3 ms·

See also: http://www.hackersdelight.org/ http://www.hackersdelight.org/

• tycho77 15y ago
  Brilliant book, picked it up not long ago. So far, my favorite thing I have learned is on page 14 - the snoob function. Snoob stands for "same number of one bits". Essentially, if x is an integer whose binary representation contains n one-bits, snoob(x) will return the next smallest integer which is also represented using n one-bits. The obvious application here is iterating through all subsets of a certain size. The function is as follows: unsigned int snoob(unsigned int x) { unsigned int smallest, ripple, ones = 0; smallest = x & -x; ripple = x + smallest; ones = x ^ ripple; ones = (ones >> 2) / smallest; return ripple | ones; } Given a set containing N elements, to generate all subsets of size K you initialize a bitmask to (1 << K) - 1 then perform a snoob N Choose K times to get all the bitmasks you need. (Disclaimer: Although neat, I've never found a use for this outside of programming competitions)