Power-of-2 functions (get the nearest, next, or previous power of 2).

pow2.cpp

/**
 * Test of getting the next/previous powers of 2
 *
 * g++ -std=c++11 -Wall -Werror -Wextra -pedantic -pedantic-errors pow2.cpp -o pow2
 */

#include <iostream>
#include <sstream>

/******************************************************************************
 *  Power of 2 functions for unsigned integers
******************************************************************************/
inline unsigned nextPow2(unsigned n) {
    if (n == 0) {
        return 0;
    }
    
    --n;
    n |= n >> 1;
    n |= n >> 2;
    n |= n >> 4;
    n |= n >> 8;
    n |= n >> 16;
    return ++n;
}

inline unsigned prevPow2(unsigned n) {
    if (n == 0) {
        return 0;
    }
    
    --n;
    n |= n >> 1;
    n |= n >> 2;
    n |= n >> 4;
    n |= n >> 8;
    n |= n >> 16;
    return n - (n >> 1);
}

inline unsigned nearPow2(unsigned n) {
    const unsigned pp2 = prevPow2(n);
    const unsigned np2 = nextPow2(n);
    const unsigned lo = n - pp2;
    const unsigned hi = np2 - n;
    
    return lo < hi ? pp2 : np2;
}

constexpr bool isPow2(unsigned n) {
    return n && !(n & (n-1));
}

/******************************************************************************
 *  Verification function to make sure that main()'s "argv[1]" is a number
******************************************************************************/
bool inputIsNumber(char* const str) {
    for (unsigned i = 0; str[i]; ++i) {
        if (str[i] < '0' || str[i] > '9') {
            return false;
        }
    }
    
    return true;
}

int main(int argc, char** argv) {
    if (argc != 2 || !inputIsNumber(argv[1])) {
        std::cerr
            << "Incorrect Syntax. Try \"pow2 n\" where \"n\" is a positive number"
            << std::endl;
        return -1;
    }
    
    std::istringstream converter(argv[1]);
    unsigned uval;
    converter >> uval;
    
    std::cout << "You entered: " << uval << '\n';
    
    if (isPow2(uval)) {
        std::cout << uval << " is a power of 2.\n";
    }
    else {
        std::cout << uval << " is NOT a power of 2.\n";
    }
    
    
    std::cout << "The next power of 2 is:\t\t" << nextPow2(uval) << '\n';
    std::cout << "The nearest power of 2 is:\t" << nearPow2(uval) << '\n';
    std::cout << "The previous power of 2 is:\t" << prevPow2(uval) << std::endl;
    
    return 0;
}