array_trie (rubygems): Trie-like, prefix-tree data structures. First, a prefix-tree based on Arrays, which differs from a traditional trie, which maps strings to values. Second, a more general prefix-tree data structure that works for any type of keys, provided those keys can be transformed to and from an array.  Both of these data structures are implemented in terms of hashes. 
Keywords: array|data-structure|ruby|trie