Initial data commit

Task/Gray-code/Mercury/gray-code-1.mercury

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+:- 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.