spdlog/include/c11log/details/fast_istostr.h

#pragma once
#include <string>

//Fast to int to string
//Source: http://stackoverflow.com/a/4351484/192001
//Modified version to pad zeros according to min_size

namespace c11log {
namespace details {

const char digit_pairs[201] = {
    "00010203040506070809"
    "10111213141516171819"
    "20212223242526272829"
    "30313233343536373839"
    "40414243444546474849"
    "50515253545556575859"
    "60616263646566676869"
    "70717273747576777879"
    "80818283848586878889"
    "90919293949596979899"
};


inline std::string& fast_itostr(int n, std::string& s, int padding)
{
    if (n == 0)
    {
        s = "0";
        return s;
    }

    int sign = -(n < 0);
    unsigned int val = (n^sign) - sign;

    int size;
    if (val >= 10000)
    {
        if (val >= 10000000)
        {
            if (val >= 1000000000)
                size = 10;
            else if (val >= 100000000)
                size = 9;
            else
                size = 8;
        }
        else
        {
            if (val >= 1000000)
                size = 7;
            else if (val >= 100000)
                size = 6;
            else
                size = 5;
        }
    }
    else
    {
        if (val >= 100)
        {
            if (val >= 1000)
                size = 4;
            else
                size = 3;
        }
        else
        {
            if (val >= 10)
                size = 2;
            else
                size = 1;
        }
    }
    size -= sign;
    if (size < padding)
        size = padding;

    s.resize(size);
    char* c = &s[0];
    if (sign)
        *c = '-';

    c += size - 1;
    while (val >= 100)
    {
        int pos = val % 100;
        val /= 100;
        *(short*)(c - 1) = *(short*)(digit_pairs + 2 * pos);
        c -= 2;
    }
    while (val > 0)
    {
        *c-- = '0' + (val % 10);
        val /= 10;
    }

    while (c >= s.data())
        *c-- = '0';
    return s;
}
}
}