(*  Title:      Provers/Arith/assoc_fold.ML
    ID:         $Id: assoc_fold.ML,v 1.11 2005/08/01 17:20:26 wenzelm Exp $
    Author:     Lawrence C Paulson, Cambridge University Computer Laboratory
    Copyright   1999  University of Cambridge

Simplification procedure for associative operators + and * on numeric types

Performs constant folding when the literals are separated, as in 3+n+4.
*)


signature ASSOC_FOLD_DATA =
sig
  val ss                : simpset       (*basic simpset of object-logtic*)
  val eq_reflection     : thm           (*object-equality to meta-equality*)
  val thy_ref           : theory_ref    (*the operator's signature*)
  val add_ac            : thm list      (*AC-rewrites for plus*)
end;


functor Assoc_Fold (Data: ASSOC_FOLD_DATA) =
struct

 val assoc_ss = Data.ss addsimps Data.add_ac;

 exception Assoc_fail;

 fun mk_sum plus []  = raise Assoc_fail
   | mk_sum plus tms = foldr1 (fn (x,y) => plus $ x $ y) tms;

 (*Separate the literals from the other terms being combined*)
 fun sift_terms plus (t, (lits,others)) =
     case t of
          Const("Numeral.number_of", _) $ _ =>
              (t::lits, others)         (*new literal*)
        | (f as Const _) $ x $ y =>
              if f = plus
              then sift_terms plus (x, sift_terms plus (y, (lits,others)))
              else (lits, t::others)    (*arbitrary summand*)
        | _ => (lits, t::others);

 val trace = ref false;

 (*Make a simproc to combine all literals in a associative nest*)
 fun proc thy ss lhs =
   let fun show t = string_of_cterm (Thm.cterm_of thy t)
       val _ = if !trace then tracing ("assoc_fold simproc: LHS = " ^ show lhs)
               else ()
       val plus =
         (case lhs of f $ _ $ _ => f | _ => error "Assoc_fold: bad pattern")
       val (lits,others) = sift_terms plus (lhs, ([],[]))
       val _ = if length lits < 2
               then raise Assoc_fail (*we can't reduce the number of terms*)
               else ()
       val rhs = plus $ mk_sum plus lits $ mk_sum plus others
       val _ = if !trace then tracing ("RHS = " ^ show rhs) else ()
       val th = Tactic.prove thy [] [] (Logic.mk_equals (lhs, rhs))
                   (fn _ => rtac Data.eq_reflection 1 THEN
                            simp_tac (Simplifier.inherit_bounds ss assoc_ss) 1)
   in SOME th end
   handle Assoc_fail => NONE;

end;


(*test data:
set timing;

Goal "(#3 * (a * #34)) * (#2 * b * #9) = (x::int)";

Goal "a + b + c + d + e + f + g + h + i + j + k + l + m + n + oo + p + q + r + s + t + u + v + (w + x + y + z + a + #2 + b + #2 + c + #2 + d + #2 + e) + #2 + f + (#2 + g + #2 + h + #2 + i) + #2 + (j + #2 + k + #2 + l + #2 + m + #2) + n + #2 + (oo + #2 + p + #2 + q + #2 + r) + #2 + s + #2 + t + #2 + u + #2 + v + #2 + w + #2 + x + #2 + y + #2 + z + #2 = (uu::nat)";
*)