confusion about tkey_extract_bits, please help.

hi:

  i had a question about ipv6 routing in 2.6.39. what the algorithm of
the function tkey_extract_bits. can anyone give me some advice on it,
thank u indeed .


static inline t_key tkey_extract_bits(t_key a, unsigned int offset,
unsigned int bits)
{
    if (offset < KEYLENGTH)
        return ((t_key)(a << offset)) >> (KEYLENGTH - bits);
    else
        return 0;
}