Perl Quiz-of-the-Week #10

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Here's an old chestnut of a computer entertainment: The
'guess-the-animal' program.  The human thinks of an animal.  The
computer tries to guess what the animal is by asking yes-or-no
questions.

The program maintains a decision tree of questions and answers, say
like this:


                Does it fly?
                     /\
                yes /  \ no
                   /    \
          Does it have   salmon
              feathers?
                 /\
            yes /  \ no
               /    \
           sparrow  fruit
                     bat

The program starts at the root and asks the question it finds there.
Depending on the user's response, it proceeds down the tree to the
'yes' or 'no' branch.  Leaf nodes in the tree represent animals; when
the program reaches one it asks (for example) "is it a sparrow?".  If
the answer is 'yes', the computer wins; if not, it prepares to extend
the tree.

To extend the tree, the computer asks a series of three questions:

        What animal were you thinking of?

                (Suppose the user replies "duck".)

        What is a question that would distinguish a duck from a sparrow?

                (Suppose the user replies "does it have webbed feet")

        For a duck, the answer would be?

                (The user replies "yes".)

The program then modifies its database to look like this:


                Does it fly?
                     /\
                yes /  \ no
                   /    \
          Does it have   salmon
              feathers?
                 /\
            yes /  \ no
               /    \
       Does it have fruit
       webbed feet?  bat
            /\
       yes /  \ no
          /    \
        duck   sparrow

Next time it plays the game, if the user says that their animal flies
and has feathers, the program will not ask right away if it was a
sparrow; instead, it will ask whether the animal has webbed feet, and
then if so it will ask if it is a duck, and if not it will ask if it
is a sparrow.

The program should record its database in a file so that it can
remember it from run to run.