gistfile1.hrl

#!/usr/bin/env escript
%% -*- erlang -*-

main(_) ->
    io:format("~p~n", [hex_to_binary("180ff")]),
    io:format("~p~n", [integer_to_binary(16#180ff)]).

hex_to_binary(Hex) ->
    fold(fun
             ([L, U|H]) -> {char_to_integer(U) bsl 4 bor char_to_integer(L), H};
             ([L])      -> {char_to_integer(L), []};
             ([])       -> ok
         end, lists:reverse(Hex)).

integer_to_binary(Integer) ->
    fold(fun
             (0) -> ok;
             (I) -> {I band 16#ff, I bsr 8}
         end, Integer).

fold(Fun, Input) ->
    fold(Fun, Input, <<>>).

fold(Fun, Input, Binary) ->
    case Fun(Input) of
        {Byte, NextInput} -> fold(Fun, NextInput, <<Byte/integer, Binary/binary>>);
        _                 -> Binary
    end.

char_to_integer(Char) when $0 =< Char, Char =< $9 ->
    Char - $0;

char_to_integer(Char) when $a =< Char, Char =< $f ->
    10 + Char - $a.