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json_sax_parser.erl
Erlang
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%%% Copyright (C) 2008 Willem de Jong
%%%
%%% This is json_parser.
%%%
%%% json_parser is free software: you can redistribute it and/or modify
%%% it under the terms of the GNU Lesser General Public License as
%%% published by the Free Software Foundation, either version 3 of
%%% the License, or (at your option) any later version.
%%%
%%% json_parser is distributed in the hope that it will be useful,
%%% but WITHOUT ANY WARRANTY; without even the implied warranty of
%%% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
%%% GNU Lesser General Public License for more details.
%%%
%%% You should have received a copy of the GNU Lesser General Public
%%% License along with json_parser. If not, see
%%% <http://www.gnu.org/licenses/>.
%%%
%%% Author contact: w.a.de.jong@gmail.com
 
% @doc `json_parser' is a parser for json documents.
%
% The parser has a SAX-like API: as soon as it has processed a meaningful part
% of the JSON document (such as a '{' that starts an object, or the name of a
% name/value pair), the parser will call a handler function that then has to
% process this information.
%
% json_parser can work in a streaming mode: when it reaches the end of the
% input data before reaching the end of the JSON document, it will call a
% 'continuation-function' to request for more data.
%
% json_parser only works on utf-8 encoded input.
%
% As an example of how the SAX API can be used, a handler function is provided
% that produces 'mochijson2'-output.
 
% @reference See www.json.org for an introduction to JSON.
%
% @reference See
% <a href="http://www.saxproject.org/apidoc/org/xml/sax/ContentHandler.html">the
% SAX API description</a> for more information SAX parsers for XML.
%
% @author Willem de Jong <w.a.de.jong@gmail.com>
% @copyright 2008 Willem de Jong
% @version 0.1
 
%% @type sax_event() = (startDocument | endDocument | startObject | endObject |
%% startArray | endArray | sax_value_event())
%% @type sax_value_event() = {value, (integer() | float() |
%% binary() | sax_atom_value())}
%% @type sax_atom_value() = null | true | false
-module(json_parser).
 
-export([event_stream_parser/3]).
-export([event_stream_parser/4]).
-export([dvm_parser/1,dvm_parser2/1]).
-export([test/0,test_all/0,profile/0]).
-export([test_dvm/0,test_dvm2/0]).
-export([test_file/1]).
 
-record(json_state,
{user_state,
callback,
continuation_state,
stack,
continuation_fun}).
 
-define(LOG(Format, Args),
io:format(Format, Args)).
 
 
-define(is_digit(C),
C > 47, C < 58).
-define(is_hex_digit(C),
C > 47, C < 58; C > 64, C < 71).
-define(is_number_char(C),
C =:= $E; C =:= $e; C =:= $-; C =:= $+; C =:= $.).
-define(is_delimiter(C),
C =:= $]; C =:= $}; C =:= $,).
-define(is_whitespace(C),
C =:= 32; C =:= 13 ; C =:= 10; C =:= 9).
 
% @equiv event_stream_parser(Json, UserState, Handler, [])
event_stream_parser(Json, UserState, Handler) ->
event_stream_parser(Json, UserState, Handler, []).
 
% @spec event_stream_parser(Json, Acc0, Handler, Options) -> {ok, Acc1, Rest}
% where Json = binary(),
% Acc0 = term(),
% Handler = (Event::sax_event(), AccIn::term()) -> AccOut,
% AccIn = term(),
% AccOut = term(),
% Options = [Option],
% Acc1 = term(),
% Rest = binary(),
% Option = {continuation_function, CFunction, C_Acc0},
% CFunction = (C_StateIn::term()) -> C_StateOut
% C_StateOut = term()
% @doc Parses a JSON document.
%
% event_stream_parser(Json, Acc0, Handler, Options) calls Handler(Event, AccIn)
% on successive "JSON SAX events" that result from parsing Json, starting with
% AccIn == Acc0. Handler/2 must return a new accumulator which is passed to the
% next call. The function returns {ok, AccOut, Tail}, where AccOut is the final
% value of the accumulator and Tail the remaining bytes that follow after the
% end of the Json document.
%
% (Notice how similar this is to lists:foldl(Fun, Acc0, Json_sax_events),
% assuming that Json_sax_events is the list of Sax events - I more or less
% copied this description from the documentation of the lists module.)
%
% CFunction should be a function that takes 1 arguments: State. It should
% return the next block of data, and a new value for State. This new value will
% be passed to the next invocation of CFunction; it can be used to keep track
% of the location in a file or something similar.
event_stream_parser(Json, Acc0, Handler, Options) when is_binary(Json) ->
{CFun, CState} = getCFunction(Options),
parse(Json,
#json_state{callback = Handler,
stack = [],
user_state = Acc0,
continuation_state = CState,
continuation_fun = CFun}).
 
