What is Project Euler

Project Euler is a website with a collection of programming and mathematical problems. Each challenge varies in difficulty and there are over 600 challenges. I highly recommend anyone interested in puzzles and programming to create an account and start solving. About Project Euler

** Stop here if you have not yet tried and solved Problem 2. **

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Problem 2

Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, …

By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

class Problem2
  require 'benchmark'

  def question
    "Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:

    1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

    By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms."
  end


  def fibonacci_even_sum(limit = 4000000)
    sum = 2
    runner = 2
    one_back = 1
    two_back = 0

    while(runner <= limit)
      sum += runner if runner.even?

      two_back = one_back
      one_back = runner
      runner = two_back + one_back
    end

    sum
  end

  def solve
    fibonacci_even_sum
  end

  def benchmark
    Benchmark.measure { solve }
  end

  def correct?
    solve === answer
  end

  private

  def answer
    4613732
  end
end

 

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