src/Pure/search.ML

tuned signature;
tuned comments;

(*  Title:      Pure/search.ML
    Author:     Lawrence C Paulson and Norbert Voelker

Search tacticals.
*)

infix 1 THEN_MAYBE THEN_MAYBE';

signature SEARCH =
sig
  val trace_DEPTH_FIRST: bool Unsynchronized.ref
  val DEPTH_FIRST: (thm -> bool) -> tactic -> tactic
  val has_fewer_prems: int -> thm -> bool
  val IF_UNSOLVED: tactic -> tactic
  val SOLVE: tactic -> tactic
  val THEN_MAYBE: tactic * tactic -> tactic
  val THEN_MAYBE': ('a -> tactic) * ('a -> tactic) -> 'a -> tactic
  val DEPTH_SOLVE: tactic -> tactic
  val DEPTH_SOLVE_1: tactic -> tactic
  val THEN_ITER_DEEPEN: int -> tactic -> (thm -> bool) -> (int -> tactic) -> tactic
  val ITER_DEEPEN: int -> (thm -> bool) -> (int -> tactic) -> tactic
  val trace_DEEPEN: bool Unsynchronized.ref
  val DEEPEN: int * int -> (int -> int -> tactic) -> int -> int -> tactic
  val trace_BEST_FIRST: bool Unsynchronized.ref
  val THEN_BEST_FIRST: tactic -> (thm -> bool) * (thm -> int) -> tactic -> tactic
  val BEST_FIRST: (thm -> bool) * (thm -> int) -> tactic -> tactic
  val BREADTH_FIRST: (thm -> bool) -> tactic -> tactic
  val QUIET_BREADTH_FIRST: (thm -> bool) -> tactic -> tactic
  val trace_ASTAR: bool Unsynchronized.ref
  val THEN_ASTAR: tactic -> (thm -> bool) * (int -> thm -> int) -> tactic -> tactic
  val ASTAR: (thm -> bool) * (int -> thm -> int) -> tactic -> tactic
end;

structure Search: SEARCH =
struct

(**** Depth-first search ****)

val trace_DEPTH_FIRST = Unsynchronized.ref false;

(*Searches until "satp" reports proof tree as satisfied.
  Suppresses duplicate solutions to minimize search space.*)
fun DEPTH_FIRST satp tac =
 let val tac = tracify trace_DEPTH_FIRST tac
     fun depth used [] = NONE
       | depth used (q::qs) =
          case Seq.pull q of
              NONE         => depth used qs
            | SOME(st,stq) =>
                if satp st andalso not (member Thm.eq_thm used st)
                then SOME(st, Seq.make
                                 (fn()=> depth (st::used) (stq::qs)))
                else depth used (tac st :: stq :: qs)
  in  traced_tac (fn st => depth [] [Seq.single st])  end;



(*Predicate: Does the rule have fewer than n premises?*)
fun has_fewer_prems n rule = (nprems_of rule < n);

(*Apply a tactic if subgoals remain, else do nothing.*)
val IF_UNSOLVED = COND (has_fewer_prems 1) all_tac;

(*Force a tactic to solve its goal completely, otherwise fail *)
fun SOLVE tac = tac THEN COND (has_fewer_prems 1) all_tac no_tac;

(*Execute tac1, but only execute tac2 if there are at least as many subgoals
  as before.  This ensures that tac2 is only applied to an outcome of tac1.*)
fun (tac1 THEN_MAYBE tac2) st =
  (tac1  THEN  COND (has_fewer_prems (nprems_of st)) all_tac tac2)  st;

fun (tac1 THEN_MAYBE' tac2) x = tac1 x THEN_MAYBE tac2 x;

(*Tactical to reduce the number of premises by 1.
  If no subgoals then it must fail! *)
fun DEPTH_SOLVE_1 tac st = st |>
    (case nprems_of st of
        0 => no_tac
      | n => DEPTH_FIRST (has_fewer_prems n) tac);

(*Uses depth-first search to solve ALL subgoals*)
val DEPTH_SOLVE = DEPTH_FIRST (has_fewer_prems 1);



(**** Iterative deepening with pruning ****)

fun has_vars (Var _) = true
  | has_vars (Abs (_,_,t)) = has_vars t
  | has_vars (f$t) = has_vars f orelse has_vars t
  | has_vars _ = false;

(*Counting of primitive inferences is APPROXIMATE, as the step tactic
  may perform >1 inference*)

