I recently went to a job interview (I'm looking for work, by the way; hire me!) and one of the questions put to me was "solve the Boggle™ board." I don't think I did very well on the question and it wasn't until I got home that I remembered "Dammit, the fastest structure for checking if a word is in a dictionary is a trie!"

Whiteboard exercises are terrible. I don't think I've ever actually programmed a trie, not even in Data Structures class I took way back in college in 19mumblemumble. I'm sure I've used them, but I've never had to write one.

So what is a trie?[


A trie is a data structure that encodes every word in a dictionary as a string of nodes, one per letter. This trie encodes the dictionary containing the words "a", "an," "ant", "art", and "aunt". If we're looking up "ant," we start at the root node and traverse to "a", then "n", and then "t". The "t" is double-circled, that is, it's marked as a terminal node, and thus "ant" is in this dictionary. If we were looking up the word "any", we would get to "a,n," and then fail, because "any" is not in this dictionary. Likewise, we might ask if "aun" is in the dictionary, and we would get to "a,u,n," but that "n" is not double-circled, and thus "aun" is also not a complete word in this dictionary.

Like a regular expression, a trie search is successful when the string is exhausted, the trie is still active, and we find ourselves on a node marked as a word terminator. If the trie exhausts before the string, or if the string exhausts on a non-terminating node, the search fails to find a complete word.

The rules of Boggle™ are very simple: on a 4x4 grid, sixteen dice are shaken and dropped into place. Each die has six letters on it, probabilistically arranged. Players have three minutes to find every valid word on the board, where "valid" means "found in the dictionary the players have agreed to use," modulo some rules like "no contractions or abbreviations." The letters of the words must be adjacent in any of the eight cardinal directions (modulo the borders of the grid), and no letter may be used twice for the same word.

So, to solve for Boggle, we need a fast lookup dictionary, and thus we need a trie. Tries are extremely fast, Ο(n) where n is the length of the word. They're memory-intensive, but these days memory is cheaper than time. Each node has two conditions: the list of letters that it leads to, and whether or not it is a terminal node. For the "list of letters," we're going to use a HashMap. The child nodes must be Boxed as we're building this on the heap (it will be huge for a full-sized dictionary) and it must be RefCell'd because we'll be mutating child nodes repeatedly as we build the trie.

For a canonical dictionary lookup, this implementation is different the Wikipedia page's description.  There's no utility to storing the node's value in itself; we know what the value is by the key we used to reach it!  We do need to know if the node is a terminator node.

There are strong feelings about tuples vs structs for values with multiple fields, with most people preferring structs.  For a value with two fields, I feel a tuple ought to be adequate.  A record with so many fields it has to name them, and therefore a struct, has a code smell and is worth examining closely for cognitive complexity.  I'll address that in the next post.

<code>pub struct Node(HashMap<char, Box<RefCell<Node>>, bool);</code>

For this structure, we're going to build it imperatively; that is, each word will be presented to the structure one at a time, and each word will be an iterator over its characters. The first thing we need to do is say, for a given node, if the word is exhausted, then we are a terminator node:

<code>impl Node {
    pub fn insert(&mut self, word: &mut Iterator<Item = char>) {
        let c = match word.next() {
            None => {
                self.1 = true;
            Some(c) => c,

Note that we're returning from this function if we're a terminator. There is no character to insert into the next node.

If we're not a terminator, we must then either access or create a child node, and then keep traversing the word until its exhausted. In the "create a new child" scenario, we create the full trie before inserting it.

<code>        match self.0.get(&c) {
            None => {
                let mut newtrie = Node::new();
                self.0.insert(c, Box::new(RefCell::new(newtrie)));
            Some(node) => {

One tricky feature of Boggle™ is that we want to end a search for a word early if the "word" found isn't really a word. That is, if on the board you find the sequence "tk", no word starts with "tk" and you want the search to terminate early. But as in our example above, "aun" is also not a word, but we do not want to terminate the search, as there may be a handy 't' nearby to finish the word.

So we want to be able to search the trie, but we have two different criteria: "is this a word" and "could this be the prefix of a word?" We want our search engine to be able to handle both.

How do we handle both? Let's go back: what are the failure conditions? The trie gets exhausted or the string gets exhausted. If both are exhausted at the same time and we're on a terminator, it's a word. If the word is exhausted but the trie is not, this is a prefix, regardless of its terminator status.

So, our search feature will be a recursive function that, letter by letter, transits the trie node by node. If the word exhausts and we're still in the trie, we check the end state (always true for a prefix, is a terminator for a word), otherwise we recurse down to the next letter and the next node.

First, let's see what we do if we run out of word. For the endstate, we're going to pass it a function that says what to do when we run out of word:

<code>    fn search(&self, word: &mut Iterator<Item = char>, endstate: &Fn(&Node) -> bool) -> bool {
        let c = match word.next() {
            None => return endstate(self),
            Some(c) => c,

Note that it's not pub! This is the search function, but it's going to be invoked by the functions that make the distinction between finding a word and affirming a prefix. Flags are a code smell, and to the extent that you use them, they should never be accessible to client code.  Search, by the way, is completely immutable with respect to the structure being searched; only the word is mutating as we iterate through it, and the per-search recursion is wholly stacked based.  Once built, the trie could be safely used by multiple threads without the need for locking mechanisms.

Finally, if the trie is not exhausted, we try to get the child node, passing the endstate handler forward:

<code>        match self.0.get(&c) {
            None => false,
            Some(n) => n.borrow().search(word, endstate),

And the two functions, find and prefix:

<code>    pub fn find(&self, word: &mut Iterator<Item = char>) -> bool {
        self.search(word, &|s| s.1)

    pub fn is_prefix(&self, word: &mut Iterator<Item = char>) -> bool {
        self.search(word, &|_s| true)

And finally, the constructor. We create a new, empty node, and then we insert words into the trie, using that node as the root of the dictionary. The root node is never a terminator.

<code>    pub fn new() -> Node {
        Node(HashMap::new(), false)

And that's it. That's the whole of a trie, just two functions: a builder and a searcher. I had one special need (whole word vs prefix), but that was handled by using a distinguishing function as a flag on the semantics of the search.

Tries are a fun data structure. This version is pretty memory-heavy; it might be possible, for those cases where a word prefix has only one child, to pack them into a denser structure. The cost of running the discriminator versus a win on space density might even be worth it.

But if you have a finite dictionary, even one as big as the Scrabble™ legal words list (178,960 words as of 2012), a trie is the fastest data structure for determining if a string of letters is a real word, or possibly the prefix of a real word.

All of this code is available on my Github, and licensed under the Mozilla Public License v. 2.0.  The code on Github is a little more generic, and will work with both char and byte strings.