optimized pow10

optimized.cpp

#include <iostream>
#include <climits>

const static int longsize = sizeof(long) * CHAR_BIT;
constexpr const static long pow10[19] = {
    1,
    10,
    100,
    1000,
    10000,
    100000,
    1000000,
    10000000,
    100000000,
    1000000000,
    10000000000,
    100000000000,
    1000000000000,
    10000000000000,
    100000000000000,
    1000000000000000,
    10000000000000000,
    100000000000000000,
    1000000000000000000
};

long getDigitsInNumber(long x) {
    if (x == 0) return 1;
    long t = ((longsize - __builtin_clzl(x)) * 1233) >> 12;
    return t + (x >= pow10[t]);
}

int main() {
    long x = 100;
    std::cout << getDigitsInNumber(x) << std::endl;
    return 0;
}

original.cpp

#include <iostream>
#include <climits>
const static long longsize = sizeof(long)*CHAR_BIT;
constexpr const static long pow10[20] = {
        1,
        10,
        100,
        1000,
        10000,
        100000,
        1000000,
        10000000,
        100000000,
        1000000000,
        10000000000,
        100000000000,
        1000000000000,
        10000000000000,
        100000000000000,
        1000000000000000,
        10000000000000000,
        100000000000000000,
        1000000000000000000
};

long getDigitsInNumber(long x) {
    long minDigits = ((__builtin_clzl(x) | (-longsize)) * -1233 >> 12);
    return minDigits + (pow10[minDigits] <= x);
}


int main() {
    long x = 100;
    std::cout << x << std::endl;
    return 0;
}

//        lzcnt   rax, rdi
//        or      rax, -64
//        imul    rax, rax, -1233
//        shr     rax, 12
//        cmp     rdi, qword ptr [8*rax + pow10]
//        sbb     rax, -1