Decoding CESU-8 in Erlang (into UTF-8).

cesu8.erl

decode_cesu8(B) ->
    decode_cesu8(B, []).

% https://en.wikipedia.org/wiki/UTF-16
%
decode_surrogate_pairs([], Acc) ->
    Acc;
decode_surrogate_pairs([Low, High | Rest], Acc) when High >= 16#D800, High =< 16#DBFF, Low >= 16#DC00, Low =< 16#DFFF ->
    decode_surrogate_pairs(Rest, [16#10000 + (((High - 16#D800) bsl 10) bor (Low - 16#DC00)) | Acc]);
decode_surrogate_pairs([Other | Rest], Acc) ->
    decode_surrogate_pairs(Rest , [Other | Acc]).

% http://www.unicode.org/reports/tr26/
%
decode_cesu8(<<>>, RevAcc) ->
    unicode:characters_to_binary(decode_surrogate_pairs(RevAcc, []), utf16);
decode_cesu8(<<2#0:1, X:7, Rest/binary>>, RevAcc) ->
    decode_cesu8(Rest, [X | RevAcc]);
decode_cesu8(<<2#110:3, Y:5, 2#10:2, X:6, Rest/binary>>, RevAcc) ->
    decode_cesu8(Rest, [(Y bsl 6) bor X | RevAcc]);
decode_cesu8(<<2#1110:4, Z:4, 2#10:2, Y:6, 2#10:2, X:6, Rest/binary>>, RevAcc) ->
    decode_cesu8(Rest, [(Z bsl 12) bor (Y bsl 6) bor X | RevAcc]).