list (x::xs) i =

      if x=i then (SOME true)

      else if x=(~i) then (SOME false)

      else assignment_from_list xs i

    (* int list -> string list -> assignment *)

    fun parse_assignment xs [] =

      assignment_from_list xs

      | parse_assignment xs (line::lines) =

      if String.isPrefix assignment_prefix line then

        parse_assignment (xs @ int_list_from_string line) lines

      else

        assignment_from_list xs

    (* string -> string -> bool *)

    fun is_substring needle haystack =

    let

      val length1 = String.size needle

      val length2 = String.size haystack

    in

      if length2 < length1 then

        false

      else if needle = String.substring (haystack, 0, length1) then

        true

      else is_substring needle (String.substring (haystack, 1, length2-1))

    end

    (* string list -> result *)

    fun parse_lines [] =

      UNKNOWN

      | parse_lines (line::lines) =

      if is_substring unsatisfiable line then

        UNSATISFIABLE NONE

      else if is_substring satisfiable line then

        SATISFIABLE (parse_assignment [] lines)

      else

        parse_lines lines

  in

    (parse_lines o (List.filter (fn l => l <> "")) o split_lines o File.read) path

  end;

(* ------------------------------------------------------------------------- *)

(* make_external_solver: call 'writefn', execute 'cmd', call 'readfn'        *)

(* ------------------------------------------------------------------------- *)

  (* string -> (PropLogic.prop_formula -> unit) -> (unit -> result) -> solver *)

  fun make_external_solver cmd writefn readfn fm =

    (writefn fm; system cmd; readfn ());

(* ------------------------------------------------------------------------- *)

(* read_dimacs_cnf_file: returns a propositional formula that corresponds to *)

(*      a SAT problem given in DIMACS CNF format                             *)

(* ------------------------------------------------------------------------- *)

  (* Path.T -> PropLogic.prop_formula *)

  fun read_dimacs_cnf_file path =

  let

    (* string list -> string list *)

    fun filter_preamble [] =

      error "problem line not found in DIMACS CNF file"

      | filter_preamble (line::lines) =

      if String.isPrefix "c " line orelse line = "c" then

        (* ignore comments *)

        filter_preamble lines

      else if String.isPrefix "p " line then

        (* ignore the problem line (which must be the last line of the preamble) *)

        (* Ignoring the problem line implies that if the file contains more clauses *)

        (* or variables than specified in its preamble, we will accept it anyway.   *)

        lines

      else

        error "preamble in DIMACS CNF file contains a line that does not begin with \"c \" or \"p \""

    (* string -> int *)

    fun int_from_string s =

      case Int.fromString s of

        SOME i => i

      | NONE   => error ("token " ^ quote s ^ "in DIMACS CNF file is not a number")

    (* int list -> int list list *)

    fun clauses xs =

      let

        val (xs1, xs2) = take_prefix (fn i => i <> 0) xs

      in

        case xs2 of

          []      => [xs1]

        | (0::[]) => [xs1]

        | (0::tl) => xs1 :: clauses tl

        | _       => sys_error "this cannot happen"

      end

    (* int -> PropLogic.prop_formula *)

    fun literal_from_int i =

      (i<>0 orelse error "variable index in DIMACS CNF file is 0";

      if i>0 then

        PropLogic.BoolVar i

      else

        PropLogic.Not (PropLogic.BoolVar (~i)))

    (* PropLogic.prop_formula list -> PropLogic.prop_formula *)

    fun disjunction [] =

      error "empty clause in DIMACS CNF file"

      | disjunction (x::xs) =

      (case xs of

        [] => x

      | _  => PropLogic.Or (x, disjunction xs))

    (* PropLogic.prop_formula list -> PropLogic.prop_formula *)

    fun conjunction [] =

      error "no clause in DIMACS CNF file"

      | conjunction (x::xs) =

      (case xs of

        [] => x

      | _  => PropLogic.And (x, conjunction xs))

  in

    (conjunction

    o (map disjunction)

    o (map (map literal_from_int))

    o clauses

    o (map int_from_string)

    o List.concat

    o (map (String.fields (fn c => c mem [#" ", #"\t", #"\n"])))

    o filter_preamble

    o (List.filter (fn l => l <> ""))

    o split_lines

    o File.read)

      path

  end;

(* ------------------------------------------------------------------------- *)

(* solvers: a (reference to a) table of all registered SAT solvers           *)

(* ------------------------------------------------------------------------- *)

  val solvers = ref ([] : (string * solver) list);

