HW: 1A2B

1. Write a program which will play the 1A2B game (also known as Bulls and Cows) with the user.
2. When the program starts, it randomly chooses 4 different digits among 0..9, and keeps them in secret. It then prompts the user to guess.
   
3. The user will input 4 digits. The computer will compare them with the secret digits and give the number of matches.
   • The minimum requirement of this homework is that, you may assume the user always supply a valid input. That is, the user will not input only 3 digits, or non-numeric characters.
   • However, you are encouraged to implement a function to check whether the input is valid.
4. If the matching digits are in their right positions, they are counted as "A"; if in different positions, they are counted as "B". Example:
   • Secret number: 4271
   • User's guess: 1234
   • Result: 1A2B
5. All the previous guesses and results will be displayed, so that the user can check easily.
   
6. When the user correctly guesses the secret digits (i.e., he/she gets "4A"), the program ends.
7. The program prints out how many guesses the user have made, and how long he/she spends.
   • Hint: You may use the time() function to get current Unix clock time.
   
   
8. Hint: You may design a function ordinal() to display the ordinal number. You may test it with the following code:

       
       for ($i=1; $i<30; $i++)
           print ordinal($i) . "\n";
       
       

   Please note it should show "12th" instead of "12nd".
9. You may play this example to get a feeling on how it may work.