A-M baby

Task/Fibonacci-sequence/Ruby/fibonacci-sequence-3.rb

@@ -0,0 +1,13 @@
+# Use the Hash#default_proc feature to
+# lazily calculate the Fibonacci numbers.
+
+fib = Hash.new do |f, n|
+  f[n] = if n <= -2
+           (-1)**(n+1) * f[n.abs]
+         elsif n <= 1
+           n.abs
+         else
+           f[n-1] + f[n-2]
+         end
+end
+# examples: fib[10] => 55, fib[-10] => (-55/1)