parse(<<>>, State) ->
getMoreData(<<>>, State, fun parse/2, []);
%% ignore byte order mark
parse(<<16#EF, 16#BB, 16#BF, Doc/binary>>, State) ->
parse(Doc, State);
parse(Doc, State) ->
State2 = wrapCallback(startDocument, State),
{Tail, State3} = parseValue(Doc, State2),
State4 = wrapCallback(endDocument, State3),
{ok, State4#json_state.user_state, Tail}.
 
parseValue(<<NextChar, Tail/binary>>, State)
when ?is_whitespace(NextChar) ->
parseValue(Tail, State);
parseValue(<<$", Tail/binary>>, State) -> %"
parseStringBinary(Tail, State, <<>>);
parseValue(<<${, Tail/binary>>, #json_state{stack = S} = State) ->
parsePair(Tail, wrapCallback(startObject, State#json_state{stack =[${ | S]}));
parseValue(<<$[, Tail/binary>>, #json_state{stack = S} = State) ->
parseValue(Tail, wrapCallback(startArray, State#json_state{stack =[$[ | S]}));
parseValue(<<"t">>, State) ->
getMoreData(<<"t">>, State, fun parseValue/2, []);
parseValue(<<"tr">>, State) ->
getMoreData(<<"tr">>, State, fun parseValue/2, []);
parseValue(<<"tru">>, State) ->
getMoreData(<<"tru">>, State, fun parseValue/2, []);
parseValue(<<"true", Tail/binary>>, State) ->
parseToDelimiter(Tail, reportValue(true, State));
parseValue(<<>>, State) ->
getMoreData(<<>>, State, fun parseValue/2, []);
parseValue(<<"f">>, State) ->
getMoreData(<<"f">>, State, fun parseValue/2, []);
parseValue(<<"fa">>, State) ->
getMoreData(<<"fa">>, State, fun parseValue/2, []);
parseValue(<<"fal">>, State) ->
getMoreData(<<"fal">>, State, fun parseValue/2, []);
parseValue(<<"fals">>, State) ->
getMoreData(<<"fals">>, State, fun parseValue/2, []);
parseValue(<<"false", Tail/binary>>, State) ->
parseToDelimiter(Tail, reportValue(false, State));
parseValue(<<"n">>, State) ->
getMoreData(<<"n">>, State, fun parseValue/2, []);
parseValue(<<"nu">>, State) ->
getMoreData(<<"nu">>, State, fun parseValue/2, []);
parseValue(<<"nul">>, State) ->
getMoreData(<<"nul">>, State, fun parseValue/2, []);
parseValue(<<"null", Tail/binary>>, State) ->
parseToDelimiter(Tail, reportValue(null, State));
parseValue(<<N, Tail/binary>>, State) when ?is_digit(N); N =:= $- ->
parseNumber(Tail, State, [N], integer);
parseValue(<<$], Tail/binary>>, #json_state{stack = [$[ | S] } = State) ->
parseToDelimiter(Tail, wrapCallback(endArray, State#json_state{stack = S})).
 
parsePair(<<NextChar, Tail/binary>>, State)
when ?is_whitespace(NextChar) ->
parsePair(Tail, State);
parsePair(<<$", Tail/binary>>, State) -> %"
parseKeyColonValue(Tail, State, <<>>);
parsePair(<<>>, State) ->
getMoreData(<<>>, State, fun parsePair/2, []);
parsePair(<<$}, Tail/binary>>, #json_state{stack = [${ | S] } = State) ->
parseToDelimiter(Tail, wrapCallback(endObject, State#json_state{stack = S})).
 