(* ------------------------------------------------------------------------- *)

(* add_solver: updates 'solvers' by adding a new solver                      *)

(* ------------------------------------------------------------------------- *)

  (* string * solver -> unit *)

    fun add_solver (name, new_solver) =

      let

        val the_solvers = !solvers;

        val _ = if AList.defined (op =) the_solvers name

          then warning ("SAT solver " ^ quote name ^ " was defined before")

          else ();

      in solvers := AList.update (op =) (name, new_solver) the_solvers end;

(* ------------------------------------------------------------------------- *)

(* invoke_solver: returns the solver associated with the given 'name'        *)

(* Note: If no solver is associated with 'name', exception 'Option' will be  *)

(*       raised.                                                             *)

(* ------------------------------------------------------------------------- *)

  (* string -> solver *)

  fun invoke_solver name =

    (the o AList.lookup (op =) (!solvers)) name;

end;  (* SatSolver *)

(* ------------------------------------------------------------------------- *)

(* Predefined SAT solvers                                                    *)

(* ------------------------------------------------------------------------- *)

(* ------------------------------------------------------------------------- *)

(* Internal SAT solver, available as 'SatSolver.invoke_solver "enumerate"'   *)

(* -- simply enumerates assignments until a satisfying assignment is found   *)

(* ------------------------------------------------------------------------- *)

let

  fun enum_solver fm =

  let

    (* int list *)

    val indices = PropLogic.indices fm

    (* int list -> int list -> int list option *)

    (* binary increment: list 'xs' of current bits, list 'ys' of all bits (lower bits first) *)

    fun next_list _ ([]:int list) =

      NONE

      | next_list [] (y::ys) =

      SOME [y]

      | next_list (x::xs) (y::ys) =

      if x=y then

        (* reset the bit, continue *)

        next_list xs ys

      else

        (* set the lowest bit that wasn't set before, keep the higher bits *)

        SOME (y::x::xs)

    (* int list -> int -> bool *)

    fun assignment_from_list xs i =

      i mem xs

    (* int list -> SatSolver.result *)

    fun solver_loop xs =

      if PropLogic.eval (assignment_from_list xs) fm then

        SatSolver.SATISFIABLE (SOME o (assignment_from_list xs))

      else

        (case next_list xs indices of

          SOME xs' => solver_loop xs'

        | NONE     => SatSolver.UNSATISFIABLE NONE)

  in

    (* start with the "first" assignment (all variables are interpreted as 'false') *)

    solver_loop []

  end

in

  SatSolver.add_solver ("enumerate", enum_solver)

end;

(* ------------------------------------------------------------------------- *)

(* Internal SAT solver, available as 'SatSolver.invoke_solver "dpll"' -- a   *)

(* simple implementation of the DPLL algorithm (cf. L. Zhang, S. Malik: "The *)

(* Quest for Efficient Boolean Satisfiability Solvers", July 2002, Fig. 1).  *)

(* ------------------------------------------------------------------------- *)

let

  local

    open PropLogic

  in

    fun dpll_solver fm =

    let

      (* We could use 'PropLogic.defcnf fm' instead of 'PropLogic.nnf fm' *)

      (* but that sometimes leads to worse performance due to the         *)

      (* introduction of additional variables.                            *)

      val fm' = PropLogic.nnf fm

      (* int list *)

      val indices = PropLogic.indices fm'

      (* int list -> int -> prop_formula *)

      fun partial_var_eval []      i = BoolVar i

        | partial_var_eval (x::xs) i = if x=i then True else if x=(~i) then False else partial_var_eval xs i

      (* int list -> prop_formula -> prop_formula *)

      fun partial_eval xs True             = True

        | partial_eval xs False            = False

        | partial_eval xs (BoolVar i)      = partial_var_eval xs i

        | partial_eval xs (Not fm)         = SNot (partial_eval xs fm)

        | partial_eval xs (Or (fm1, fm2))  = SOr (partial_eval xs fm1, partial_eval xs fm2)

        | partial_eval xs (And (fm1, fm2)) = SAnd (partial_eval xs fm1, partial_eval xs fm2)

      (* prop_formula -> int list *)

      fun forced_vars True              = []

        | forced_vars False             = []

        | forced_vars (BoolVar i)       = [i]

        | forced_vars (Not (BoolVar i)) = [~i]

        | forced_vars (Or (fm1, fm2))   = (forced_vars fm1) inter_int (forced_vars fm2)

        | forced_vars (And (fm1, fm2))  = (forced_vars fm1) union_int (forced_vars fm2)

