pgm(P2) image reformatter

pgm_p2_reformater.cpp

#include <iostream>
#include <sstream>
#include <string>
int proc_main(std::istream& is, std::ostream& os)
{
    std::string pn;
    if(!std::getline(is, pn)) return 1;
    if("P2" != pn) return 2;
    std::string tmp;
    if(!std::getline(is, tmp)) return 3;
    if(tmp.empty() || '#' != tmp[0]) return 4;
    std::string size;
    if(!std::getline(is, size)) return 5;
    const auto width = std::size_t(std::stoul(size.substr(0, size.find_first_of(' '))));
    const auto height = std::size_t(std::stoul(size.substr(size.find_first_not_of(' ', size.find_first_of(' ')))));
    for(
        std::size_t i = 0, j = 0;
        [&is](std::string& buf) -> std::istream& {
            while(auto& re = std::getline(is, buf)) if(buf.empty() || '#' != buf[0]) return re;
            return is;
        }(tmp);
    ){
        for(
            std::size_t pos = 0, pre_pos = 0;
            std::string::npos != pre_pos;
            pre_pos = (std::string::npos == pos) ? pos : tmp.find_first_not_of(' ', pos)
        ){
            pos = tmp.find_first_of(' ', pre_pos);
            if(0 != j) os << ' ';
            os << tmp.substr(pre_pos, pos - pre_pos);
            if(++j == width){
                if(++i == height){
                    os << std::flush;
                    return 0;
                }
                j = 0;
                os << std::endl;
            }
        }
    }
    os << std::flush;
    return 0;
}
constexpr auto input = R"(P2
# Created by IrfanView
7 4
255 172 9
31 7 9 14
221 187 1
0 92 33 7
182 21 37
22 3 5 71
92 23 143
22 5 7 21)";
int main()
{
    std::istringstream iss(input);
    return proc_main(iss, std::cout);
}

https://wandbox.org/permlink/SzJ55T77lushcySA

https://teratail.com/questions/127103