
Now we need to generate the random numbers according to
the above distribution.

We create a lot of map tasks, each of which takes at least
one \"line\" of the distribution.  (That is, a certain number
X is to be generated Y number of times.)

A map task emits Y key/val pairs.  The val is X.  The key
is a randomly-generated number.

The reduce task gets its input sorted by key.  That is, sorted
in random order.  It then emits a single line of text that
for the given values.  It does not emit the key.

Because there's just one reduce task, we emit a single big
file of random numbers.
