c++ - Isolating a string of 1's in a character -

i need come function takes char , index of set bit in , isolates string of 1's containing bit.

i.e.

char isolate(unsigned char arg, int i); 

for example:

isolate(221,2) return 28 (11011101 >>> 00011100)

isolate(221,6) return 192 (11011101 >>> 1100000)

a lookup table seems clumsy solution require ~256*8=2048 entries.

i thinking of examining each individual bit left , right of index:

char isolate(char arg, int i) {    char result=0;    char mask = 1<<i;    for(char mask = 1<<i; arg & mask != 0; mask>>=1)       result |= mask;    for(char mask = 1<<i; arg & mask != 0; mask<<=1)       result |= mask;    return result; } 

but seems bit ugly. how can better this?

warning: none of tested or relevant*, may interesting.

isolating rightmost run of 1s easy, this: x ^ (x & ((x|(x-1))+1)) (explanation below), let's work that.

first x|(x-1) smears rightmost 1 right, adding 1 turns bits 0 including rightmost run of 1's, anding x removes rightmost run of 1's, , finally, xoring x leaves rightmost run of 1s.

then need make sure range we're looking rightmost one. that's less amenable simple bitmath, if there's count leading zeros (clz), it's not hard:

int shift = 32 - clz(~x & ((1 << i) - 1));  //replace 32 word size x = (x >> shift) << shift; 

((1 << i) - 1) makes mask of part right-end of run we're looking in (it just miss end, that's ok), clz looks first 0 right of i in x, shifts remove bits don't want at.

apply first formula, isolating rightmost run of 1s, result of run of ones i in. i had better in run, or things go sideways (more accurately, return first run of 1s starts @ index higher i)

*: question, none of matters. 2kb table not clumsy solution unless have tiny amount of memory available, , if that's case, input short loops aren't bad.