parseStringBinary(<<$", Tail/binary>>, State, StringSoFar) -> %"
parseToDelimiter(Tail, reportValue(StringSoFar, State));
parseStringBinary(<<"\\">>, State, StringSoFar) -> %"
getMoreData(<<"\\">>, State, fun parseStringBinary/3, [StringSoFar]); %"
parseStringBinary(<<$\\, $u, Tail/binary>>, State, StringSoFar) ->
parseCodePoint(Tail, State, StringSoFar);
parseStringBinary(<<$\\, Esc, Tail/binary>>, State, StringSoFar) ->
EscapedChar = mapEscapedChar(Esc),
parseStringBinary(Tail, State, <<StringSoFar/binary, EscapedChar>>);
parseStringBinary(<<Char, Tail/binary>>, State, StringSoFar) ->
%% note: there is no test on illegal or incomplete utf-8 encoding
parseStringBinary(Tail, State, <<StringSoFar/binary, Char>>);
parseStringBinary(<<>>, State, StringSoFar) ->
getMoreData(<<>>, State, fun parseStringBinary/3, [StringSoFar]).
 
parseKeyColonValue(<<$", Tail/binary>>, State, StringSoFar) -> %"
parseColonValue(Tail, wrapCallback({key, StringSoFar}, State));
parseKeyColonValue(<<$\\>>, State, StringSoFar) ->
getMoreData(<<$\\>>, State, fun parseKeyColonValue/3, [StringSoFar]);
parseKeyColonValue(<<$\\, $u, Tail/binary>>, State, StringSoFar) ->
parseCodePoint(Tail, State, StringSoFar);
parseKeyColonValue(<<$\\, Esc, Tail/binary>>, State, StringSoFar) ->
EscapedChar = mapEscapedChar(Esc),
parseKeyColonValue(Tail, State, <<StringSoFar/binary, EscapedChar>>);
parseKeyColonValue(<<Char, Tail/binary>>, State, StringSoFar) ->
%% note: there is no test on illegal or incomplete utf-8 encoding
parseKeyColonValue(Tail, State, <<StringSoFar/binary, Char>>);
parseKeyColonValue(<<>>, State, StringSoFar) ->
getMoreData(<<>>, State, fun parseKeyColonValue/3, [StringSoFar]).
 
parseColonValue(<<NextChar, Tail/binary>>, State)
when ?is_whitespace(NextChar) ->
parseColonValue(Tail, State);
parseColonValue(<<$:, Tail/binary>>, State) ->
parseValue(Tail, State);
parseColonValue(<<>>, State) ->
getMoreData(<<>>, State, fun parseColonValue/2, []).
 
parseNumber(<<$, ,Tail/binary>>,
#json_state{stack = [${ | _] } = State, Acc, Type) ->
parsePair(Tail, reportNumber(Acc, Type, State));
parseNumber(<<$, ,Tail/binary>>, State, Acc, Type) ->
parseValue(Tail, reportNumber(Acc, Type, State));
parseNumber(<<$}, Tail/binary>>,
#json_state{stack = [${ | S] } = State,
Acc, Type) ->
State2 = reportNumber(Acc, Type, State),
parseToDelimiter(Tail, wrapCallback(endObject, State2#json_state{stack = S}));
parseNumber(<<$], Tail/binary>>,
#json_state{stack = [$[ | S] } = State,
Acc, Type) ->
State2 = reportNumber(Acc, Type, State),
parseToDelimiter(Tail, wrapCallback(endArray, State2#json_state{stack = S}));
parseNumber(<<NextChar, Tail/binary>>, State, Acc, Type) when
?is_whitespace(NextChar) ->
parseToDelimiter(Tail, reportNumber(Acc, Type, State));
parseNumber(<<NextChar, Tail/binary>>, State, Acc, Type) when
?is_digit(NextChar) ->
parseNumber(Tail, State, [NextChar | Acc], Type);
parseNumber(<<NextChar, Tail/binary>>, State, Acc, _Type) when
?is_number_char(NextChar) ->
parseNumber(Tail, State, [NextChar | Acc], float);
parseNumber(<<>>, State, Acc, Type) ->
getMoreData(<<>>, State, fun parseNumber/4, [Acc, Type]).
 
