utf8.erl

-module(utf8).
-export([from_binary/1, to_binary/1]).
    
%% Given a binary of UTF-8 encoded text, return a UTF-32 String
%% (i.e. each element is a unicode code point).
from_binary(Bin) ->
    decode_binary(Bin, []).

decode_binary(<<>>, Str) ->
    {ok, lists:reverse(Str)};

decode_binary(Bin, Str) ->
    {B1,B2} = split_binary(Bin, 1),
    case B1 of
        %% 0-7F  0zzzzzzz
        <<0:1,Z:7>> ->
            decode_binary(B2, [Z|Str]);

        %% 110yyyyy 10zzzzzz
        <<2#110:3,Y:5>> ->
            {<<2#10:2,Z:6>>,B3} = split_binary(B2, 1),
            U32 = (Y bsl 6) bor Z,
            decode_binary(B3, [U32|Str]);

        %% 1110xxxx 10yyyyyy 10zzzzzz
        <<2#1110:4,X:4>> ->
            {<<2#10:2,Y:6,2#10:2,Z:6>>,B3} = split_binary(B2, 2),
            U32 = (X bsl 12) bor (Y bsl 6) bor Z,
            decode_binary(B3, [U32|Str]);

        %% 11110www 10xxxxxx 10yyyyyy 10zzzzzz
        <<2#11110:5,W:3>> ->
            {<<2#10:2,X:6,2#10:2,Y:6,2#10:2,Z:6>>,B3} = split_binary(B2, 3),
            U32 = (W bsl 18) bor (X bsl 12) bor (Y bsl 6) bor Z,
            decode_binary(B3, [U32|Str]);

        %% an exception will be raised if the utf8 encoding is off
        %% and causes a match error
        true ->
            {bad_octet, B1}
    end.

%% Given a list of unicode code points, return a binary of UTF-8
%% encoded text.
to_binary(Str) ->
    encode_utf32(Str, []).

encode_utf32([], Utf8) ->
    {ok, list_to_binary(lists:reverse(Utf8))};
encode_utf32([U32|Str], Utf8) ->
    if
        %% 0-7F  0zzzzzzz
        U32 < 16#80 ->
            encode_utf32(Str, [U32|Utf8]);

        %% 110yyyyy 10zzzzzz
        U32 < 16#800 ->
            Y = 2#11000000 bor ((U32 band 16#7c0) bsr 6),
            Z = 2#10000000 bor  (U32 band 16#3f),
            encode_utf32(Str, [Z|[Y|Utf8]]);

        %% 1110xxxx 10yyyyyy 10zzzzzz
        U32 < 16#10000  ->
            X = 2#11100000 bor ((U32 band 16#f000) bsr 12),
            Y = 2#10000000 bor ((U32 band 16#fc0) bsr 6),
            Z = 2#10000000 bor  (U32 band 16#3f),
            encode_utf32(Str, [Z|[Y|[X|Utf8]]]);

        %% 11110www 10xxxxxx 10yyyyyy 10zzzzzz
        U32 < 16#110000 ->
            W = 2#11110000 bor ((U32 band 16#1c0000) bsr 18),
            X = 2#10000000 bor ((U32 band 16#3f000) bsr 12),
            Y = 2#10000000 bor ((U32 band 16#fc0) bsr 6),
            Z = 2#10000000 bor  (U32 band 16#3f),
            encode_utf32(Str, [Z|[Y|[X|[W|Utf8]]]]);

        %% past allocated code points
        true ->
            {bad_code_point, U32}
    end.