[math-fun] reversing the order of bits in a word

ulong jj3_revbin(ulong x) { ulong z; z = (x ^ (x >> 2)) & 0x9249249249249249UL; x ^= z ^ (z << 2); // low 3 bits reversed z = (x ^ (x >> 6)) & 0x01c0e070381c0e07UL; x ^= z ^ (z << 6); // low 9 bits reversed z = (x ^ (x >> 18)) & 0x00001ff0000001ffUL; x ^= z ^ (z << 18); // low 27 bits reversed z = (x ^ (x >> 36)) & 0x0000000007ffffffUL; x ^= z ^ (z << 36); // low 63 bits reversed x = (x>>63) | (x<<1); // rotate left by 1 return x; } Can explain if anybody is interested later, got a visitor now. Did run automated testing, but would appreciate independent "OK"s. Yes, tricky. Best, jj