reportNumber(Number, Type, State) ->
reportValue(case Type of
integer -> list_to_integer(lists:reverse(Number));
_ -> list_to_float(lists:reverse(Number))
end, State).
 
reportValue(Value, State) ->
wrapCallback({value, Value}, State).
 
parseToDelimiter(<<Tail/binary>>, #json_state{stack = []} = State) ->
{Tail, State};
parseToDelimiter(<<NextChar, Tail/binary>>, State)
when ?is_whitespace(NextChar) ->
parseToDelimiter(Tail, State);
parseToDelimiter(<<$, , Tail/binary>>,
#json_state{stack = [${ | _S] } = State) ->
parsePair(Tail, State);
parseToDelimiter(<<$, , Tail/binary>>,
#json_state{stack = [$[ | _S] } = State) ->
parseValue(Tail, State);
parseToDelimiter(<<$}, Tail/binary>>,
#json_state{stack = [${ | S] } = State) ->
parseToDelimiter(Tail, wrapCallback(endObject, State#json_state{stack = S}));
parseToDelimiter(<<$], Tail/binary>>,
#json_state{stack = [$[ | S] } = State) ->
parseToDelimiter(Tail, wrapCallback(endArray, State#json_state{stack = S}));
parseToDelimiter(<<>>, State) ->
getMoreData(<<>>, State, fun parseToDelimiter/2, []).
 
parseCodePoint(<<Tail/binary>>, State, StringSoFar) ->
parseCodePoint(Tail, State, StringSoFar, 3, 0).
 
parseCodePoint(<<Digit, Tail/binary>>, State, StringSoFar, N, Acc) when
?is_hex_digit(Digit) ->
case N of
0 ->
CodePoint = list_to_binary(xmerl_ucs:to_utf8(Acc + hexToDec(Digit))),
parseStringBinary(Tail, State, <<StringSoFar/binary, CodePoint/binary>>);
_ ->
parseCodePoint(Tail, State, StringSoFar, N-1,
Acc + pow(16,N)*hexToDec(Digit))
end;
parseCodePoint(<<>>, State, StringSoFar, N, Acc) ->
getMoreData(<<>>, State, fun parseCodePoint/5, [StringSoFar, N, Acc]).
 
hexToDec(N) when ?is_digit(N) -> N - 48;
hexToDec(N) -> N - 55.
pow(_, 0) -> 1;
pow(X, N) -> X * pow(X, N-1).
mapEscapedChar($") -> $"; %"
mapEscapedChar($\\) -> $\\;
mapEscapedChar($/) -> $/;
mapEscapedChar($b) -> 8;
mapEscapedChar($f) -> 12;
mapEscapedChar($r) -> 13;
mapEscapedChar($n) -> 10;
mapEscapedChar($t) -> 9;
mapEscapedChar(_) -> throw({error, "Malformed: Illegal escaped character"}).
 
wrapCallback(Event, #json_state{callback = Callback, user_state = UserState} = State) ->
State#json_state{user_state = Callback(Event, UserState)}.
 
getCFunction(Options) ->
case lists:keysearch(continuation_function,1,Options) of
{value, {_, F, S}} -> {F, S};
false -> {fun(T, S) -> {T, S} end, undefined}
end.
 
getMoreData(Tail, State, ParseFun, Args) ->
{Data, ContinuationState2} =
(State#json_state.continuation_fun)(State#json_state.continuation_state),
case Data of
<<>> -> throw({error, "Malformed: Unexpected end of data"});
_ ->
apply(ParseFun,
[<<Tail/binary, Data/binary>>,
State#json_state{continuation_state = ContinuationState2} |Args])
end.
 
%% ----- anything below this line is intended for testing or as example ------
 
% @hidden A very simple test.
test() ->
Doc = doc(),
{ok, R, _} = event_stream_parser(list_to_binary(Doc), [],
fun(Event, State) -> [Event | State] end),
lists:reverse(R).
 
