Description: Trie data structure

Latest: trie-0.2.6.el, 2012-Nov-29, 69.5kB

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Quick Overview -------------- A trie is a data structure used to store keys that are ordered sequences of elements (vectors, lists or strings in Elisp; strings are by far the most common), in such a way that both storage and retrieval are space- and time-efficient. But, more importantly, a variety of more advanced queries can also be performed efficiently: for example, returning all strings with a given prefix, searching for keys matching a given wildcard pattern or regular expression, or searching for all keys that match any of the above to within a given Lewenstein distance (though this last is not yet implemented in this package - code contributions welcome!). You create a trie using `make-trie', create an association using `trie-insert', retrieve an association using `trie-lookup', and map over a trie using `trie-map', `trie-mapc', `trie-mapcar', or `trie-mapf'. You can find completions of a prefix sequence using `trie-complete', or search for keys matching a regular expression using `trie-regexp-search'. Using `trie-stack', you can create an object that allows the contents of the trie to be used like a stack, useful for building other algorithms on top of tries; `trie-stack-pop' pops elements off the stack one-by-one, in "lexical" order, whilst `trie-stack-push' pushes things onto the stack. Similarly, `trie-complete-stack', and `trie-regexp-stack' create "lexically-ordered" stacks of query results. Note that there are two uses for a trie: as a lookup table, in which case only the presence or absence of a key in the trie is significant, or as an associative array, in which case each key carries some associated data. Libraries for other data structure often only implement lookup tables, leaving it up to you to implement an associative array on top of this (by storing key+data pairs in the data structure's keys, then defining a comparison function that only compares the key part). For a trie, however, the underlying data structures naturally support associative arrays at no extra cost, so this package does the opposite: it implements associative arrays, and leaves it up to you to use them as lookup tables if you so desire. Different Types of Trie ----------------------- There are numerous ways to implement trie data structures internally, each with its own time- and space-efficiency trade-offs. By viewing a trie as a tree whose nodes are themselves lookup tables for key elements, this package is able to support all types of trie in a uniform manner. This relies on there existing (or you writing!) an Elisp implementation of the corresponding type of lookup table. The best type of trie to use will depend on what trade-offs are appropriate for your particular application. The following gives an overview of the advantages and disadvantages of various types of trie. (Not all of the underlying lookup tables have been implemented in Elisp yet, so using some of the trie types described below would require writing the missing Elisp package!) One of the most effective all-round implementations of a trie is a ternary search tree, which can be viewed as a tree of binary trees. If basic binary search trees are used for the nodes of the trie, we get a standard ternary search tree. If self-balancing binary trees are used (e.g. AVL or red-black trees), we get a self-balancing ternary search tree. If splay trees are used, we get yet another self-organising variant of a ternary search tree. All ternary search trees have, in common, good space-efficiency. The time-efficiency of the various trie operations is also good, assuming the underlying binary trees are balanced. Under that assumption, all variants of ternary search trees described below have the same asymptotic time-complexity for all trie operations. Self-balancing trees ensure the underlying binary trees are always close to perfectly balanced, with the usual trade-offs between the different the types of self-balancing binary tree: AVL trees are slightly more efficient for lookup operations than red-black trees, at a cost of slightly less efficienct insertion operations, and less efficient deletion operations. Splay trees give good average-case complexity and are simpler to implement than AVL or red-black trees (which can mean they're faster in practice!), at the expense of poor worst-case complexity. If your tries are going to be static (i.e. created once and rarely modified), then using perfectly balanced binary search trees might be appropriate. Perfectly balancing the binary trees is very inefficient, but it only has to be when the trie is first created or modified. Lookup operations will then be as efficient as possible for ternary search trees, and the implementation will also be simpler (so probably faster) than a self-balancing tree, without the space and time overhead required to keep track of rebalancing. On the other hand, adding data to a binary search tree in a random order usually results in a reasonably balanced tree. If this is the likely scenario, using a basic binary tree without bothering to balance it at all might be quite efficient, and, being even simpler to implement, could be quite fast overall. A digital trie is a different implementation of a trie, which can be viewed as a tree of arrays, and has different space- and time-complexities than a ternary search tree. Roughly speaking, a digital trie has worse space-complexity, but better time-complexity. Using hash tables instead of arrays for the nodes gives something similar to a digital trie, potentially with better space-complexity and the same amortised time-complexity, but at the expense of occasional significant inefficiency when inserting and deleting (whenever a hash table has to be resized). Indeed, an array can be viewed as a perfect hash table, but as such it requires the number of possible values to be known in advance. Finally, if you really need optimal efficiency from your trie, you could even write a custom type of underlying lookup table, optimised for your specific needs. This package uses the AVL tree package avl-tree.el, the tagged NFA package tNFA.el, and the heap package heap.el.