        (* Above, i *and* ~i may be forced.  In this case the first occurrence takes   *)

        (* precedence, and the next partial evaluation of the formula returns 'False'. *)

        | forced_vars _                 = error "formula is not in negation normal form"

      (* int list -> prop_formula -> (int list * prop_formula) *)

      fun eval_and_force xs fm =

      let

        val fm' = partial_eval xs fm

        val xs' = forced_vars fm'

      in

        if null xs' then

          (xs, fm')

        else

          eval_and_force (xs@xs') fm'  (* xs and xs' should be distinct, so '@' here should have *)

                                       (* the same effect as 'union_int'                         *)

      end

      (* int list -> int option *)

      fun fresh_var xs =

        Library.find_first (fn i => not (i mem_int xs) andalso not ((~i) mem_int xs)) indices

      (* int list -> prop_formula -> int list option *)

      (* partial assignment 'xs' *)

      fun dpll xs fm =

      let

        val (xs', fm') = eval_and_force xs fm

      in

        case fm' of

          True  => SOME xs'

        | False => NONE

        | _     =>

          let

            val x = valOf (fresh_var xs')  (* a fresh variable must exist since 'fm' did not evaluate to 'True'/'False' *)

          in

            case dpll (x::xs') fm' of  (* passing fm' rather than fm should save some simplification work *)

              SOME xs'' => SOME xs''

            | NONE      => dpll ((~x)::xs') fm'  (* now try interpreting 'x' as 'False' *)

          end

      end

      (* int list -> assignment *)

      fun assignment_from_list [] i =

        NONE  (* the DPLL procedure didn't provide a value for this variable *)

        | assignment_from_list (x::xs) i =

        if x=i then (SOME true)

        else if x=(~i) then (SOME false)

        else assignment_from_list xs i

    in

      (* initially, no variable is interpreted yet *)

      case dpll [] fm' of

        SOME assignment => SatSolver.SATISFIABLE (assignment_from_list assignment)

      | NONE            => SatSolver.UNSATISFIABLE NONE

    end

  end  (* local *)

in

  SatSolver.add_solver ("dpll", dpll_solver)

end;

(* ------------------------------------------------------------------------- *)

(* Internal SAT solver, available as 'SatSolver.invoke_solver "auto"': uses  *)

(* the last installed solver (other than "auto" itself) that does not raise  *)

(* 'NOT_CONFIGURED'.  (However, the solver may return 'UNKNOWN'.)            *)

(* ------------------------------------------------------------------------- *)

let

  fun auto_solver fm =

  let

    fun loop [] =

      SatSolver.UNKNOWN

      | loop ((name, solver)::solvers) =

      if name="auto" then

        (* do not call solver "auto" from within "auto" *)

        loop solvers

      else (

        (if name="dpll" orelse name="enumerate" then

          warning ("Using SAT solver " ^ quote name ^ "; for better performance, consider using an external solver.")

        else

          tracing ("Using SAT solver " ^ quote name ^ "."));

        (* apply 'solver' to 'fm' *)

        solver fm

          handle SatSolver.NOT_CONFIGURED => loop solvers

      )

  in

    loop (!SatSolver.solvers)

  end

in

  SatSolver.add_solver ("auto", auto_solver)

end;

(* ------------------------------------------------------------------------- *)

(* MiniSat 1.14                                                              *)

(* (http://www.cs.chalmers.se/Cs/Research/FormalMethods/MiniSat/)            *)

(* ------------------------------------------------------------------------- *)

(* ------------------------------------------------------------------------- *)

(* "minisat_with_proofs" requires a modified version of MiniSat 1.14 by John *)

(* Matthews, which can output ASCII proof traces.  Replaying binary proof    *)

(* traces generated by MiniSat-p_v1.14 has _not_ been implemented.           *)

(* ------------------------------------------------------------------------- *)

(* add "minisat_with_proofs" _before_ "minisat" to the available solvers, so *)

(* that the latter is preferred by the "auto" solver                         *)

(* There is a complication that is dealt with in the code below: MiniSat     *)

(* introduces IDs for original clauses in the proof trace.  It does not (in  *)

(* general) follow the convention that the original clauses are numbered     *)

