parse_json.m |
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function data = parse_json(string) |
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DATA = PARSE_JSON(string) This function parses a JSON string and returns a cell array with the parsed data. JSON objects are converted to structures and JSON arrays are converted to cell arrays. |
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F. Glineur, 2009 (inspired by the JSON parser by Joel Feenstra on MATLAB File Exchange (http://www.mathworks.com/matlabcentral/fileexchange/20565) but with faster handling of strings) |
pos = 1;
len = length(string); |
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String delimiters and escape characters are identified beforehand to improve speed |
esc = regexp(string, '["\\]'); index_esc = 1; len_esc = length(esc);
if pos <= len
switch(next_char)
case '{'
data = parse_object;
case '['
data = parse_array;
otherwise
error_pos('Outer level structure must be an object or an array');
end
end
function object = parse_object
parse_char('{');
object = [];
if next_char ~= '}'
while 1
str = parse_string;
if isempty(str)
error_pos('Name of value at position %d cannot be empty');
end
parse_char(':');
val = parse_value;
object.(valid_field(str)) = val;
if next_char == '}'
break;
end
parse_char(',');
end
end
parse_char('}');
end
function object = parse_array
parse_char('[');
object = cell(0, 1);
if next_char ~= ']'
while 1
val = parse_value;
object{end+1} = val;
if next_char == ']'
break;
end
parse_char(',');
end
end
parse_char(']');
end
function parse_char(c)
skip_whitespace;
if pos > len || string(pos) ~= c
error_pos(sprintf('Expected %c at position %%d', c));
else
pos = pos + 1;
skip_whitespace;
end
end
function c = next_char
skip_whitespace;
if pos > len
c = [];
else
c = string(pos);
end
end
function skip_whitespace
while pos <= len && isspace(string(pos))
pos = pos + 1;
end
end
function str = parse_string
if string(pos) ~= '"'
error_pos('String starting with " expected at position %d');
else
pos = pos + 1;
end
str = '';
while pos <= len
while index_esc <= len_esc && esc(index_esc) < pos
index_esc = index_esc + 1;
end
if index_esc > len_esc
str = [str string(pos:end)];
pos = len + 1;
break;
else
str = [str string(pos:esc(index_esc)-1)];
pos = esc(index_esc);
end
switch string(pos)
case '"'
pos = pos + 1;
return;
case '\'
if pos+1 > len
error_pos('End of file reached right after escape character');
end
pos = pos + 1;
switch string(pos)
case {'"' '\' '/'}
str(end+1) = string(pos);
pos = pos + 1;
case {'b' 'f' 'n' 'r' 't'}
str(end+1) = sprintf(['\' string(pos)]);
pos = pos + 1;
case 'u'
if pos+4 > len
error_pos('End of file reached in escaped unicode character');
end
str(end+1:end+6) = string(pos-1:pos+4);
pos = pos + 5;
end
otherwise % should never happen
str(end+1) = string(pos);
pos = pos + 1;
end
end
error_pos('End of file while expecting end of string');
end
function num = parse_number
[num, one, err, delta] = sscanf(string(pos:min(len,pos+20)), '%f', 1); % TODO : compare with json(pos:end)
if ~isempty(err)
error_pos('Error reading number at position %d');
end
pos = pos + delta-1;
end
function val = parse_value
switch(string(pos))
case '"'
val = parse_string;
return;
case '['
val = parse_array;
return;
case '{'
val = parse_object;
return;
case {'-','0','1','2','3','4','5','6','7','8','9'}
val = parse_number;
return;
case 't'
if pos+3 <= len && strcmpi(string(pos:pos+3), 'true')
val = true;
pos = pos + 4;
return;
end
case 'f'
if pos+4 <= len && strcmpi(string(pos:pos+4), 'false')
val = false;
pos = pos + 5;
return;
end
case 'n'
if pos+3 <= len && strcmpi(string(pos:pos+3), 'null')
val = [];
pos = pos + 4;
return;
end
end
error_pos('Value expected at position %d');
end
function error_pos(msg)
poss = max(min([pos-15 pos-1 pos pos+20],len),1);
if poss(3) == poss(2)
poss(3:4) = poss(2)+[0 -1]; % display nothing after
end
msg = [sprintf(msg, pos) ' : ... ' string(poss(1):poss(2)) '<error>' string(poss(3):poss(4)) ' ... '];
ME = MException('JSONparser:invalidFormat', msg);
throw(ME);
end
function str = valid_field(str) |
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From MATLAB doc: field names must begin with a letter, which may be followed by any combination of letters, digits, and underscores. Invalid characters will be converted to underscores, and the prefix “alpha_” will be added if first character is not a letter. |
if ~isletter(str(1))
str = ['alpha_' str];
end
str(~isletter(str) & ~('0' <= str & str <= '9')) = '_';
end
end |