Ruby solution

test

def get_last_index(n, m, x)
  return nil if x > m # nil if received invalid input
  last_person_index = -1
  while m > 0 do
    m -= x
    m > x ? x += 1 : x = m
    last_person_index == n-1 ? last_person_index = 0 : last_person_index += 1
  end
  return last_person_index
end

describe "get_last_index " do
  context "when passed a valid input" do
    it "should return the expteced result" do
      r = get_last_index(2, 11, 2)
      expect(r).to eq 1
    end
    it "should return the expteced result" do
      r = get_last_index(3, 11, 2)
      expect(r).to eq 0
    end
  end

  context "when passed invalid input" do
    it "should return nil" do
      r = get_last_index(2, 2, 11)
      expect(r).to be nil
    end
  end

end