src/Provers/Arith/combine_numerals.ML
 author desharna Mon, 20 Sep 2021 20:24:43 +0200 changeset 74329 949054d78a77 parent 70527 095e6459d3da permissions -rw-r--r--
merged
```
(*  Title:      Provers/Arith/combine_numerals.ML
Author:     Lawrence C Paulson, Cambridge University Computer Laboratory

Combine coefficients in expressions:

i + #m*u + j ... + #n*u + k  ==  #(m+n)*u + (i + (j + k))

It works by (a) massaging the sum to bring the selected terms to the front:

#m*u + (#n*u + (i + (j + k)))

(b) then using left_distrib to reach

#(m+n)*u + (i + (j + k))
*)

signature COMBINE_NUMERALS_DATA =
sig
(*abstract syntax*)
eqtype coeff
val iszero: coeff -> bool
val mk_sum: typ -> term list -> term
val dest_sum: term -> term list
val mk_coeff: coeff * term -> term
val dest_coeff: term -> coeff * term
(*rules*)
val left_distrib: thm
(*proof tools*)
val prove_conv: tactic list -> Proof.context -> term * term -> thm option
val trans_tac: Proof.context -> thm option -> tactic            (*applies the initial lemma*)
val norm_tac: Proof.context -> tactic          (*proves the initial lemma*)
val numeral_simp_tac: Proof.context -> tactic  (*proves the final theorem*)
val simplify_meta_eq: Proof.context -> thm -> thm  (*simplifies the final theorem*)
end;

functor CombineNumeralsFun(Data: COMBINE_NUMERALS_DATA):
sig
val proc: Proof.context -> cterm -> thm option
end
=
struct

(*Remove the first occurrence of #m*u from the term list*)
fun remove (_, _, []) = (*impossible, since #m*u was found by find_repeated*)
raise TERM("combine_numerals: remove", [])
| remove (m, u, t::terms) =
case try Data.dest_coeff t of
SOME(n,v) => if m=n andalso u aconv v then terms
else t :: remove (m, u, terms)
| NONE      =>  t :: remove (m, u, terms);

(*a left-to-right scan of terms, seeking another term of the form #n*u, where
#m*u is already in terms for some m*)
fun find_repeated (tab, _, []) = raise TERM("find_repeated", [])
| find_repeated (tab, past, t::terms) =
case try Data.dest_coeff t of
SOME(n,u) =>
(case Termtab.lookup tab u of
SOME m => (u, m, n, rev (remove (m,u,past)) @ terms)
| NONE => find_repeated (Termtab.update (u, n) tab,
t::past,  terms))
| NONE => find_repeated (tab, t::past, terms);

(*the simplification procedure*)
fun proc ctxt ct =
let
val t = Thm.term_of ct
val (t', ctxt') = yield_singleton (Variable.import_terms true) t ctxt

val (u,m,n,terms) = find_repeated (Termtab.empty, [], Data.dest_sum t')
val T = Term.fastype_of u

val reshape =  (*Move i*u to the front and put j*u into standard form
i + #m + j + k == #m + i + (j + k) *)
if Data.iszero m orelse Data.iszero n then   (*trivial, so do nothing*)
raise TERM("combine_numerals", [])
else Data.prove_conv [Data.norm_tac ctxt'] ctxt'
(t', Data.mk_sum T ([Data.mk_coeff (m, u), Data.mk_coeff (n, u)] @ terms))
in
Data.prove_conv
[Data.trans_tac ctxt' reshape, resolve_tac ctxt' [Data.left_distrib] 1,
Data.numeral_simp_tac ctxt'] ctxt'
(t', Data.mk_sum T (Data.mk_coeff(Data.add(m,n), u) :: terms))
|> Option.map
(Data.simplify_meta_eq ctxt' #>
singleton (Variable.export ctxt' ctxt))
end
(* FIXME avoid handling of generic exceptions *)
handle TERM _ => NONE
| TYPE _ => NONE;   (*Typically (if thy doesn't include Numeral)
Undeclared type constructor "Numeral.bin"*)

end;
```