(* from 0 to n-1 (where n is the number of clauses in the formula).          *)

let

  exception INVALID_PROOF of string

  fun minisat_with_proofs fm =

  let

    val _          = if (getenv "MINISAT_HOME") = "" then raise SatSolver.NOT_CONFIGURED else ()

    val inpath     = File.tmp_path (Path.explode "isabelle.cnf")

    val outpath    = File.tmp_path (Path.explode "result")

    val proofpath  = File.tmp_path (Path.explode "result.prf")

    val cmd        = (getenv "MINISAT_HOME") ^ "/minisat " ^ (Path.implode inpath) ^ " -r " ^ (Path.implode outpath) ^ " -t " ^ (Path.implode proofpath) ^ "> /dev/null"

    fun writefn fm = SatSolver.write_dimacs_cnf_file inpath fm

    fun readfn ()  = SatSolver.read_std_result_file outpath ("SAT", "", "UNSAT")

    val _          = if File.exists inpath then warning ("overwriting existing file " ^ quote (Path.implode inpath)) else ()

    val _          = if File.exists outpath then warning ("overwriting existing file " ^ quote (Path.implode outpath)) else ()

    val cnf        = PropLogic.defcnf fm

    val result     = SatSolver.make_external_solver cmd writefn readfn cnf

    val _          = try File.rm inpath

    val _          = try File.rm outpath

  in  case result of

    SatSolver.UNSATISFIABLE NONE =>

    (let

      (* string list *)

      val proof_lines = (split_lines o File.read) proofpath

        handle IO.Io _ => raise INVALID_PROOF "Could not read file \"result.prf\""

      (* representation of clauses as ordered lists of literals (with duplicates removed) *)

      (* prop_formula -> int list *)

      fun clause_to_lit_list (PropLogic.Or (fm1, fm2)) =

        OrdList.union int_ord (clause_to_lit_list fm1) (clause_to_lit_list fm2)

        | clause_to_lit_list (PropLogic.BoolVar i) =

        [i]

        | clause_to_lit_list (PropLogic.Not (PropLogic.BoolVar i)) =

        [~i]

        | clause_to_lit_list _ =

        raise INVALID_PROOF "Error: invalid clause in CNF formula."

      (* prop_formula -> int *)

      fun cnf_number_of_clauses (PropLogic.And (fm1, fm2)) =

        cnf_number_of_clauses fm1 + cnf_number_of_clauses fm2

        | cnf_number_of_clauses _ =

        1

      val number_of_clauses = cnf_number_of_clauses cnf

      (* int list array *)

      val clauses = Array.array (number_of_clauses, [])

      (* initialize the 'clauses' array *)

      (* prop_formula * int -> int *)

      fun init_array (PropLogic.And (fm1, fm2), n) =

        init_array (fm2, init_array (fm1, n))

        | init_array (fm, n) =

        (Array.update (clauses, n, clause_to_lit_list fm); n+1)

      val _ = init_array (cnf, 0)

      (* optimization for the common case where MiniSat "R"s clauses in their *)

      (* original order:                                                      *)

      val last_ref_clause = ref (number_of_clauses - 1)

      (* search the 'clauses' array for the given list of literals 'lits', *)

      (* starting at index '!last_ref_clause + 1'                          *)

      (* int list -> int option *)

      fun original_clause_id lits =

      let

        fun original_clause_id_from index =

          if index = number_of_clauses then

            (* search from beginning again *)

            original_clause_id_from 0

          (* both 'lits' and the list of literals used in 'clauses' are sorted, so *)

          (* testing for equality should suffice -- barring duplicate literals     *)

          else if Array.sub (clauses, index) = lits then (

            (* success *)

            last_ref_clause := index;

            SOME index

          ) else if index = !last_ref_clause then

            (* failure *)

            NONE

          else

            (* continue search *)

            original_clause_id_from (index + 1)

      in

        original_clause_id_from (!last_ref_clause + 1)

      end

      (* string -> int *)

      fun int_from_string s = (

        case Int.fromString s of

          SOME i => i

        | NONE   => raise INVALID_PROOF ("File format error: number expected (" ^ quote s ^ " encountered).")

      )

      (* parse the proof file *)

      val clause_table  = ref (Inttab.empty : int list Inttab.table)

      val empty_id      = ref ~1

      (* contains a mapping from clause IDs as used by MiniSat to clause IDs in *)

      (* our proof format, where original clauses are numbered starting from 0  *)

      val clause_id_map = ref (Inttab.empty : int Inttab.table)

      fun sat_to_proof id = (

        case Inttab.lookup (!clause_id_map) id of

          SOME id' => id'

        | NONE     => raise INVALID_PROOF ("Clause ID " ^ Int.toString id ^ " used, but not defined.")

