Fibonacci number

fib.rb

class Fib
  def at(n)
    raise "invalid index" unless n > 0
    if n == 1
      0
    elsif n == 2
      1
    else
      at(n - 1) + at(n - 2)
    end
  end
end

fib_spec.rb

require './fib'

describe Fib do
  describe '#at' do
    context 'with 1' do
      it 'should return 0' do
        Fib.new.at(1).should == 0
      end
    end

    context 'with 2' do
      it 'should return 1' do
        Fib.new.at(2).should == 1
      end
    end

    context 'with 0' do
      it 'should raise RuntimeError' do
        lambda { Fib.new.at(0) }.should raise_error(RuntimeError)
      end
    end
  end
end

fib_test.rb

require 'test/unit'
require './fib'

class FibTest < Test::Unit::TestCase
  def setup
    @fib = Fib.new
  end

  def test_at1
    r = @fib.at 1
    assert_equal(0, r)
  end

  def test_at7
    r = @fib.at 7
    assert_equal(8, r)
  end

  def test_at0
    assert_raise RuntimeError do
      @fib.at 0
    end
  end

  def teardown; end;
end