There are a couple of small optimizations for your version. By reversing the roles of True and False, you can change “if flags[i] is False:” to “if flags[i]:“. And the starting value for the second range statement can be i*i instead of i*3. Your original version takes 0.166 seconds on my system. With those changes, … Read more
int(‘{:08b}’.format(n)[::-1], 2) You can specify any filling length in place of the 8. If you want to get really fancy, b = ‘{:0{width}b}’.format(n, width=width) int(b[::-1], 2) lets you specify the width programmatically.
It’s a portable way to set all the binary bits in an integer to 1 bits without having to know how many bits are in the integer on the current architecture.
The guy who said “leave it to the compiler” was right, but I don’t have the “reputation” to mod him up or comment. I asked gcc to compile int test(int a) { return a / 3; } for an ix86 and then disassembled the output. Just for academic interest, what it’s doing is roughly multiplying … Read more
Well, the official Java tutorial Bitwise and Bit Shift Operators covers the actual operations that are available in Java, and how to invoke them. If you’re wondering “what can I do with bit-shifting”, then that’s not Java specific, and since it’s a low-level technique I’m not aware of any list of “cool things you can” … Read more
Whoa. You’re really over-complicating it by a very long distance. Try: >>> print(hex(0x12ef ^ 0xabcd)) 0xb922 You seem to be ignoring these handy facts, at least: Python has native support for hexadecimal integer literals, with the 0x prefix. “Hexadecimal” is just a presentation detail; the arithmetic is done in binary, and then the result is … Read more
Think about how you multiply in decimal using pencil and paper: 12 x 26 —- 72 24 —- 312 What does multiplication look like in binary? 0111 x 0101 ——- 0111 0000 0111 ——- 100011 Notice anything? Unlike multiplication in decimal, where you need to memorize the “times table,” when multiplying in binary, you are … Read more
Unless you’re using an ancient compiler, it can already handle this level of conversion on its own. That is to say, a modern compiler can and will implement i % 2 using a bitwise AND instruction, provided it makes sense to do so on the target CPU (which, in fairness, it usually will). In other … Read more
GCC has __builtin_clz that translates to BSR on x86/x64, CLZ on ARM, etc. and emulates the instruction if the hardware does not implement it. Visual C++ 2005 and up has _BitScanReverse.
The code is found in _spin_lock_contested, which is called from _spin_lock_quick when someone else is attempting to obtain the lock : count = atomic_fetchadd_int(&spin->counta, 1); if (__predict_false(count != 0)) { _spin_lock_contested(spin, ident, count); } If there’s no contest, then count (the previous value) should be 0, but it isn’t. This count value is passed as … Read more