      )

      val next_id = ref (number_of_clauses - 1)

      (* string list -> unit *)

      fun process_tokens [] =

        ()

        | process_tokens (tok::toks) =

        if tok="R" then (

          case toks of

            id::sep::lits =>

            let

              val _        = if !empty_id = ~1 then () else raise INVALID_PROOF "File format error: \"R\" disallowed after \"X\"."

              val cid      = int_from_string id

              val _        = if sep = "<=" then () else raise INVALID_PROOF ("File format error: \"<=\" expected (" ^ quote sep ^ " encountered).")

              val ls       = sort int_ord (map int_from_string lits)

              val proof_id = case original_clause_id ls of

                               SOME orig_id => orig_id

                             | NONE         => raise INVALID_PROOF ("Original clause (new ID is " ^ id ^ ") not found.")

            in

              (* extend the mapping of clause IDs with this newly defined ID *)

              clause_id_map := Inttab.update_new (cid, proof_id) (!clause_id_map)

                handle Inttab.DUP _ => raise INVALID_PROOF ("File format error: clause " ^ id ^ " defined more than once (in \"R\").")

              (* the proof itself doesn't change *)

            end

          | _ =>

            raise INVALID_PROOF "File format error: \"R\" followed by an insufficient number of tokens."

        ) else if tok="C" then (

          case toks of

            id::sep::ids =>

            let

              val _        = if !empty_id = ~1 then () else raise INVALID_PROOF "File format error: \"C\" disallowed after \"X\"."

              val cid      = int_from_string id

              val _        = if sep = "<=" then () else raise INVALID_PROOF ("File format error: \"<=\" expected (" ^ quote sep ^ " encountered).")

              (* ignore the pivot literals in MiniSat's trace *)

              fun unevens []             = raise INVALID_PROOF "File format error: \"C\" followed by an even number of IDs."

                | unevens (x :: [])      = x :: []

                | unevens (x :: _ :: xs) = x :: unevens xs

              val rs       = (map sat_to_proof o unevens o map int_from_string) ids

              (* extend the mapping of clause IDs with this newly defined ID *)

              val proof_id = inc next_id

              val _        = clause_id_map := Inttab.update_new (cid, proof_id) (!clause_id_map)

                               handle Inttab.DUP _ => raise INVALID_PROOF ("File format error: clause " ^ id ^ " defined more than once (in \"C\").")

            in

              (* update clause table *)

              clause_table := Inttab.update_new (proof_id, rs) (!clause_table)

                handle Inttab.DUP _ => raise INVALID_PROOF ("Error: internal ID for clause " ^ id ^ " already used.")

            end

          | _ =>

            raise INVALID_PROOF "File format error: \"C\" followed by an insufficient number of tokens."

        ) else if tok="D" then (

          case toks of

            [id] =>

            let

              val _ = if !empty_id = ~1 then () else raise INVALID_PROOF "File format error: \"D\" disallowed after \"X\"."

              val _ = sat_to_proof (int_from_string id)

            in

              (* simply ignore "D" *)

              ()

            end

          | _ =>

            raise INVALID_PROOF "File format error: \"D\" followed by an illegal number of tokens."

        ) else if tok="X" then (

          case toks of

            [id1, id2] =>

            let

              val _            = if !empty_id = ~1 then () else raise INVALID_PROOF "File format error: more than one end-of-proof statement."

              val _            = sat_to_proof (int_from_string id1)

              val new_empty_id = sat_to_proof (int_from_string id2)

            in

              (* update conflict id *)

              empty_id := new_empty_id

            end

          | _ =>

            raise INVALID_PROOF "File format error: \"X\" followed by an illegal number of tokens."

        ) else

          raise INVALID_PROOF ("File format error: unknown token " ^ quote tok ^ " encountered.")

      (* string list -> unit *)

      fun process_lines [] =

        ()

        | process_lines (l::ls) = (

          process_tokens (String.tokens (fn c => c = #" " orelse c = #"\t") l);

          process_lines ls

        )

      (* proof *)

      val _ = process_lines proof_lines

      val _ = if !empty_id <> ~1 then () else raise INVALID_PROOF "File format error: no conflicting clause specified."

