#include <iostream.h>
#include <stdlib.h>
#include <math.h>
#include <limits.h>


bool event_has_occured (double probability_of_event) {

  return (rand() / (RAND_MAX + 1.0)) < probability_of_event;

}


// returns true if team A wins and false otherwise
bool run_one_tug_of_war (double initial_probability,
                         double probability_increment,
                         int distance_to_win, long &seconds) {

  int position = 0;
  double probability = initial_probability;

  seconds = 0;

  for (;;) {

    seconds++;

    if (event_has_occured (probability)) {
      position++;
      if (position >= distance_to_win) {
        return true; // team A has won
      }
    } else {
      position--;
      if (position <= -distance_to_win) {
        return false; // team B has won
      }
    }

    // update probability, maling sure that it never becomes
    // greater than one or less than zero.
    probability += probability_increment;
    if (probability > 1) {
      probability = 1;
    } else if (probability < 0) {
      probability = 0;
    }

  }

}


// returns probability (0 to 1) of team A winning.
// also "returns" average length of all simulated contents.
double run_many_tugs_of_war (long trials,
                             double initial_probability,
                             double probability_increment,
                             int distance_to_win,
                             double &average_time) {

  long i, seconds, victories = 0;
  double total_seconds = 0;

  for (i = 0; i < trials; i++) {
    if (run_one_tug_of_war (initial_probability, probability_increment,
                            distance_to_win, seconds)) {
      victories++; // another win for team A
    }
    total_seconds += seconds;
  }

  average_time = total_seconds / trials;
  return ((double) victories) / trials;

}


void main (void) {

  double initial_probability, probability_increment,
         probability_of_winning, average_time;
  int distance_to_win;
  long trials;

  randomize();

  for (;;) {

    cout << "\nEnter number of trials (zero to terminate): ";
    cin >> trials;

    if (!cin.fail() && trials == 0) {
      // valid input and zero entered - time to quit
      return;
    }

    // continue with the input process
    cout << "\nEnter initial probability and probability increment: ";
    cin >> initial_probability >> probability_increment;
    cout << "\nEnter distance to win: ";
    cin >> distance_to_win;

	 if ( cin.fail() ||
			(initial_probability < 0) || (initial_probability > 1) ||
			(fabs(probability_increment) > 0.01) ||
         (trials < 0) || (distance_to_win <= 0) ) {

		cout << "\n**** Invalid input ****" << endl;
		// clean up things in case we had total garbage
		cin.clear(); cin.ignore (INT_MAX, '\n');

	 } else {

      // valid data available - go ahead and do the computations
      
      probability_of_winning =
          run_many_tugs_of_war (trials,
                                initial_probability, probability_increment,
                                distance_to_win, average_time);

		cout << "\nThere is a " << probability_of_winning * 100
			  << " per cent chance of team A winning."
			  << "\nThe expected duration of the tug of war is "
			  << average_time <<  " seconds." << endl;

	 }

  }

}