Hi,
I've got a problem to solve regarding binary data strings, which is an area
I don't have a lot of experience in. If anyone can help, I would be
grateful.
Here's the problem in a nutshell:
I am getting a binary string from a third-party server that I need to encode
into a PNG image. The string arrives double-quoted, so double quotes which
may occur in the binary string have been escaped. I need to parse through
the string and extract only the PNG data, and convert whatever has been
escaped into usable data.
I cannot control the output from the third-party -- I've got to deal with
what they're sending. (I would much rather have them send a base64 encoded
PNG, but unfortunately I'm not able to influence their methods.)
Here's the problem as described by the API I'm working with:
____________________________________________________________________________
The rule for encoding the PNG image in the returned buffer was designed to
eliminate certain illegal characters (byte values) by replacing them with an
escape sequence. Those values that are not allowed include:
* NULL (0x00 hex)
* Double Quote (0x22 hex)
The percent character '%' (0x25 hex) is used as the escape character, and
therefore it must be replaced where it occurs naturally. Whenever a
disallowed character or the escape character '%' is encountered, it is
replaced by the escape character '%' (0x25 hex) followed by two characters
comprising the hexadecimal numeral representing the value of the character.
For example, the NULL character (0x00 hex) is replaced by three characters:
* '%' (0x25)
* '0' (0x30)
* '0' (0x30)
The percent character 0x25 is replaced by three characters:
* '%' (0x25)
* '2' (0x32)
* '5' (0x35)
The algorithm for decoding the PNG image in the returned buffer is as
follows. Read bytes from the buffer one at a time. When a byte read is not
equal to the '%' character (0x25 hex), pass it through unchanged. When a
byte is read that is equal to the '%' character (0x25 hex), read an
additional two bytes, which will each take a value from zero (0x30 hex)
through nine (0x39 hex) or 'A' (0x41 hex) through 'F' (0x46 hex). These two
bytes are interpreted as a character representation of a two-digit
hexadecimal numeral ranging from 0x00 through 0xFF. The single byte having
the integral value represented by that numeral is appended to your output.
For example, when the 3-byte string '%22' is encountered, '"' (0x22) - the
double quote character - is passed out. When the 3 bytes '%00' are read, the
null character is written.
In essence, the developer will need to take the data received and store it
in a buffer, which has sufficient memory to hold the entire data stream.
Once the data has been received, the program must call a function similar to
the one described below in order to parse the data in the buffer and extract
only the PNG image data.
____________________________________________________________________________
The API then presents an example in C, which I've tried to translate into
PHP as best as I can. It's pretty close, I think -- but I'm still not
getting a PNG out when I'm done.
Here's what I've written, based on the C example:
____________________________________________________________________________
$buf = [binary data received];
$len = strlen($buf);
$out = '';
for ($c = 1; $c < $len; $c++) {
$data = $buf{$c};
if ($data != '%')
$out .= $data;
if ($data == '%') {
for ($e = 0; $e < 2; $e++) {
$c++;
$data = $buf{$c};
if ((($data >= 0x30) && ($data <= 0x39))
|| (($data >= 0x41) && ($data <= 0x46))) {
if ($e == 1) {
$d = $data;
$d = $d & 0x0f;
if (($data >= 0x41) && ($data <= 0x46)) {
$d += 9;
}
$store = $store | $d;
} else {
$d = $data;
$d = $d & 0x0f;
if (($data >= 0x41) && ($data <= 0x46)) {
$d += 9;
}
$store = $d << 4;
}
}
}
$out .= $store;
}
}
Header("Content-type: image/png");
echo $out;
____________________________________________________________________________
I'm just getting a blank screen at the end here -- not a PNG image.
Can anyone with more experience dealing with binary data offer any
suggestions? I'm at a loss and would appreciate the help! I would be happy
to send the example C code from the API if that would be helpful.
Thank you!
-Clay
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