Fibonacci One-Liner

My favorite is:

def fib(n)
  (0..n).inject([1,0]) { |(a,b), _| [b, a+b] }[0]
end

from https://gist.github.com/1007228

My favorite solution to this is to use a Hash, the values of which can be determined by an anonymous function:

fibonacci = Hash.new{ |h,k| h[k] = k < 2 ? k : h[k-1] + h[k-2] }

fibonacci[6]  # => 8 
fibonacci[50] # => 12586269025

It's a "genuine" one-liner and very Ruby-ish.

Inspired on Alex's answer:

# Ruby 1.8.7
f = lambda { |x| x < 2 ? x : f.call(x-1) + f.call(x-2) }
puts f.call(6)   #=> 8

# Ruby 1.9.2
f = ->(x){ x < 2 ? x : f[x-1] + f[x-2] }
puts f[6]        #=> 8

Using a Ruby 1.9 Enumerator:

fib = Enumerator.new do |yielder|
  i = 0
  j = 1
  loop do
    i, j = j, i + j
    yielder.yield i
  end
end

p fib.take_while { |n| n <= 4E6 }
# => [1, 1, 2 ... 1346269, 2178309, 3524578]

As one line:

p Enumerator.new { |yielder| i, j = 0, 1; loop {i, j = j, i + j; yielder.yield i} }.take_while { |n| n <= 4E6}