/////////////////////////////////////////////
// MT4Q4.cpp - implementing pushbacks

#include <iostream>
#include <queue>
#include <sstream>

////////////////////////////////////////////////////////////
// std::istream only guarantees that a single putback char
// will work, provided that the user extracts one or more chars
// before the next putback, e.g., there may only be storage in
// the stream for a single putback character.
//
// Ideal solutions for this problem would be to override
// virtual functions sbumpc and getc in a class derived from
// streambuf, as those functions are used to supply chars
// to istream.  Unfortunately that plot is foiled because
// std::streambuf does not make them virtual - sigh!
//
// Overriding get(), getc(), and all the other variants in
// std::istream is also foiled as they are not virtual.
// That only leaves us with the option of tacking on an
// extended interface to accomplish multiple putbacks, as
// illustrated below.
// 
namespace CSE687
{
  class istreamWithPushback : public std::istream 
  {
  public:
    istreamWithPushback(std::streambuf* pSb) : std::istream(pSb) {}
    void pushback(int ch);
    int getback();
  private:
    std::queue<int> backChars;
  };

  void istreamWithPushback::pushback(int ch)
  {
    backChars.push(ch);
  }

  int istreamWithPushback::getback()
  {
    int ch;
    if(backChars.size() > 0)
    {
      ch = backChars.front();
      backChars.pop();
      return ch;
    }
    return std::istream::get();
  }
}

using namespace::CSE687;

void main()
{
  char ch, ch1, ch2;
  std::istringstream ss("abcdefghijklmnopqrstuvwxyz");
  istreamWithPushback iwpb(ss.rdbuf());
  std::cout << "\n  the input stream contains:";
  std::cout << "\n  " << ss.rdbuf();
  ss.seekg(0);
  iwpb >> ch;
  std::cout << "\n  read: " << ch;
  iwpb >> ch;
  std::cout << "\n  read: " << ch;
  iwpb >> ch1;
  std::cout << "\n  read: " << ch1;
  iwpb >> ch2;
  std::cout << "\n  read: " << ch2;
  std::cout << "\n  pushing back " << ch1 << ":";
  iwpb.pushback(ch1);
  std::cout << "\n  pushing back " << ch2 << ":";
  iwpb.pushback(ch2);
  std::cout << "\n  the input stream now contains:";
  std::cout << "\n  ";
  while(iwpb.good())
  {
    ch = iwpb.getback();
    std::cout << ch;
  }
  std::cout << "\n\n";
}