To get a bit mask: var mask = 1 << 5; // gets the 6th bit To test if a bit is set: if ((n & mask) != 0) { // bit is set } else { // bit is not set } To set a bit: n |= mask; To clear a bit: n … Read more
No, it will not be fully-portable. Packing options for structs are extensions, and are themselves not fully portable. In addition to that, C99 ยง6.7.2.1, paragraph 10 says: “The order of allocation of bit-fields within a unit (high-order to low-order or low-order to high-order) is implementation-defined.” Even a single compiler might lay the bit field out … Read more
To multiply in terms of adding and shifting you want to decompose one of the numbers by powers of two, like so: 21 * 5 = 10101_2 * 101_2 (Initial step) = 10101_2 * (1 * 2^2 + 0 * 2^1 + 1 * 2^0) = 10101_2 * 2^2 + 10101_2 * 2^0 = 10101_2 … Read more
(n & (n – 1)) == 0 is best. However, note that it will incorrectly return true for n=0, so if that is possible, you will want to check for it explicitly. http://www.graphics.stanford.edu/~seander/bithacks.html has a large collection of clever bit-twiddling algorithms, including this one.
Bitwise operators are operators that work on multi-bit values, but conceptually one bit at a time. AND is 1 only if both of its inputs are 1, otherwise it’s 0. OR is 1 if one or both of its inputs are 1, otherwise it’s 0. XOR is 1 only if exactly one of its inputs … Read more
CHAR_BIT is the number of bits in char. These days, almost all architectures use 8 bits per byte but it is not the case always. Some older machines used to have 7-bit byte. It can be found in <limits.h>.
Best version is c++17: template< unsigned char… indexes > constexpr unsigned long long mask(){ return ((1ull<<indexes)|…|0ull); } Then void apply_known_mask(std::bitset<64> &bits) { constexpr auto m = mask<B,D,E,H,K,M,L,O>(); bits &= m; } back in c++14, we can do this strange trick: template< unsigned char… indexes > constexpr unsigned long long mask(){ auto r = 0ull; using … Read more
See also an earlier version of this answer on another rotate question with some more details about what asm gcc/clang produce for x86. The most compiler-friendly way to express a rotate in C and C++ that avoids any Undefined Behaviour seems to be John Regehr’s implementation. I’ve adapted it to rotate by the width of … Read more
In C, if you want to hide bit manipulation, you can write a macro: #define CHECK_BIT(var,pos) ((var) & (1<<(pos))) and use it this way to check the nth bit from the right end: CHECK_BIT(temp, n – 1) In C++, you can use std::bitset.
Combine the strategies to reduce the disadvantages of a single approach. I work in embedded systems so the following solution is based on the fact that integer and bitwise operators are fast, low memory & low in flash usage. Place the enum in a namespace to prevent the constants from polluting the global namespace. namespace … Read more