memoized-fibonacci.rb

require 'rspec'

class Fibonacci
  def self.memoize(method_name)
    slow_method_name = "slow_#{method_name}".to_sym
    alias_method slow_method_name, method_name
    cache ||= {}

    define_method method_name do |index|
      unless cache.key?(index)
        cache[index] = send(slow_method_name, index)
      end
      
      cache[index]
    end
  end

  def at_index(index)
    return index if index <= 1
    at_index(index - 2) + at_index(index - 1)
  end

  memoize :at_index
end

describe 'Fibonacci#at_index' do
  SEQUENCE = {
    0 => 0,
    1 => 1,
    2 => 1,
    3 => 2,
    4 => 3,
    5 => 5,
    6 => 8,
    35 => 9227465
  }
  SEQUENCE.each do |index, expected_result|
    it "receives #{index} and returns #{expected_result}" do
      expect(Fibonacci.new.at_index(index)).to eq expected_result
    end
  end
end