(*Pruning of rigid ancestor to prevent backtracking*)
fun prune (new as (k', np':int, rgd', stq), qs) =
    let fun prune_aux (qs, []) = new::qs
          | prune_aux (qs, (k,np,rgd,q)::rqs) =
              if np'+1 = np andalso rgd then
                  (if !trace_DEPTH_FIRST then
                       tracing ("Pruning " ^
                                string_of_int (1+length rqs) ^ " levels")
                   else ();
                   (*Use OLD k: zero-cost solution; see Stickel, p 365*)
                   (k, np', rgd', stq) :: qs)
              else prune_aux ((k,np,rgd,q)::qs, rqs)
        fun take ([], rqs) = ([], rqs)
          | take (arg as ((k,np,rgd,stq)::qs, rqs)) =
                if np' < np then take (qs, (k,np,rgd,stq)::rqs)
                            else arg
    in  prune_aux (take (qs, []))  end;


(*Depth-first iterative deepening search for a state that satisfies satp
  tactic tac0 sets up the initial goal queue, while tac1 searches it.
  The solution sequence is redundant: the cutoff heuristic makes it impossible
  to suppress solutions arising from earlier searches, as the accumulated cost
  (k) can be wrong.*)
fun THEN_ITER_DEEPEN lim tac0 satp tac1 = traced_tac (fn st =>
 let val countr = Unsynchronized.ref 0
     and tf = tracify trace_DEPTH_FIRST (tac1 1)
     and qs0 = tac0 st
     (*bnd = depth bound; inc = estimate of increment required next*)
     fun depth (bnd,inc) [] =
          if bnd > lim then
             (tracing (string_of_int (!countr) ^
                       " inferences so far.  Giving up at " ^ string_of_int bnd);
              NONE)
          else
             (tracing (string_of_int (!countr) ^
                       " inferences so far.  Searching to depth " ^
                       string_of_int bnd);
              (*larger increments make it run slower for the hard problems*)
              depth (bnd+inc, 10)) [(0, 1, false, qs0)]
       | depth (bnd,inc) ((k,np,rgd,q)::qs) =
          if k>=bnd then depth (bnd,inc) qs
          else
          case (Unsynchronized.inc countr;
                if !trace_DEPTH_FIRST then
                    tracing (string_of_int np ^ implode (map (fn _ => "*") qs))
                else ();
                Seq.pull q) of
             NONE         => depth (bnd,inc) qs
           | SOME(st,stq) =>
               if satp st       (*solution!*)
               then SOME(st, Seq.make
                         (fn()=> depth (bnd,inc) ((k,np,rgd,stq)::qs)))

               else
               let val np' = nprems_of st
                     (*rgd' calculation assumes tactic operates on subgoal 1*)
                   val rgd' = not (has_vars (hd (prems_of st)))
                   val k' = k+np'-np+1  (*difference in # of subgoals, +1*)
               in  if k'+np' >= bnd
                   then depth (bnd, Int.min(inc, k'+np'+1-bnd)) qs
                   else if np' < np (*solved a subgoal; prune rigid ancestors*)
                   then depth (bnd,inc)
                         (prune ((k', np', rgd', tf st), (k,np,rgd,stq) :: qs))
                   else depth (bnd,inc) ((k', np', rgd', tf st) ::
                                         (k,np,rgd,stq) :: qs)
               end
  in depth (0,5) [] end);

fun ITER_DEEPEN lim = THEN_ITER_DEEPEN lim all_tac;


(*Simple iterative deepening tactical.  It merely "deepens" any search tactic
  using increment "inc" up to limit "lim". *)
val trace_DEEPEN = Unsynchronized.ref false;

fun DEEPEN (inc, lim) tacf m i =
  let
    fun dpn m st =
      st |>
       (if has_fewer_prems i st then no_tac
        else if m > lim then
          (if !trace_DEEPEN then tracing "Search depth limit exceeded: giving up" else ();
            no_tac)
        else
          (if !trace_DEEPEN then tracing ("Search depth = " ^ string_of_int m) else ();
            tacf m i  ORELSE  dpn (m+inc)))
  in  dpn m  end;


(*** Best-first search ***)

(*total ordering on theorems, allowing duplicates to be found*)
structure Thm_Heap = Heap
(
  type elem = int * thm;
  val ord = prod_ord int_ord (Term_Ord.term_ord o pairself Thm.prop_of);
);

val trace_BEST_FIRST = Unsynchronized.ref false;

