/* reverse.cpp - Matt Mahoney, mmahoney@cs.fit.edu

This program reads lines of text from a file and writes them
to another file in reverse order.  The input and output file names
are given on the command line.  For instance, if input.txt contains:

  This
  is a
  test.

Then the command:

  reverse input.txt output.txt

creates output.txt as follows:

  test.
  is a
  This

If the file names are not specified or cannot be found or created, then
an error message is printed to standard error.
*/

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
using namespace std;

/* A Stack stores strings.  Strings are removed (popped) in the
reverse order in which they were are added (pushed).  Attempting to pop
an empty stack throws Stack::Error.

  Stack st;      // Initially empty stack of strings
  st.push(s);    // Push string s onto Stack st
  s = st.pop();  // Pop and return string s.  If empty, throws Stack::Error
  st.empty();    // true if empty
  Stack::Error;  // An exception type
*/

class Stack {
private:
  vector<string> v;  // The top is at the back
public:
  class Error {};    // Exception type
  bool empty() const {return v.empty();}
  void push(const string& s) {v.push_back(s);}
  string pop();
};

string Stack::pop() {
  if (empty())
    throw Error();
  string s=v.back();
  v.pop_back();
  return s;
}

// Read file argv[1] and write to argv[2] with the lines in reverse order.
int main(int argc, char** argv) {

  // Check for 2 file name arguments, exit with an error if not.
  if (argc<3) {
    cerr << "Usage: reverse inputfile outputfile\n";
    return 1;
  }

  // Open input file, check for error.
  ifstream in(argv[1]);
  if (!in) {
    cerr << "File not found: " << argv[1] << endl;
    return 1;
  }

  // Open output file, check for error.
  ofstream out(argv[2]);
  if (!out) {
    cerr << "Cannot create file: " << argv[2] << endl;
    return 1;
  }

  // Read input file into stack
  Stack st;
  string s;
  while (getline(in, s))
    st.push(s);

  // Write stack contents to output file
  try {
    while (true)
      out << st.pop() << endl;
  }
  catch (Stack::Error x) {}
  return 0;
}