From: C Erler <erlercw@...>
Date: 2006-05-27T08:13:37+09:00
Subject: Fast Fibonacci method

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The fastest pure-Ruby Fibonacci method I have been able to come up with is =
a
copy of the GMP algorithm described at
http://www.swox.com/gmp/manual/Fibonacci-Numbers-Algorithm.html

Is there a Ruby implementation that gets better time results for the
millionth Fibonacci number (timing code included at bottom) ?

class Integer
  FibonacciCache =3D Hash.new do |hash, key|
    subkey =3D key.div(2)
    case key.modulo(4)
      when 1
        hash[key] =3D (2*hash[subkey] + hash[subkey - 1])*(2*hash[subkey] -
hash[subkey - 1]) + 2
      when 3
        hash[key] =3D (2*hash[subkey] + hash[subkey - 1])*(2*hash[subkey] -
hash[subkey - 1]) - 2
      else
        hash[key] =3D hash[subkey] * (hash[subkey] + 2*hash[subkey - 1])
    end
  end
  FibonacciCache[0] =3D 0
  FibonacciCache[1] =3D 1

  def fib
    return FibonacciCache[self]
  end

  def fib_uncached
    return FibonacciCache.dup[self]
  end
end

start_time =3D Time.now
1_000_000.fib
p Time.now - start_time
puts "Doesn't work !" unless (1..10).map { |i| i.fib } =3D=3D [1, 1, 2, 3, =
5, 8,
13, 21, 34, 55]

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