read file bytewise and find information

Question

i am a hobby php-dev and have the following problem:

i have a rar-file, whose header is damaged, therefor extracting is not fully possible. the contents are non-compressed, and can be read with eg. a hex-editor. the archive contains jpeg-files, but because of the damaged header, some of them are not extractable in winrar. i have tried repairing, but a good portion of the file remains broken.

i want to read the file in, and look for the bytes that indicate start and end of a jpeg (from what i know they are FFD8FF for start and FFD9FF for end). using a hex-editor i have managed to find some of those bytes, and also extracting the image into a file and view it does work. since the file is 500mb, i want to do this automatically, and since i am php-friendly, i would like to do it there :-)

i know how to read a file bytewise (fread), what i am having difficulties is, parsing the file correctly in hex, so i can identify the starts and ends. what i am thinking about is something like this (in pseudocode):

while( READ FILE UNTIL EOF ){
    if( CURRENTBYTES == FFD8FF ){
        $jpeg_file = READ FILE UNTIL CURRENTBYTES == FFD9FF
        fwrite($jpeg_file, "xyz.jpg");
        // return to while, looking for next FFD8FF
    }
}

could someone give me a hint, how the reading and identifying would be done best? the main question is, how can i read a file until a start-byte, then save the following, then look for the next start-byte? efficiency, security, code beauty are no concern, i just want all the pics :-)

many thanx for any helps

Answer 1

Since efficiency is no concern, and 500mb is not that large anymore, the simplest option would be:

• read the file into string $blob (hint: file_get_contents).
• get each separate part into an array $parts. Ugly solution: preg_match_all('/\xff\xd8\xff(.*?)\xff\xd9\xff/', $blob, $matches), and inspect the shape of the list $matches afterwards.
• safe each $part in a separate file, by for instance using the md5sum of the file as filename. (md5sum ought to be good enough for this)

This should be enough I suppose.

Answer 2

I would step through the file, instead of reading it all at once. Look for the first byte (0xFF), and when found, see if the next byte is 0xD8. If so, open output file and write those two bytes, followed by each byte until you reach the end pattern. Look for it the same way as the start bytes and close the file output when encountered.

Memory usage will be much less. If should run much faster than using a regex.

This is basically a very simple state machine.