add varuint module

utils/vw/src/vu.erl

@@ -0,0 +1,56 @@
+% @doc Bitcoin varuint
+% https://en.bitcoin.it/wiki/Protocol_documentation#Variable_length_integer
+-module(vu).
+
+-compile([export_all, nowarn_export_all]).
+
+-spec encode(Integer) -> Result
+    when Integer :: integer(),
+         Result  :: {ok, Encoded :: binary()}
+                  | {error, Reason :: term()}.
+
+encode(N) when N < 0 ->
+    {error, {negative_N, N}};
+encode(N) when N < 16#FD ->
+    {ok, <<N>>};
+encode(N) when N =< 16#FFFF ->
+    NBytes = eu(N, 2),
+    {ok, <<16#FD, NBytes/binary>>};
+encode(N) when N =< 16#FFFF_FFFF ->
+    NBytes = eu(N, 4),
+    {ok, <<16#FE, NBytes/binary>>};
+encode(N) when N < (2 bsl 64) ->
+    NBytes = eu(N, 8),
+    {ok, <<16#FF, NBytes/binary>>}.
+
+% add bytes to the end as needed
+eu(N, Size) ->
+    Bytes = binary:encode_unsigned(N, little),
+    NExtraZeros = Size - byte_size(Bytes),
+    ExtraZeros = << <<0>> || _ <- lists:seq(1, NExtraZeros) >>,
+    <<Bytes/binary, ExtraZeros/binary>>.
+
+
+
+-spec decode(Encoded) -> Result
+    when Encoded :: binary(),
+         Result  :: {ok, Integer :: integer()}
+                  | {error, Reason :: term()}.
+
+decode(<<N>>) when N < 16#FD ->
+    {ok, N};
+decode(<<_, Rest/binary>>) ->
+    {ok, binary:decode_unsigned(Rest, little)};
+decode(_) ->
+    {error, ur_mom}.
+
+
+% See https://github.com/aeternity/aepp-sdk-js/blob/370f1e30064ad0239ba59931908d9aba0a2e86b6/src/utils/crypto.ts#L171-L175
+salted_msg(Msg) when is_binary(Msg) ->
+    P = <<"aeternity Signed Message:\n">>,
+    {ok, SP}   = encode(byte_size(P)),
+    {ok, SMsg} = encode(byte_size(Msg)),
+    <<SP/binary,
+      P/binary,
+      SMsg/binary,
+      Msg/binary>>.