(*For creating output sequence*)
fun some_of_list []     = NONE
  | some_of_list (x::l) = SOME (x, Seq.make (fn () => some_of_list l));

(*Check for and delete duplicate proof states*)
fun delete_all_min prf heap =
  if Thm_Heap.is_empty heap then heap
  else if Thm.eq_thm (prf, #2 (Thm_Heap.min heap))
  then delete_all_min prf (Thm_Heap.delete_min heap)
  else heap;

(*Best-first search for a state that satisfies satp (incl initial state)
  Function sizef estimates size of problem remaining (smaller means better).
  tactic tac0 sets up the initial priority queue, while tac1 searches it. *)
fun THEN_BEST_FIRST tac0 (satp, sizef) tac1 =
  let val tac = tracify trace_BEST_FIRST tac1
      fun pairsize th = (sizef th, th);
      fun bfs (news,nprf_heap) =
           (case  List.partition satp news  of
                ([],nonsats) => next(fold_rev Thm_Heap.insert (map pairsize nonsats) nprf_heap)
              | (sats,_)  => some_of_list sats)
      and next nprf_heap =
            if Thm_Heap.is_empty nprf_heap then NONE
            else
            let val (n,prf) = Thm_Heap.min nprf_heap
            in if !trace_BEST_FIRST
               then tracing("state size = " ^ string_of_int n)
               else ();
               bfs (Seq.list_of (tac prf),
                    delete_all_min prf (Thm_Heap.delete_min nprf_heap))
            end
      fun btac st = bfs (Seq.list_of (tac0 st), Thm_Heap.empty)
  in traced_tac btac end;

(*Ordinary best-first search, with no initial tactic*)
val BEST_FIRST = THEN_BEST_FIRST all_tac;

(*Breadth-first search to satisfy satpred (including initial state)
  SLOW -- SHOULD NOT USE APPEND!*)
fun gen_BREADTH_FIRST message satpred (tac:tactic) =
  let val tacf = Seq.list_of o tac;
      fun bfs prfs =
         (case  List.partition satpred prfs  of
              ([],[]) => []
            | ([],nonsats) =>
                  (message("breadth=" ^ string_of_int(length nonsats));
                   bfs (maps tacf nonsats))
            | (sats,_)  => sats)
  in (fn st => Seq.of_list (bfs [st])) end;

val BREADTH_FIRST = gen_BREADTH_FIRST tracing;
val QUIET_BREADTH_FIRST = gen_BREADTH_FIRST (K ());


(*  Author:     Norbert Voelker, FernUniversitaet Hagen
    Remarks:    Implementation of A*-like proof procedure by modification
                of the existing code for BEST_FIRST and best_tac so that the
                current level of search is taken into account.
*)

(*Insertion into priority queue of states, marked with level *)
fun insert_with_level (lnth: int*int*thm) [] = [lnth]
  | insert_with_level (l,m,th) ((l',n,th') :: nths) =
      if  n<m then (l',n,th') :: insert_with_level (l,m,th) nths
      else if  n=m andalso Thm.eq_thm(th,th')
              then (l',n,th')::nths
              else (l,m,th)::(l',n,th')::nths;

(*For creating output sequence*)
fun some_of_list []     = NONE
  | some_of_list (x::l) = SOME (x, Seq.make (fn () => some_of_list l));

val trace_ASTAR = Unsynchronized.ref false;

fun THEN_ASTAR tac0 (satp, costf) tac1 =
  let val tf = tracify trace_ASTAR tac1;
      fun bfs (news,nprfs,level) =
      let fun cost thm = (level, costf level thm, thm)
      in (case  List.partition satp news  of
            ([],nonsats)
                 => next (fold_rev (insert_with_level o cost) nonsats nprfs)
          | (sats,_)  => some_of_list sats)
      end and
      next []  = NONE
        | next ((level,n,prf)::nprfs)  =
            (if !trace_ASTAR
               then tracing("level = " ^ string_of_int level ^
                         "  cost = " ^ string_of_int n ^
                         "  queue length =" ^ string_of_int (length nprfs))
               else ();
             bfs (Seq.list_of (tf prf), nprfs,level+1))
      fun tf st = bfs (Seq.list_of (tac0 st), [], 0)
  in traced_tac tf end;

(*Ordinary ASTAR, with no initial tactic*)
val ASTAR = THEN_ASTAR all_tac;

end;

open Search;