:- module gray.

:- interface.
:- import_module int.

:- type gray.

% VALUE conversion functions
:- func gray.from_int(int) = gray.
:- func gray.to_int(gray) = int.

% REPRESENTATION conversion predicate
:- pred gray.coerce(gray, int).
:- mode gray.coerce(in,   out) is det.
:- mode gray.coerce(out,  in)  is det.

:- implementation.

:- import_module list.

:- type gray
  ---> gray(int).

gray.from_int(X) = gray(X `xor` (X >> 1)).

gray.to_int(gray(G)) = (G > 0 -> G `xor` gray.to_int(gray(G >> 1))
                        ;        G).
gray.coerce(gray(I), I).

:- end_module gray.