# Theory HOL-Library.Code_Abstract_Nat

(*  Title:      HOL/Library/Code_Abstract_Nat.thy
Author:     Stefan Berghofer, Florian Haftmann, TU Muenchen
*)

section ‹Avoidance of pattern matching on natural numbers›

theory Code_Abstract_Nat
imports Main
begin

text ‹
When natural numbers are implemented in another than the
conventional inductive term0::nat/termSuc representation,
it is necessary to avoid all pattern matching on natural numbers
altogether.  This is accomplished by this theory (up to a certain
extent).
›

subsection ‹Case analysis›

text ‹
Case analysis on natural numbers is rephrased using a conditional
expression:
›

lemma [code, code_unfold]:
"case_nat = (λf g n. if n = 0 then f else g (n - 1))"
by (auto simp add: fun_eq_iff dest!: gr0_implies_Suc)

subsection ‹Preprocessors›

text ‹
The term termSuc n is no longer a valid pattern.  Therefore,
all occurrences of this term in a position where a pattern is
expected (i.e.~on the left-hand side of a code equation) must be
eliminated.  This can be accomplished -- as far as possible -- by
applying the following transformation rule:
›

lemma Suc_if_eq:
assumes "n. f (Suc n)  h n"
assumes "f 0  g"
shows "f n  if n = 0 then g else h (n - 1)"
by (rule eq_reflection) (cases n, insert assms, simp_all)

text ‹
The rule above is built into a preprocessor that is plugged into
the code generator.
›

setup let

val Suc_if_eq = Thm.incr_indexes 1 @{thm Suc_if_eq};

fun remove_suc ctxt thms =
let
val vname = singleton (Name.variant_list (map fst
(fold (Term.add_var_names o Thm.full_prop_of) thms []))) "n";
val cv = Thm.cterm_of ctxt (Var ((vname, 0), HOLogic.natT));
val lhs_of = snd o Thm.dest_comb o fst o Thm.dest_comb o Thm.cprop_of;
val rhs_of = snd o Thm.dest_comb o Thm.cprop_of;
fun find_vars ct = (case Thm.term_of ct of
(Const (const_nameSuc, _) \$ Var _) => [(cv, snd (Thm.dest_comb ct))]
| _ \$ _ =>
let val (ct1, ct2) = Thm.dest_comb ct
in
map (apfst (fn ct => Thm.apply ct ct2)) (find_vars ct1) @
map (apfst (Thm.apply ct1)) (find_vars ct2)
end
| _ => []);
val eqs = maps
(fn thm => map (pair thm) (find_vars (lhs_of thm))) thms;
fun mk_thms (thm, (ct, cv')) =
let
val thm' =
Thm.implies_elim
(Conv.fconv_rule (Thm.beta_conversion true)
(Thm.instantiate'
[SOME (Thm.ctyp_of_cterm ct)] [SOME (Thm.lambda cv ct),
SOME (Thm.lambda cv' (rhs_of thm)), NONE, SOME cv']
Suc_if_eq)) (Thm.forall_intr cv' thm)
in
case map_filter (fn thm'' =>
SOME (thm'', singleton
(Variable.trade (K (fn [thm'''] => [thm''' RS thm']))
(Variable.declare_thm thm'' ctxt)) thm'')
handle THM _ => NONE) thms of
[] => NONE
| thmps =>
let val (thms1, thms2) = split_list thmps
in SOME (subtract Thm.eq_thm (thm :: thms1) thms @ thms2) end
end
in get_first mk_thms eqs end;

fun eqn_suc_base_preproc ctxt thms =
let
val dest = fst o Logic.dest_equals o Thm.prop_of;
val contains_suc = exists_Const (fn (c, _) => c = const_nameSuc);
in
if forall (can dest) thms andalso exists (contains_suc o dest) thms
then thms |> perhaps_loop (remove_suc ctxt) |> (Option.map o map) Drule.zero_var_indexes
else NONE
end;

val eqn_suc_preproc = Code_Preproc.simple_functrans eqn_suc_base_preproc;

in

end

subsection ‹Candidates which need special treatment›

lemma drop_bit_int_code [code]:
drop_bit n k = k div 2 ^ n for k :: int
by (fact drop_bit_eq_div)

lemma take_bit_num_code [code]:
take_bit_num n Num.One =
(case n of 0  None | Suc n  Some Num.One)
take_bit_num n (Num.Bit0 m) =
(case n of 0  None | Suc n  (case take_bit_num n m of None  None | Some q  Some (Num.Bit0 q)))
take_bit_num n (Num.Bit1 m) =
(case n of 0  None | Suc n  Some (case take_bit_num n m of None  Num.One | Some q  Num.Bit1 q))
apply (cases n; simp)+
done

end