(*  Title:      HOL/Library/Efficient_Nat.thy

     Author:     Stefan Berghofer, Florian Haftmann, TU Muenchen

 *)

 header {* Implementation of natural numbers by target-language integers *}

 theory Efficient_Nat

 imports Code_Integer Main

 begin

 text {*

   When generating code for functions on natural numbers, the

   canonical representation using @{term "0::nat"} and

   @{term Suc} is unsuitable for computations involving large

   numbers.  The efficiency of the generated code can be improved

   drastically by implementing natural numbers by target-language

   integers.  To do this, just include this theory.

 *}

 subsection {* Basic arithmetic *}

 text {*

   Most standard arithmetic functions on natural numbers are implemented

   using their counterparts on the integers:

 *}

 code_datatype number_nat_inst.number_of_nat

 lemma zero_nat_code [code, code_unfold]:

   "0 = (Numeral0 :: nat)"

   by simp

 lemma one_nat_code [code, code_unfold]:

   "1 = (Numeral1 :: nat)"

   by simp

 lemma Suc_code [code]:

   "Suc n = n + 1"

   by simp

 lemma plus_nat_code [code]:

   "n + m = nat (of_nat n + of_nat m)"

   by simp

 lemma minus_nat_code [code]:

   "n - m = nat (of_nat n - of_nat m)"

   by simp

 lemma times_nat_code [code]:

   "n * m = nat (of_nat n * of_nat m)"

   unfolding of_nat_mult [symmetric] by simp

 lemma divmod_nat_code [code]:

   "divmod_nat n m = map_pair nat nat (pdivmod (of_nat n) (of_nat m))"

   by (simp add: map_pair_def split_def pdivmod_def nat_div_distrib nat_mod_distrib divmod_nat_div_mod)

 lemma eq_nat_code [code]:

   "HOL.equal n m \<longleftrightarrow> HOL.equal (of_nat n \<Colon> int) (of_nat m)"

   by (simp add: equal)

 lemma eq_nat_refl [code nbe]:

   "HOL.equal (n::nat) n \<longleftrightarrow> True"

   by (rule equal_refl)

 lemma less_eq_nat_code [code]:

   "n \<le> m \<longleftrightarrow> (of_nat n \<Colon> int) \<le> of_nat m"

   by simp

 lemma less_nat_code [code]:

   "n < m \<longleftrightarrow> (of_nat n \<Colon> int) < of_nat m"

   by simp

 subsection {* Case analysis *}

 text {*

   Case analysis on natural numbers is rephrased using a conditional

   expression:

 *}

 lemma [code, code_unfold]:

   "nat_case = (\<lambda>f 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 {*

   In contrast to @{term "Suc n"}, the term @{term "n + (1::nat)"} is no longer

   a constructor term. Therefore, all occurrences of this term in a position

   where a pattern is expected (i.e.\ on the left-hand side of a recursion

   equation or in the arguments of an inductive relation in an introduction

   rule) must be eliminated.

   This can be accomplished by applying the following transformation rules:

 *}

 lemma Suc_if_eq: "(\<And>n. f (Suc n) \<equiv> h n) \<Longrightarrow> f 0 \<equiv> g \<Longrightarrow>

   f n \<equiv> if n = 0 then g else h (n - 1)"

   by (rule eq_reflection) (cases n, simp_all)

 lemma Suc_clause: "(\<And>n. P n (Suc n)) \<Longrightarrow> n \<noteq> 0 \<Longrightarrow> P (n - 1) n"

   by (cases n) simp_all

 text {*

   The rules above are built into a preprocessor that is plugged into

   the code generator. Since the preprocessor for introduction rules

   does not know anything about modes, some of the modes that worked

   for the canonical representation of natural numbers may no longer work.

 *}

 (*<*)

 setup {*

 let

 fun remove_suc thy thms =

   let

     val vname = singleton (Name.variant_list (map fst

       (fold (Term.add_var_names o Thm.full_prop_of) thms []))) "n";

     val cv = cterm_of thy (Var ((vname, 0), HOLogic.natT));

     fun lhs_of th = snd (Thm.dest_comb

       (fst (Thm.dest_comb (cprop_of th))));

     fun rhs_of th = snd (Thm.dest_comb (cprop_of th));

     fun find_vars ct = (case term_of ct of

         (Const (@{const_name Suc}, _) $ Var _) => [(cv, snd (Thm.dest_comb ct))]

       | _ $ _ =>

         let val (ct1, ct2) = Thm.dest_comb ct

         in 

           map (apfst (fn ct => Thm.capply ct ct2)) (find_vars ct1) @

           map (apfst (Thm.capply ct1)) (find_vars ct2)

         end

       | _ => []);

     val eqs = maps

       (fn th => map (pair th) (find_vars (lhs_of th))) thms;

     fun mk_thms (th, (ct, cv')) =

       let

         val th' =

           Thm.implies_elim

            (Conv.fconv_rule (Thm.beta_conversion true)

              (Drule.instantiate'

                [SOME (ctyp_of_term ct)] [SOME (Thm.cabs cv ct),

                  SOME (Thm.cabs cv' (rhs_of th)), NONE, SOME cv']

