Memoized Fibonacci numbers with Hash default values

fib.rb

Fib = Hash.new { |hash, key| hash[key] = hash[key - 2] + hash[key - 1] }
Fib.merge! 0 => 0, 1 => 1

require 'minitest'
require 'minitest/autorun'
 
describe Fib do
  it 'fibs' do
    assert_equal Fib[0], 0
    assert_equal Fib[1], 1
    assert_equal Fib[2], 1
    assert_equal Fib[5], 5
    assert_equal Fib[10], 55
    assert_equal Fib[20], 6765
  end
end

output.txt

$ ruby fib.rb
Run options: --seed 52550

# Running:

.

Finished in 0.006770s, 147.7105 runs/s, 886.2629 assertions/s.

1 runs, 6 assertions, 0 failures, 0 errors, 0 skips