    in

      SatSolver.UNSATISFIABLE (SOME (!clause_table, !empty_id))

    end handle INVALID_PROOF reason => (warning reason; SatSolver.UNSATISFIABLE NONE))

  | result =>

    result

  end

in

  SatSolver.add_solver ("minisat_with_proofs", minisat_with_proofs)

end;

let

  fun minisat fm =

  let

    val _          = if (getenv "MINISAT_HOME") = "" then raise SatSolver.NOT_CONFIGURED else ()

    val inpath     = File.tmp_path (Path.explode "isabelle.cnf")

    val outpath    = File.tmp_path (Path.explode "result")

    val cmd        = (getenv "MINISAT_HOME") ^ "/minisat " ^ (Path.implode inpath) ^ " -r " ^ (Path.implode outpath) ^ " > /dev/null"

    fun writefn fm = SatSolver.write_dimacs_cnf_file inpath (PropLogic.defcnf fm)

    fun readfn ()  = SatSolver.read_std_result_file outpath ("SAT", "", "UNSAT")

    val _          = if File.exists inpath then warning ("overwriting existing file " ^ quote (Path.implode inpath)) else ()

    val _          = if File.exists outpath then warning ("overwriting existing file " ^ quote (Path.implode outpath)) else ()

    val result     = SatSolver.make_external_solver cmd writefn readfn fm

    val _          = try File.rm inpath

    val _          = try File.rm outpath

  in

    result

  end

in

  SatSolver.add_solver ("minisat", minisat)

end;

(* ------------------------------------------------------------------------- *)

(* zChaff (http://www.princeton.edu/~chaff/zchaff.html)                      *)

(* ------------------------------------------------------------------------- *)

(* ------------------------------------------------------------------------- *)

(* 'zchaff_with_proofs' applies the "zchaff" prover to a formula, and if     *)

(* zChaff finds that the formula is unsatisfiable, a proof of this is read   *)

(* from a file "resolve_trace" that was generated by zChaff.  See the code   *)

(* below for the expected format of the "resolve_trace" file.  Aside from    *)

(* some basic syntactic checks, no verification of the proof is performed.   *)

(* ------------------------------------------------------------------------- *)

(* add "zchaff_with_proofs" _before_ "zchaff" to the available solvers, so   *)

(* that the latter is preferred by the "auto" solver                         *)

let

  exception INVALID_PROOF of string

  fun zchaff_with_proofs fm =

  case SatSolver.invoke_solver "zchaff" fm of

    SatSolver.UNSATISFIABLE NONE =>

    (let

      (* string list *)

      val proof_lines = ((split_lines o File.read) (Path.explode "resolve_trace"))

        handle IO.Io _ => raise INVALID_PROOF "Could not read file \"resolve_trace\""

      (* PropLogic.prop_formula -> int *)

      fun cnf_number_of_clauses (PropLogic.And (fm1, fm2)) = cnf_number_of_clauses fm1 + cnf_number_of_clauses fm2

        | cnf_number_of_clauses _                          = 1

      (* string -> int *)

      fun int_from_string s = (

        case Int.fromString s of

          SOME i => i

        | NONE   => raise INVALID_PROOF ("File format error: number expected (" ^ quote s ^ " encountered).")

      )

      (* parse the "resolve_trace" file *)

      val clause_offset = ref ~1

      val clause_table  = ref (Inttab.empty : int list Inttab.table)

      val empty_id      = ref ~1

      (* string list -> unit *)

      fun process_tokens [] =

        ()

        | process_tokens (tok::toks) =

        if tok="CL:" then (

          case toks of

            id::sep::ids =>

            let

              val _   = if !clause_offset = ~1 then () else raise INVALID_PROOF ("File format error: \"CL:\" disallowed after \"VAR:\".")

              val _   = if !empty_id = ~1 then () else raise INVALID_PROOF ("File format error: \"CL:\" disallowed after \"CONF:\".")

              val cid = int_from_string id

              val _   = if sep = "<=" then () else raise INVALID_PROOF ("File format error: \"<=\" expected (" ^ quote sep ^ " encountered).")

              val rs  = map int_from_string ids

            in

              (* update clause table *)

              clause_table := Inttab.update_new (cid, rs) (!clause_table)

                handle Inttab.DUP _ => raise INVALID_PROOF ("File format error: clause " ^ id ^ " defined more than once.")

            end

          | _ =>

            raise INVALID_PROOF "File format error: \"CL:\" followed by an insufficient number of tokens."

        ) else if tok="VAR:" then (

          case toks of

            id::levsep::levid::valsep::valid::antesep::anteid::litsep::lits =>

            let

              val _   = if !empty_id = ~1 then () else raise INVALID_PROOF ("File format error: \"VAR:\" disallowed after \"CONF:\".")

