fib.rb

require 'rspec'

describe "Fib" do
  it "should return 0" do
    expect(fib 0).to eq 0
  end 

  it "should return 1" do
    expect(fib 1).to eq 1
  end 

  it "should return 1" do
    expect(fib 2).to eq 1
  end

  it "should return 2" do
    expect(fib 3).to eq 2
  end

  it "should return 3" do
    expect(fib 4).to eq 3
  end

  it "should return 5" do
    expect(fib 5).to eq 5
  end

  it "should return 8" do
    expect(fib 6).to eq 8
  end

  it "should return 89" do
    expect(fib 11).to eq 89
  end

  it "should return 6765" do
    expect(fib 20).to eq 6765
  end
end

def fib n
  i, fibs = 0, []
  while i <= n
    if i == 0
      fibs.push 0
    elsif i == 1 || i == 2
      fibs.push 1
    else
      fibs.push(fibs.last + fibs[-2])
    end
    i += 1
  end
  fibs.last
end