A Ruby function to calculate the nth Fibonacci number using recursion with memoization.

fibonacci.rb

# Initialize the memoization array
@cache = []

# Calculate the nth Fibonacci number
# using recursion with memoization
def fibonacci(n)
	return n if (0..1).include? n
	(@cache[n-1] ? @cache[n-1] : @cache[n-1] = fibonacci(n - 1)) +
		(@cache[n-2] ? @cache[n-2] : @cache[n-2] = fibonacci(n - 2))
end