              (* set 'clause_offset' to the largest used clause ID *)

              val _   = if !clause_offset = ~1 then clause_offset :=

                (case Inttab.max_key (!clause_table) of

                  SOME id => id

                | NONE    => cnf_number_of_clauses (PropLogic.defcnf fm) - 1  (* the first clause ID is 0, not 1 *))

                else

                  ()

              val vid = int_from_string id

              val _   = if levsep = "L:" then () else raise INVALID_PROOF ("File format error: \"L:\" expected (" ^ quote levsep ^ " encountered).")

              val _   = int_from_string levid

              val _   = if valsep = "V:" then () else raise INVALID_PROOF ("File format error: \"V:\" expected (" ^ quote valsep ^ " encountered).")

              val _   = int_from_string valid

              val _   = if antesep = "A:" then () else raise INVALID_PROOF ("File format error: \"A:\" expected (" ^ quote antesep ^ " encountered).")

              val aid = int_from_string anteid

              val _   = if litsep = "Lits:" then () else raise INVALID_PROOF ("File format error: \"Lits:\" expected (" ^ quote litsep ^ " encountered).")

              val ls  = map int_from_string lits

              (* convert the data provided by zChaff to our resolution-style proof format *)

              (* each "VAR:" line defines a unit clause, the resolvents are implicitly    *)

              (* given by the literals in the antecedent clause                           *)

              (* we use the sum of '!clause_offset' and the variable ID as clause ID for the unit clause *)

              val cid = !clause_offset + vid

              (* the low bit of each literal gives its sign (positive/negative), therefore  *)

              (* we have to divide each literal by 2 to obtain the proper variable ID; then *)

              (* we add '!clause_offset' to obtain the ID of the corresponding unit clause  *)

              val vids = filter (not_equal vid) (map (fn l => l div 2) ls)

              val rs   = aid :: map (fn v => !clause_offset + v) vids

            in

              (* update clause table *)

              clause_table := Inttab.update_new (cid, rs) (!clause_table)

                handle Inttab.DUP _ => raise INVALID_PROOF ("File format error: clause " ^ string_of_int cid ^ " (derived from antecedent for variable " ^ id ^ ") already defined.")

            end

          | _ =>

            raise INVALID_PROOF "File format error: \"VAR:\" followed by an insufficient number of tokens."

        ) else if tok="CONF:" then (

          case toks of

            id::sep::ids =>

            let

              val _   = if !empty_id = ~1 then () else raise INVALID_PROOF "File format error: more than one conflicting clause specified."

              val cid = int_from_string id

              val _   = if sep = "==" then () else raise INVALID_PROOF ("File format error: \"==\" expected (" ^ quote sep ^ " encountered).")

              val ls  = map int_from_string ids

              (* the conflict clause must be resolved with the unit clauses *)

              (* for its literals to obtain the empty clause                *)

              val vids         = map (fn l => l div 2) ls

              val rs           = cid :: map (fn v => !clause_offset + v) vids

              val new_empty_id = getOpt (Inttab.max_key (!clause_table), !clause_offset) + 1

            in

              (* update clause table and conflict id *)

              clause_table := Inttab.update_new (new_empty_id, rs) (!clause_table)

                handle Inttab.DUP _ => raise INVALID_PROOF ("File format error: clause " ^ string_of_int new_empty_id ^ " (empty clause derived from clause " ^ id ^ ") already defined.");

              empty_id     := new_empty_id

            end

          | _ =>

            raise INVALID_PROOF "File format error: \"CONF:\" followed by an insufficient number of tokens."

        ) else

          raise INVALID_PROOF ("File format error: unknown token " ^ quote tok ^ " encountered.")

      (* string list -> unit *)

      fun process_lines [] =

        ()

        | process_lines (l::ls) = (

          process_tokens (String.tokens (fn c => c = #" " orelse c = #"\t") l);

          process_lines ls

        )

      (* proof *)

      val _ = process_lines proof_lines

      val _ = if !empty_id <> ~1 then () else raise INVALID_PROOF "File format error: no conflicting clause specified."

    in

      SatSolver.UNSATISFIABLE (SOME (!clause_table, !empty_id))

    end handle INVALID_PROOF reason => (warning reason; SatSolver.UNSATISFIABLE NONE))

  | result =>

    result

in

  SatSolver.add_solver ("zchaff_with_proofs", zchaff_with_proofs)

end;

let

  fun zchaff fm =

  let

    val _          = if (getenv "ZCHAFF_HOME") = "" then raise SatSolver.NOT_CONFIGURED else ()

    val _          = if (getenv "ZCHAFF_VERSION") <> "2004.5.13" andalso

                        (getenv "ZCHAFF_VERSION") <> "2004.11.15" then raise SatSolver.NOT_CONFIGURED else ()