                @{thm Suc_if_eq})) (Thm.forall_intr cv' th)

       in

         case map_filter (fn th'' =>

             SOME (th'', singleton

               (Variable.trade (K (fn [th'''] => [th''' RS th']))

                 (Variable.global_thm_context th'')) th'')

           handle THM _ => NONE) thms of

             [] => NONE

           | thps =>

               let val (ths1, ths2) = split_list thps

               in SOME (subtract Thm.eq_thm (th :: ths1) thms @ ths2) end

       end

   in get_first mk_thms eqs end;

 fun eqn_suc_base_preproc thy thms =

   let

     val dest = fst o Logic.dest_equals o prop_of;

     val contains_suc = exists_Const (fn (c, _) => c = @{const_name Suc});

   in

     if forall (can dest) thms andalso exists (contains_suc o dest) thms

       then thms |> perhaps_loop (remove_suc thy) |> (Option.map o map) Drule.zero_var_indexes

        else NONE

   end;

 val eqn_suc_preproc = Code_Preproc.simple_functrans eqn_suc_base_preproc;

 in

   Code_Preproc.add_functrans ("eqn_Suc", eqn_suc_preproc)

 end;

 *}

 (*>*)

 subsection {* Target language setup *}

 text {*

   For ML, we map @{typ nat} to target language integers, where we

   ensure that values are always non-negative.

 *}

 code_type nat

   (SML "IntInf.int")

   (OCaml "Big'_int.big'_int")

   (Eval "int")

 text {*

   For Haskell and Scala we define our own @{typ nat} type.  The reason

   is that we have to distinguish type class instances for @{typ nat}

   and @{typ int}.

 *}

 code_include Haskell "Nat"

 {*newtype Nat = Nat Integer deriving (Eq, Show, Read);

 instance Num Nat where {

   fromInteger k = Nat (if k >= 0 then k else 0);

   Nat n + Nat m = Nat (n + m);

   Nat n - Nat m = fromInteger (n - m);

   Nat n * Nat m = Nat (n * m);

   abs n = n;

   signum _ = 1;

   negate n = error "negate Nat";

 };

 instance Ord Nat where {

   Nat n <= Nat m = n <= m;

   Nat n < Nat m = n < m;

 };

 instance Real Nat where {

   toRational (Nat n) = toRational n;

 };

 instance Enum Nat where {

   toEnum k = fromInteger (toEnum k);

   fromEnum (Nat n) = fromEnum n;

 };

 instance Integral Nat where {

   toInteger (Nat n) = n;

   divMod n m = quotRem n m;

   quotRem (Nat n) (Nat m)

     | (m == 0) = (0, Nat n)

     | otherwise = (Nat k, Nat l) where (k, l) = quotRem n m;

 };

 *}

 code_reserved Haskell Nat

 code_include Scala "Nat"

 {*object Nat {

   def apply(numeral: BigInt): Nat = new Nat(numeral max 0)

   def apply(numeral: Int): Nat = Nat(BigInt(numeral))

   def apply(numeral: String): Nat = Nat(BigInt(numeral))

 }

 class Nat private(private val value: BigInt) {

   override def hashCode(): Int = this.value.hashCode()

   override def equals(that: Any): Boolean = that match {

     case that: Nat => this equals that

     case _ => false

   }

   override def toString(): String = this.value.toString

   def equals(that: Nat): Boolean = this.value == that.value

   def as_BigInt: BigInt = this.value

   def as_Int: Int = if (this.value >= scala.Int.MinValue && this.value <= scala.Int.MaxValue)

       this.value.intValue

     else error("Int value out of range: " + this.value.toString)

   def +(that: Nat): Nat = new Nat(this.value + that.value)

   def -(that: Nat): Nat = Nat(this.value - that.value)

   def *(that: Nat): Nat = new Nat(this.value * that.value)

   def /%(that: Nat): (Nat, Nat) = if (that.value == 0) (new Nat(0), this)

     else {

       val (k, l) = this.value /% that.value

       (new Nat(k), new Nat(l))

     }

   def <=(that: Nat): Boolean = this.value <= that.value

   def <(that: Nat): Boolean = this.value < that.value

 }

 *}

 code_reserved Scala Nat

 code_type nat

   (Haskell "Nat.Nat")

   (Scala "Nat")

 code_instance nat :: equal

   (Haskell -)

 text {*

   Natural numerals.

 *}

 lemma [code_abbrev]:

   "number_nat_inst.number_of_nat i = nat (number_of i)"

   -- {* this interacts as desired with @{thm nat_number_of_def} *}

   by (simp add: number_nat_inst.number_of_nat)

 setup {*

   fold (Numeral.add_code @{const_name number_nat_inst.number_of_nat}

     false Code_Printer.literal_positive_numeral) ["SML", "OCaml", "Haskell", "Scala"]

 *}

 text {*

   Since natural numbers are implemented

   using integers in ML, the coercion function @{const "of_nat"} of type

   @{typ "nat \<Rightarrow> int"} is simply implemented by the identity function.

