/*
 * Prof. Sloan model solution to Program 5b, CS 202, Nov. 2002
 */


#include <iostream>
#include <fstream>
#include <string>
#include <list>

#include "Graph.hh"

using namespace std;

bool offByOne(const string& w1, const string& w2);


int
main (int argc, char* argv[])
{
  const string defaultDictionary = "/usr/dict/words";
  const string noPathMessage = "IMPOSSIBLE";

  string word;			// where dictionary words go
  Graph wordGraph;		// graph with words on nodes

  string begin = argv[1];
  string end = argv[2];

  if (argc != 4)
    {
      cout << argv[0] << ": usage is " << argv[0] 
	   << " word1 word2 dict-file" << endl;  
      return 0;
    }

  int wordLength = begin.size();
  if (end.size() != wordLength)
    {
      cout << "puz: words must have same length" << endl;
      return 1;
    }

  ifstream dictionary(argv[3]);

  cout << "Building graph on words of length " << wordLength <<
    ". . . ." << endl;


  do
  {
    dictionary >> word;
    if (word.size() == wordLength)
      {
	wordGraph.insertVertex(word);
	// cout << word;
      }
  }
  while (dictionary);

  if (!(wordGraph.containsVertex(begin) && wordGraph.containsVertex(end)))
    {
      cerr << "puz: One or both input words not in Dictionary." << endl;
      return 1;
    }


  Graph::iterator i, j;		// to walk through & insert edges
  for (i = wordGraph.begin(); i != wordGraph.end(); i++)
    for (j = wordGraph.begin(); j != wordGraph.end(); j++)
      if (i != j && offByOne(*i, *j) )
	wordGraph.insertEdge( *i, *j);

  cout << "Searching graph for transformation. . . ." << endl;

  list<string> wordList;
  Graph::dfs_reach_iterator itr;
  for (itr = wordGraph.dfs_reach_begin(begin); 
       itr != wordGraph.dfs_reach_end(); itr++)
    {
      wordList.push_back(*itr);
      if ((*itr) == end)
	break;
    }

  if (itr == wordGraph.dfs_reach_end())
    {
      cout << noPathMessage << endl;
      return 0;
    }

//   cout << "Transformation possible.  Long list is" << endl;

//   list<string>::iterator debugItr;
//   for (debugItr = wordList.begin(); debugItr != wordList.end();
//        ++debugItr)
//     cout << *debugItr << " ";
//   cout << endl;

  cout << endl << "Now constructing output. . . ." << endl;


  // Prune dfsList to get simple path
  list<string>::iterator listItrFront = wordList.begin();
  list<string>::iterator listItrBack = --wordList.end();
  list<string>::iterator listItrPenultimate;
  list<string>::iterator listItrWalk;

  while (1)
    {
      listItrPenultimate = listItrBack;
      --listItrPenultimate;	// Now points to 2nd last
      if (listItrFront == listItrPenultimate || 
	  listItrFront == listItrBack)
	break;
      listItrWalk = listItrBack;
      while (! wordGraph.containsEdge(*listItrFront, *listItrWalk))
	--listItrWalk;		// walk back till edge from Front->Walk
      cout << "Found edge (" << *listItrFront << ", " << *listItrWalk 
	   << ")" << endl;
      if ( ++listItrFront != listItrWalk) // some can be cut out!
	listItrFront = wordList.erase(listItrFront, listItrWalk);
    }

  for (listItrWalk = wordList.begin(); listItrWalk != wordList.end();
       listItrWalk++) 
    cout << *listItrWalk << endl;

  return 0;

}

//Returns true if and only if w1 and w2 are of same length and
//different by exactly one char
bool
offByOne(const string& w1, const string& w2)
{
  if (w1.size() != w2.size())
    return false;

  int numDifferent = 0;
  for (int i = 0; i < w1.size(); i++)
    if (w1[i] != w2[i])
      ++numDifferent;
  
  return (numDifferent == 1);
}