      (* both versions of zChaff appear to have the same interface, so we do *)

      (* not actually need to distinguish between them in the following code *)

    val inpath     = File.tmp_path (Path.explode "isabelle.cnf")

    val outpath    = File.tmp_path (Path.explode "result")

    val cmd        = (getenv "ZCHAFF_HOME") ^ "/zchaff " ^ (Path.implode inpath) ^ " > " ^ (Path.implode outpath)

    fun writefn fm = SatSolver.write_dimacs_cnf_file inpath (PropLogic.defcnf fm)

    fun readfn ()  = SatSolver.read_std_result_file outpath ("Instance Satisfiable", "", "Instance Unsatisfiable")

    val _          = if File.exists inpath then warning ("overwriting existing file " ^ quote (Path.implode inpath)) else ()

    val _          = if File.exists outpath then warning ("overwriting existing file " ^ quote (Path.implode outpath)) else ()

    val result     = SatSolver.make_external_solver cmd writefn readfn fm

    val _          = try File.rm inpath

    val _          = try File.rm outpath

  in

    result

  end

in

  SatSolver.add_solver ("zchaff", zchaff)

end;

(* ------------------------------------------------------------------------- *)

(* BerkMin 561 (http://eigold.tripod.com/BerkMin.html)                       *)

(* ------------------------------------------------------------------------- *)

let

  fun berkmin fm =

  let

    val _          = if (getenv "BERKMIN_HOME") = "" orelse (getenv "BERKMIN_EXE") = "" then raise SatSolver.NOT_CONFIGURED else ()

    val inpath     = File.tmp_path (Path.explode "isabelle.cnf")

    val outpath    = File.tmp_path (Path.explode "result")

    val cmd        = (getenv "BERKMIN_HOME") ^ "/" ^ (getenv "BERKMIN_EXE") ^ " " ^ (Path.implode inpath) ^ " > " ^ (Path.implode outpath)

    fun writefn fm = SatSolver.write_dimacs_cnf_file inpath (PropLogic.defcnf fm)

    fun readfn ()  = SatSolver.read_std_result_file outpath ("Satisfiable          !!", "solution =", "UNSATISFIABLE          !!")

    val _          = if File.exists inpath then warning ("overwriting existing file " ^ quote (Path.implode inpath)) else ()

    val _          = if File.exists outpath then warning ("overwriting existing file " ^ quote (Path.implode outpath)) else ()

    val result     = SatSolver.make_external_solver cmd writefn readfn fm

    val _          = try File.rm inpath

    val _          = try File.rm outpath

  in

    result

  end

in

  SatSolver.add_solver ("berkmin", berkmin)

end;

(* ------------------------------------------------------------------------- *)

(* Jerusat 1.3 (http://www.cs.tau.ac.il/~ale1/)                              *)

(* ------------------------------------------------------------------------- *)

let

  fun jerusat fm =

  let

    val _          = if (getenv "JERUSAT_HOME") = "" then raise SatSolver.NOT_CONFIGURED else ()

    val inpath     = File.tmp_path (Path.explode "isabelle.cnf")

    val outpath    = File.tmp_path (Path.explode "result")

    val cmd        = (getenv "JERUSAT_HOME") ^ "/Jerusat1.3 " ^ (Path.implode inpath) ^ " > " ^ (Path.implode outpath)

    fun writefn fm = SatSolver.write_dimacs_cnf_file inpath (PropLogic.defcnf fm)

    fun readfn ()  = SatSolver.read_std_result_file outpath ("s SATISFIABLE", "v ", "s UNSATISFIABLE")

    val _          = if File.exists inpath then warning ("overwriting existing file " ^ quote (Path.implode inpath)) else ()

    val _          = if File.exists outpath then warning ("overwriting existing file " ^ quote (Path.implode outpath)) else ()

    val result     = SatSolver.make_external_solver cmd writefn readfn fm

    val _          = try File.rm inpath

    val _          = try File.rm outpath

  in

    result

  end

in

  SatSolver.add_solver ("jerusat", jerusat)

end;

(* ------------------------------------------------------------------------- *)

(* Add code for other SAT solvers below.                                     *)

(* ------------------------------------------------------------------------- *)

(*

let

  fun mysolver fm = ...

in