This is an implementation of double-array structure for representing trie,
as proposed by Junichi Aoe.

Trie is a kind of digital search tree, an efficient indexing method with
O(1) time complexity for searching.  Comparably as efficient as hashing,
trie also provides flexibility on incremental matching and key spelling
manipulation.  This makes it ideal for lexical analyzers, as well as
spelling dictionaries.

WWW: https://linux.thai.net/projects/datrie/