/*
  Birthday
  written by Keith Fenske
  Wednesday, 15 May 2002
  Copyright (c) 2002 by Keith Fenske.  All rights reserved.

  When you have a group of people, there is a probability that two or more
  people have the same birthday.  This program calculates the minimum size
  of a group for the probability to reach a certain number.

  Obviously, the first person has a unique birthday.  After we know the
  first person's birthday, there are 364 remaining days in the year.  (We
  assume that each year has 365 days: no leap years.)  The second person
  has a 364/365 probability of *not* being born on the same day.  The third
  has a 364/365 x 363/365 probability of *not* being born on either of the
  first two days.  And so on.

  Here is a chart of some results:

    goal    n     probability that all birthdays are different

            22    0.5243
    0.50    23    0.4927

            28    0.3455
    0.33    29    0.3190

            31    0.2695
    0.25    32    0.2467

  In other words, if you have a group of 23 people (or more), then you have
  greater than a 50% chance of two people sharing the same birthday.
*/

import cs1.Keyboard;              // import Lewis/Loftus keyboard class
//                           http://duke.csc.villanova.edu/jss/keyboard.html

public class Birthday
{
  public static void main(String[] args)
  {
    double current;               // current probability
    double goal;                  // probability that we are looking for
    int n;                        // group size

    System.out.println("\nThis program calculates the smallest group size n");
    System.out.println("where the probability that n people do *not* have");
    System.out.println("the same birthday is less than a probability given");
    System.out.println("by the user.");

    do
    {
      System.out.println("\nPlease type a probability from 0.0 to 1.0");
      System.out.println("(or any other number to quit):");

      goal = Keyboard.readDouble();
//    System.out.println("goal = " + goal);
      if ((goal >= 0.0) && (goal <= 1.0))
      {
        current = 1.0;            // first person has 1.0 probability
        n = 1;                    // start with one person
        while ((current > goal) && (n < 365))
        {
//        System.out.println("current = " + current + " n = " + n);
          n = n + 1;              // add one more person
          current = current * (365.0 - n + 1.0) / 365.0;
        }
        System.out.println("The probability that " + n + " people have different");
        System.out.println("birthdays is " + current);
      }
    }
    while ((goal >= 0.0) && (goal <= 1.0));

  } // end of main()

} // end of class