find-closest-bitstring.c

#include <stdint.h>

#ifdef __GNUC__
#define LOG2(X)                                                                \
    ((uint32_t)(8 * sizeof(unsigned long long) - __builtin_clzll((X)) - 1))
#else
#ifdef _MSC_VER
#include <intrin.h>
inline uint32_t LOG2(uint64_t X) {
    unsigned long out;
    _BitScanReverse64(&out, X);
    return out;
}
#endif
#endif

uint64_t get_closest_bitstring(uint64_t x) {
    uint32_t bits = LOG2(x);
    return ((uint64_t)1 << (bits + 1)) - 1;
}