How to get a random number with a given discrete distribution in Ruby

I'm coding my university assignment that is somewhat connected with distributions and random roll stuff. So the question is: how to get a random number with a given discrete distribution in Ruby.

To be more specific: in trivial example with normal discrete distribution like (0 with P=1/2; 1000 with P=1/2) I could write such a function:

 def chooseNumber(a,b)
  rval = Random.rand(0..1)
  return a if rval == 0
  return b
 end 

First question: is there any way to write it using native Random class?

Second question: what is the best way to deal with distributions like (0 with P=1/5; 2 with P=2/5; 1000 with P=2/5) or even worse (0 with P=0,33; 2 with P=0,49; 1000 with P=0,18)?