linear probability draw card

lid.rb

def draw b, c, d, q = rand
  pc = 2.0 / ((d + 1) * (c - b + 1))
  s = pb = d * pc
  dp = (d - 1) * pc / (c - b)
  while s < q
    s += pb -= dp
    b += 1
  end
  b
end

p Array.new(20) { draw 1, 200, 100 }