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)
[+] [-] ppruitt|14 years ago|reply
[+] [-] tycho77|14 years ago|reply
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) {
}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)