问用于lzw压缩的缓冲区中存储9位代码的问题

EN

byte[] buffer = new byte[512];

function getWord(int wordOfs) {  
// Gets wordOfs'th 9-bit symbol from buffer, For buffer of 512 bytes, wordOfs = 0 -> 454
   if (wordOfs<0 || wordOfs>454) throw InvalidArgumentException;

   int bitsOfs = wordOfs * 9;  // Offset (in bits) of the desired 9 bit word 
   int idx = bitsOfs / 8;      // buffer[idx] contains bit 0 of the desired word
   int ofs = bitsOfs % 8;      // ... shifted this many places to the right

   // Grab at least 8 bits beyond the calculated starting point 
   unsigned word val = buffer[idx] | (buffer[idx+1]>>8);  
   // Shift and mask it down to the desired 9 bits for return
   return (val << ofs) & 0x01FF;
}