#include <string>
#include <iostream>
#include <stdexcept>
#include <cassert>

using namespace std;

class PathString_for_ptree
{
   string filename;
   string::size_type start, size;

public:
   PathString_for_ptree( std::string const& s ) :
      filename(s), start(0), size(filename.size())
   {}

   string dump() const
   {
      return filename.substr(start);
   }

   bool empty() const
   {
      return start == size;
   }

   // aka pop_front and return the fragment
   // modifies the path
   string reduce()
   {
      assert(not empty());

      // don't crash in production, just in case
      if (empty())
         return string();

      string::size_type next_sep = filename.find('/', start);

      // no separator, then pretend separator is at the end
      if (next_sep == string::npos)
         next_sep = size-1;

      // include the trailing separator
      string::size_type old_start = start;
      start = next_sep+1;

      return filename.substr(old_start, start-old_start);
   }

   bool single() const
   {
      // will return true if empty, it should not be asked for empty paths anyway

      // it is a "single" path if there is a slash BEFORE the last character.
      // if the last character is /, then it can be a "single" directory.
      //
      std::string::size_type idx = filename.find('/', start);
      return idx == std::string::npos or idx == size-1;
   }
   
};

static void test( string f, string should_frag, string should_remainder )
{
   PathString_for_ptree f2(f);
   cout << f2.dump() << (f2.single() ? " SINGLE" : "") << " --> ";
   string frag = f2.reduce();
   cout << frag << " && " << f2.dump() << endl;
   if (frag != should_frag)
      throw logic_error("err frag");
   if (f2.dump() != should_remainder)
      throw logic_error("err remainder");
}

static void descend( string f )
{
   PathString_for_ptree f2(f);
   while (not f2.empty())
   {
      cout << f2.dump() << (f2.single() ? " SINGLE" : " more") << " --> ";
      string frag = f2.reduce();
      cout << frag << " && " << f2.dump() << endl;
   }
}


int main()
{
   test("hello/there/you.txt", "hello/", "there/you.txt");
   test("hello/there/you/", "hello/", "there/you/");
   test("hello/there/", "hello/", "there/");
   test("hello/there.txt", "hello/", "there.txt");
   test("there.txt", "there.txt", "");
   test("a", "a", "");
   test("a/", "a/", "");
   test("b/a", "b/", "a");
   test("b/a/", "b/", "a/");
   test("/a/", "/", "a/");
   test("./a/", "./", "a/");

   descend("hello/there/you.txt");

   return 0;
}