% @spec dvm_parser(Json::binary()) -> term()
%
% @doc An example that shows how json_event_parser() can be used to create
% normal 'document value model'-type of output.
%
% The output created by this function is equal (or very similar) to the output
% produced by mochijson2.erl. The function can easily be modified to produce a
% different output format.
dvm_parser(Doc) -> event_stream_parser(Doc, ok, fun dvm/2).
 
dvm(startDocument, _) ->
start;
dvm(startObject, Stack) ->
[[]| Stack];
dvm(startArray, Stack) ->
[[]| Stack];
dvm({key, _} = Event, Stack) ->
[Event|Stack];
dvm({value, Value}, start) ->
{value, Value};
dvm({value, Value}, [{key, Key}, List | T]) ->
[[{Key, Value} | List] | T];
dvm({value, Value}, [List | T]) ->
[[Value | List] | T];
dvm(endObject, [List | T]) ->
dvm({value, {struct, lists:reverse(List)}}, T);
dvm(endArray, [List | T]) ->
dvm({value, lists:reverse(List)}, T);
dvm(endDocument, {value, R}) ->
R.
 
% @hidden a simple test for the dvm function
test_dvm() ->
Doc = list_to_binary(doc()),
dvm_parser(Doc).
 
% @spec dvm_parser(Json::binary()) -> term()
%
% @doc An example that shows how json_event_parser() can be used to create
% normal 'document value model'-type of output.
%
% The output created by this function is equal (or very similar) to the output
% produced by mochijson2.erl. The function can easily be modified to produce a
% different output format.
dvm_parser2(Doc) -> event_stream_parser(Doc, ok, fun dvm2/2).
 
dvm2(startDocument, _) ->
start;
dvm2(startObject, Stack) ->
[[]| Stack];
dvm2(startArray, Stack) ->
[[]| Stack];
dvm2({key, _} = Event, Stack) ->
[Event|Stack];
dvm2({value, Value}, start) ->
{value, Value};
dvm2({value, Value}, [{key, Key}, List | T]) ->
[[{Key, Value} | List] | T];
dvm2({value, Value}, [List | T]) ->
[[Value | List] | T];
dvm2(endObject, [List | T]) ->
dvm({value, {lists:reverse(List)}}, T);
dvm2(endArray, [List | T]) ->
dvm({value, lists:reverse(List)}, T);
dvm2(endDocument, {value, R}) ->
R.
 
% @hidden a simple test for the dvm function
test_dvm2() ->
Doc = list_to_binary(doc()),
dvm_parser2(Doc).
 
% @doc a simple example of the use of a continuation function.
test_file(File) ->
F = fun(Event, State) -> io:format("event: ~p~n", [Event]), State end,
G = fun continue_file/1, %% the callback function that returns the next
%% chunk of data
{ok, Handle} = file:open(File, [read, raw, binary]),
Position = 0,
Chunk = 7, %% the number of bytes read - should be tuned, but 7 is
%% definitely far too small - this is just for testing.
CState = {Handle, Position, Chunk},
SaxCallbackState = undefined,
{ok, _Result, _TrailingBytes} =
event_stream_parser(<<>>, SaxCallbackState, F,
[{continuation_function, G, CState}]),
ok = file:close(Handle).
 
%% this is a continuation function that reads chunks of data
%% from a file.
continue_file({Handle, Offset, Chunk}) ->
%% read the next chunk
Data = case file:pread(Handle, Offset, Chunk) of
{ok, Bytes} -> Bytes;
eof -> <<>>
end,
{Data, {Handle, Offset + Chunk, Chunk}}.
%% some test data
doc() ->
"{\"menu\" : {"
"\"id\" : \"file\","
"\"number\": -123.123E3,"
"\"value\" : {},"
"\"array\" : [],"
"\"popup\" : {"
"\"menuitem\" : [false, true, null, \"codepoint (A): \\u0041B\", \"aap\", 13, \"newline: \\n linefeed: \\r tab: \\t quote: \\\"\", "
"{\"value\": \"New\", \"onclick\": \"CreateNewDoc()\"},"
"{\"value\": \"Open\", \"onclick\": 123},"
"{\"value\": \"Close\", \"onclick\": true}]"
"}}}".
 
 
%% Test for equivalence of Erlang terms.
%% Due to arbitrary order of construction, equivalent objects might
%% compare unequal as erlang terms, so we need to carefully recurse
%% through aggregates (tuples and objects).
 
equiv(T1, T2) when is_list(T1), is_list(T2) ->
equiv_list(T1, T2);
equiv({T1}, {T2}) when is_list(T1), is_list(T2) ->
equiv_object(T1, T2);
equiv(N1, N2) when is_number(N1), is_number(N2) -> N1 == N2;
equiv(S1, S2) when is_binary(S1), is_binary(S2) -> S1 == S2;
equiv(true, true) -> true;
equiv(false, false) -> true;
equiv(null, null) -> true.
 
