Decoding Function C++ source code in 31 lines

get_jpeg_size.c

//Gets the JPEG size from the array of data passed to the function, file reference: http://www.obrador.com/essentialjpeg/headerinfo.htm
static char get_jpeg_size(unsigned char* data, unsigned int data_size, unsigned short *width, unsigned short *height) {
   //Check for valid JPEG image
   int i=0;   // Keeps track of the position within the file
   if(data[i] == 0xFF && data[i+1] == 0xD8 && data[i+2] == 0xFF && data[i+3] == 0xE0) {
      i += 4;
      // Check for valid JPEG header (null terminated JFIF)
      if(data[i+2] == 'J' && data[i+3] == 'F' && data[i+4] == 'I' && data[i+5] == 'F' && data[i+6] == 0x00) {
         //Retrieve the block length of the first block since the first block will not contain the size of file
         unsigned short block_length = data[i] * 256 + data[i+1];
         while(i<data_size) {
            i+=block_length;               //Increase the file index to get to the next block
            if(i >= data_size) return false;   //Check to protect against segmentation faults
            if(data[i] != 0xFF) return false;   //Check that we are truly at the start of another block
            if(data[i+1] == 0xC0) {            //0xFFC0 is the "Start of frame" marker which contains the file size
               //The structure of the 0xFFC0 block is quite simple [0xFFC0][ushort length][uchar precision][ushort x][ushort y]
               *height = data[i+5]*256 + data[i+6];
               *width = data[i+7]*256 + data[i+8];
               return true;
            }
            else
            {
               i+=2;                              //Skip the block marker
               block_length = data[i] * 256 + data[i+1];   //Go to the next block
            }
         }
         return false;                     //If this point is reached then no size was found
      }else{ return false; }                  //Not a valid JFIF string
         
   }else{ return false; }                     //Not a valid SOI header
}