Fibonacci numbers using 4 different algorithms

fibonacci.rb

require 'rspec'

class Integer

  def to_fib
    return to_fib_formula unless self < 10
    to_fib_tail
  end

  def to_fib_recursive
    return self if zero? or one?
    (self-1).to_fib_recursive + (self-2).to_fib_recursive
  end

  def to_fib_tail
  return self if zero? or one?
    first,second = 0,1
    (self-1).times do
      first,second = second, first + second
    end
    second
  end

  def to_fib_inject
    return self if zero? or one?
    fib = (self-1).times.inject( [0,1] ) do | group, n |
       [ group[1], group[0] + group[1] ]
    end
    fib[1]
  end

  def to_fib_formula
    ( ( (GOLDEN_RATIO**self) - ((-GOLDEN_RATIO)**(-self)) ) / ROOT_FIVE ).to_i
  end

  def one?
      self == 1
  end

  ROOT_FIVE = Math.sqrt(5)
  GOLDEN_RATIO = (1 + ROOT_FIVE) / 2

end


describe "Fibonacci" do

  context "testing basic Fibonacci numbers." do

    it "Fibonacci number 0 is 0" do
      0.to_fib.should == 0
    end

    it "Fibonacci number 1 is 1" do
      1.to_fib.should == 1
    end

    it "Fibonacci number 2 is 1" do
      2.to_fib.should == 1
    end

    it "Fibonacci number 3 is 2" do
      3.to_fib.should == 2
    end

    it "Fibonacci number 10 is 55" do
      10.to_fib.should == 55
    end

    it "Fibonacci number 30 is 832040" do
         30.to_fib.should == 832040
       end

    it "Fibonacci number 38 is 39088169" do
         38.to_fib.should == 39088169
    end

  end

  context "testing all the implementations." do

    0.upto(30) do |i|
      
      it "recursive impl for number #{i}" do
        i.to_fib_recursive.should == i.to_fib
      end

      it "tail impl for number #{i}" do
        i.to_fib_tail.should == i.to_fib
      end

      it "inject impl for number #{i}" do
        i.to_fib_inject.should == i.to_fib
      end

      it "formula impl for number #{i}" do
        i.to_fib_formula.should == i.to_fib
      end

    end

  end

end