   For the @{const nat} function for converting an integer to a natural

   number, we give a specific implementation using an ML function that

   returns its input value, provided that it is non-negative, and otherwise

   returns @{text "0"}.

 *}

 definition int :: "nat \<Rightarrow> int" where

   [code del, code_abbrev]: "int = of_nat"

 lemma int_code' [code]:

   "int (number_of l) = (if neg (number_of l \<Colon> int) then 0 else number_of l)"

   unfolding int_nat_number_of [folded int_def] ..

 lemma nat_code' [code]:

   "nat (number_of l) = (if neg (number_of l \<Colon> int) then 0 else number_of l)"

   unfolding nat_number_of_def number_of_is_id neg_def by simp

 lemma of_nat_int: (* FIXME delete candidate *)

   "of_nat = int" by (simp add: int_def)

 lemma of_nat_aux_int [code_unfold]:

   "of_nat_aux (\<lambda>i. i + 1) k 0 = int k"

   by (simp add: int_def Nat.of_nat_code)

 code_const int

   (SML "_")

   (OCaml "_")

 code_const nat

   (SML "IntInf.max/ (0,/ _)")

   (OCaml "Big'_int.max'_big'_int/ Big'_int.zero'_big'_int")

   (Eval "Integer.max/ _/ 0")

 text {* For Haskell and Scala, things are slightly different again. *}

 code_const int and nat

   (Haskell "toInteger" and "fromInteger")

   (Scala "!_.as'_BigInt" and "Nat")

 text {* Conversion from and to code numerals. *}

 code_const Code_Numeral.of_nat

   (SML "IntInf.toInt")

   (OCaml "_")

   (Haskell "!(fromInteger/ ./ toInteger)")

   (Scala "!Natural(_.as'_BigInt)")

   (Eval "_")

 code_const Code_Numeral.nat_of

   (SML "IntInf.fromInt")

   (OCaml "_")

   (Haskell "!(fromInteger/ ./ toInteger)")

   (Scala "!Nat(_.as'_BigInt)")

   (Eval "_")

 text {* Using target language arithmetic operations whenever appropriate *}

 code_const "op + \<Colon> nat \<Rightarrow> nat \<Rightarrow> nat"

   (SML "IntInf.+ ((_), (_))")

   (OCaml "Big'_int.add'_big'_int")

   (Haskell infixl 6 "+")

   (Scala infixl 7 "+")

   (Eval infixl 8 "+")

 code_const "op - \<Colon> nat \<Rightarrow> nat \<Rightarrow> nat"

   (Haskell infixl 6 "-")

   (Scala infixl 7 "-")

 code_const "op * \<Colon> nat \<Rightarrow> nat \<Rightarrow> nat"

   (SML "IntInf.* ((_), (_))")

   (OCaml "Big'_int.mult'_big'_int")

   (Haskell infixl 7 "*")

   (Scala infixl 8 "*")

   (Eval infixl 9 "*")

 code_const divmod_nat

   (SML "IntInf.divMod/ ((_),/ (_))")

   (OCaml "Big'_int.quomod'_big'_int")

   (Haskell "divMod")

   (Scala infixl 8 "/%")

   (Eval "Integer.div'_mod")

 code_const "HOL.equal \<Colon> nat \<Rightarrow> nat \<Rightarrow> bool"

   (SML "!((_ : IntInf.int) = _)")

   (OCaml "Big'_int.eq'_big'_int")

   (Haskell infix 4 "==")

   (Scala infixl 5 "==")

   (Eval infixl 6 "=")

 code_const "op \<le> \<Colon> nat \<Rightarrow> nat \<Rightarrow> bool"

   (SML "IntInf.<= ((_), (_))")

   (OCaml "Big'_int.le'_big'_int")

   (Haskell infix 4 "<=")

   (Scala infixl 4 "<=")

   (Eval infixl 6 "<=")

 code_const "op < \<Colon> nat \<Rightarrow> nat \<Rightarrow> bool"

   (SML "IntInf.< ((_), (_))")

   (OCaml "Big'_int.lt'_big'_int")

   (Haskell infix 4 "<")

   (Scala infixl 4 "<")

   (Eval infixl 6 "<")

 text {* Evaluation *}

 lemma [code, code del]:

   "(Code_Evaluation.term_of \<Colon> nat \<Rightarrow> term) = Code_Evaluation.term_of" ..

 code_const "Code_Evaluation.term_of \<Colon> nat \<Rightarrow> term"

   (SML "HOLogic.mk'_number/ HOLogic.natT")

 text {* Evaluation with @{text "Quickcheck_Narrowing"} does not work, as

   @{text "code_module"} is very aggressive leading to bad Haskell code.

   Therefore, we simply deactivate the narrowing-based quickcheck from here on.

 *}

 declare [[quickcheck_narrowing_active = false]] 

 text {* Module names *}

 code_modulename SML

   Efficient_Nat Arith

 code_modulename OCaml

   Efficient_Nat Arith

 code_modulename Haskell

   Efficient_Nat Arith

 hide_const int

 end