(** {1 Hexadecimal encoding and decoding of a string}

  Implementation of functions to encode and decode strings into
  hexadecimal.

  The length of the encoded string is always twice the length of the
  input string.
*)

use mach.int.Int
use mach.int.Int63
use string.String
use string.Char
use string.OCaml
use string.StringBuffer

(** `valid_hex_char c` is true, if and only if, `c` is a valid
hexadecimal character *)
predicate valid_hex_char (c: char) =
  48 <= code c < 58 || 65 <= code c < 71

(** `hex i` gives the `i`th hexadecimal value, for `0 <= i < 16` or
`0` otherwise. *)
function hex (i: int) : char =
  if 0 <= i < 10 then chr (i + 48)
  else if 10 <= i < 16 then chr (i + 55)
  else "\000"[0]

let hex (i: int63) : char
  requires { 0 <= i < 16 }
  ensures  { result = hex i }
= if i < 10 then chr (i + 48) else chr (i + 55)

(** `xeh c` gives the index of the hexadecimal value `c`, if `c` is a
valid hexadecimal value or `-1` otherwise *)
function xeh (c: char) : int =
  if 48 <= code c < 58 then code c - 48
  else if 65 <= code c < 71 then code c - 55
  else -1

let xeh (c: char) : int63
  requires { valid_hex_char c }
  ensures  { result = xeh c }
= if 48 <= code c < 58 then code c - 48
  else code c - 55

(** checks whether a string contains only valid hexadecimal characters *)
predicate valid_hex (s : string) =
  mod (length s) 2 = 0 &&
  ( forall i. 0 <= i < length s -> valid_hex_char s[i] )

(** `encoding s1 s2` is true, if and only if, `s2` is an encoding of
`s1` *)
predicate encoding (s1 s2: string) =
  length s2 = 2 * length s1 &&
  (forall i. 0 <= i < length s1 ->
             s1[i] = chr (xeh s2[2 * i] * 16 + xeh s2[2 * i + 1])) &&
  valid_hex s2

(** the encoding of a string is unique *)
lemma decode_unique: forall s1 s2 s3.
  encoding s1 s2 /\ encoding s3 s2 -> s1 = s3

let partial encode (s: string) : string
  ensures { encoding s result }
= let ref i = 0 in
  let r = StringBuffer.create (length s) in
  while i < OCaml.length s do
    variant { length s - i }
    invariant { 0 <= i <= length s }
    invariant { length r = 2 * i }
    invariant { forall j. 0 <= j < i  ->
                  r[2 * j] = hex (div (code s[j]) 16) &&
                  r[2 * j + 1] = hex (mod (code s[j]) 16)
    }
    invariant { forall j. 0 <= j < 2*i -> valid_hex_char r[j]}
    let v = code s[i] in
    StringBuffer.add_char r (hex (v / 16));
    StringBuffer.add_char r (hex (v % 16));
    i <- i + 1
  done;
  StringBuffer.contents r

let partial decode (s: string) : string
  requires { valid_hex s }
  ensures  { encoding result s }
= let ref i = 0 in
  let r = StringBuffer.create (length s / 2) in
  while i < length s do
    variant {length s - i}
    invariant { mod i 2 = 0 }
    invariant { 0 <= i <= length s }
    invariant { length r = div i 2 }
    invariant { forall j. 0 <= j < div i 2 ->
                  r[j] = chr (xeh s[2 * j] * 16 + xeh s[2 * j + 1]) }
    let v_i = xeh s[i] in
    let v_ii = xeh s[i + 1] in
    StringBuffer.add_char r (chr (v_i * 16 + v_ii));
    i <- i + 2
  done;
  StringBuffer.contents r

let partial decode_encode (s: string) : unit
= let s1 = encode s in
  let s2 = decode s1 in
  assert { s = s2 }