#include <iostream>
#include <set>
#include <cassert>

std::set<std::string, std::less<> > extract_prefix(const std::set<std::string, std::less<> > &all, const std::string_view &prefix)
{
    assert( prefix[prefix.size()-1] == '/');
    // replace / with next ascii char higher
    std::string upper { prefix.substr(0,prefix.size()-1) };
    upper += std::string(1, prefix[prefix.size()-1] + 1);
    
    std::set<std::string, std::less<> > res;
    for(auto iter=all.lower_bound(prefix); iter!=all.upper_bound(upper); ++iter)
    {
        auto &entry = *iter;
        if(entry == prefix)
            continue;
        std::string_view remainder( entry.c_str()+prefix.size(), entry.size()-prefix.size());
        auto pos = remainder.find('/');
        if(pos == std::string_view::npos || pos == remainder.size()-1)
            res.emplace(*iter);
    }
    return std::move(res);
}

int main() {
    std::set<std::string, std::less<> > names =
    {
    "/",
    "/dir1/",
    "/dir1/file1",
    "/file2",
    "/dir1/file2",
    "/dir1/dir2/",
    "/dir1/dir3/",
    "/dir1/dir3/file3"
    };
    auto res = extract_prefix(names, "/dir1/");
    for(auto &s: res)
        fprintf(stderr,"%s\n", s.c_str());
        
    names = res;
    return 0;
}