%% Object representation and traversal order is unknown.
%% Use the sledgehammer and sort property lists.
 
equiv_object(Props1, Props2) ->
L1 = lists:keysort(1, Props1),
L2 = lists:keysort(1, Props2),
Pairs = lists:zip(L1, L2),
true = lists:all(fun({{K1, V1}, {K2, V2}}) ->
equiv(K1, K2) and equiv(V1, V2)
end, Pairs).
 
%% Recursively compare tuple elements for equivalence.
 
equiv_list([], []) ->
true;
equiv_list([V1 | L1], [V2 | L2]) ->
case equiv(V1, V2) of
true ->
equiv_list(L1, L2);
false ->
false
end.
 
profile() ->
{ok, Tracer} = fprof:profile(start),
fprof:trace([start, {tracer, Tracer}]),
profile_next(tests(binary)),
fprof:trace(stop),
fprof:analyse().
 
 
profile_next([]) -> ok;
 
profile_next([{_,J}|Rest]) ->
{ok, Term, _} = dvm_parser2(list_to_binary(J)),
% term_to_json(Term),
profile_next(Rest).
 
test_all() ->
test_next(tests(binary)).
 
test_next([]) -> {ok, passed};
 
test_next([{E,J}|Rest]) ->
?LOG("Test: Term ~p JSON ~s~n", [E, J]),
{ok, Term, _} = dvm_parser2(list_to_binary(J)),
?LOG("Decoded ~p~n", [Term]),
true = equiv(E, Term),
% Json = term_to_json(Term),
% ?LOG("Encoded ~s~n~n", [Json]),
% true = equiv(Term, json_to_term(Json)),
test_next(Rest).
 
tests(binary) ->
[
{{[{<<"key">>,<<"value">>}]}, "{\"key\":\"value\"}"},
{{[]},"{}"},
{[], "[]"},
{[1], "[1]"},
{[3.1416], "[3.14160]"}, % text representation may truncate, trail zeroes
{[-1], "[-1]"},
{[-3.1416], "[-3.14160]"},
{{[{<<"number">>, 12.0e10}]}, "{\"number\":1.20000e+11}"},
{[1.234E+10], "[1.23400e+10]"},
{[-1.234E-10], "[-1.23400e-10]"},
{[10.0], "[1.0e+01]"},
{[123.456], "[1.23456E+2]"},
% this one fails
% {[10.0], "[1e1]"},
{[<<"foo">>], "[\"foo\"]"},
{[<<"">>], "[\"\"]"},
{[<<"1/4">>], "[\"1\/4\"]"},
{[<<"name is \"Quentin\"">>], "[\"name is \\\"Quentin\\\"\"]"},
{[<<"\n\n\n">>], "[\"\\n\\n\\n\"]"},
{[iolist_to_binary("foo" ++ [5] ++ "bar")], "[\"foo\\u0005bar\"]"},
{{[{<<"foo">>, <<"bar">>}]}, "{\"foo\":\"bar\"}"},
{{[{<<"foo">>, <<"bar">>}, {<<"baz">>, 123}]}, "{\"foo\":\"bar\",\"baz\":123}"},
{[[]], "[[]]"},
{[1, <<"foo">>], "[1,\"foo\"]"},
 
% json array in a json object
{{[{<<"foo">>, [123]}]}, "{\"foo\":[123]}"},
 
% json object in a json object
{{[{<<"foo">>, {[{<<"bar">>, true}]}}]},
"{\"foo\":{\"bar\":true}}"},
 
% fold evaluation order
{{[{<<"foo">>, []},
{<<"bar">>, {[{<<"baz">>, true}]}},
{<<"alice">>, <<"bob">>}]},
"{\"foo\":[],\"bar\":{\"baz\":true},\"alice\":\"bob\"}"},
 
% json object in a json array
{[-123, <<"foo">>, {[{<<"bar">>, []}]}, null],
"[-123,\"foo\",{\"bar\":[]},null]"}
].

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