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

def fib
return FibonacciCache[self]
end

def fib_uncached
return FibonacciCache.dup[self]
end
end

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

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```