(** Turning a list into an array.

  This is a small program to illustrate the use of Why3's Peano
  numbers (see module mach.peano.Peano).

  It turns a list into an array. The point is that we use machine
  integers (for the list length and for array indices) but we avoid
  having to prove the absence of arithmetic overflow.

  Author: Jean-Christophe Filliâtre (CNRS)
*)

use int.Int
use option.Option
use list.List
use seq.Seq
use mach.peano.Peano
use mach.peano.Int63
use mach.int.Int63
use mach.array.Array63 as A
use list.NthLength

(** The length of a list computed with Peano numbers. *)
let rec length (l: list 'a) : Peano.t
  variant { l } ensures { result = length l }
= match l with
  | Nil      -> Peano.zero
  | Cons _ l -> Peano.succ (length l)
  end

(** To turn a list into an array, we first compute the length of the
    list with the function above, then we build an array of that size,
    and finally we fill it with the elements from the list.

    Note: `Array.make` requires a value of type 'a, so the code below
    requires a non-empty list. (In OCaml, we would return the empty
    array [||] when the list is empty, but there is no such empty
    array in Why3's library.)
*)
let partial array_of_list (l: list 'a) : (a: A.array 'a)
  requires { l <> Nil }
  ensures  { A.length a = length l }
  ensures  { forall i. 0 <= i < length l -> Some a[i] = nth i l }
= let n = to_int63 (length l) in
  match l with Nil -> absurd | Cons x ll ->
  let a = A.make n x in
  let rec fill (i: int63) (ll: list 'a) : unit
    requires { i >= 1 && n - i = length ll }
    requires { forall j. 0 <= j < i -> Some a[j] = nth j l }
    requires { forall j. i <= j < n -> nth (j-i) ll = nth j l }
    variant  { n - i }
    ensures  { forall j. 0 <= j < n -> Some a[j] = nth j l }
  = match ll with Nil -> ()
    | Cons x ll -> A.(a[i] <- x); fill (i+1) ll
    end in
  fill 1 ll;
  return a
  end