Лента радостей жизни - 2

Once upon a time, in a kingdom not far from here, a king summoned two
of his advisors for a test. He showed them both a shiny metal box
with two slots in the top, a control knob and a lever. "What do you
think it is?"

One advisor, an engineer, answered first.

"It is a toaster" he said.

The king asked, "How would you design an embedded computer for it?"

The engineer replied, "Using a four-bit microcontroller, I would
write a simple program that reads the darkness knob and quantizes its
position to one of 16 shades of darkness, from snow white to coal
black. The program would use that darkness level as the index to a
16-element table of initial timer values. Then it would turn on the
heating elements and start the timer with the initial value selected
from the table. At the end of the time delay, it would turn off the
heat and pop up the toast. Come back next week, and I'll show you a
working prototype."

The second advisor, a computer scientist, immediately recognized the
danger of such short-sighted thinking. He said, "Toasters don't just
turn bread into toast, they are also used to warm frozen
waffles. What you see before you is really a breakfast food
cooker. As the subjects of your kingdom become more sophisticated,
they will demand more capabilities. They will need a breakfast food
cooker that can also cook sausage, fry bacon and make scrambled
eggs. A toaster that only makes toast will soon be obsolete. If we
don't look to the future, we will have to completely redesign the
toaster in just a few years."

"With this in mind, we can formulate a more intelligent solution to
the problem. First, create a class of breakfast foods. Specialize
this class into subclasses: grains, pork and poultry. The
specialization process should be repeated with grains divided into
toast, muffins, pancakes and waffles; pork divided into sausage,
links and bacon; and poultry divided into scrambled eggs, hard-boiled
eggs, poached eggs, fried eggs and various omelet classes."

"The ham and cheese omelet class is worth special attention because
it must inherit characteristics from the pork, dairy and poultry
classes. Thus, we see that the problem cannot be properly solved
without multiple inheritance. At run time, the program must create
the proper object and send a message to the object that says, 'Cook
yourself'. The semantics of this message depend, of course, on the
kind of object, so they have a different meaning to a piece of toast
than to scrambled eggs."

"Reviewing the process so far, we see that the analysis phase has
revealed that the primary requirement is to cook any kind of
breakfast food. In the design phase, we have discovered some derived
requirements. Specifically, we need an object-oriented language with
multiple inheritance. Of course, users don't want the eggs to get
cold while the bacon if frying, so concurrent processing is required, too."

"We must not forget the user interface. The lever that lowers the
food lacks versatility, and the darkness knob is confusing. Users
won't buy the product unless it has a user-friendly, graphical
interface. When the breakfast cooker is plugged in, users should see
a cowboy boot on the screen. Users click on it, and the message
'Booting Ubuntu 11.02' appears on the screen. (Ubuntu 11.03 should
be out by the time the product gets to the market.) Users can pull
down a menu and click on the foods they want to cook."

"Having made the wise decision of specifying the software first in
the design phase, all that remains is to pick an adequate hardware
platform the implementation phase. An ARM 9 processor with 16Gb of
memory and a 500Gb hard disk should be sufficient. If you select a
multitasking, object oriented language that supports multiple
inheritance and has a built in GUI, writing the program will be a
snap. (Imagine the difficulty we would have had if we had foolishly
allowed a hardware-first design strategy to lock us into a four-bit
microcontroller!)"

The king wisely had the computer scientist beheaded, and they all
lived happily ever after.