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# Quartiles Solver: Part I

15 Aug, 2024

## Quartiles Solver: Part I

In this three-part series and the associated project, we are going to spoil enhance the fun of Apple News+ Quartiles by developing an efficient solver for it. We will use the Rust programming language and the Ratatui terminal user interface (TUI) library. Along the way, we’ll learn a bit about Rust, game theory, combinatorics, benchmarking, and TUIs. To experiment with the finished app, you’ll need to pull the project locally. You can follow the detailed instructions in the README.md to build, use, test, benchmark, or install the project on Windows, macOS, or Linux.

In this first post, we’ll introduce Quartiles, analyze why it’s a good candidate for automated solution, and finally build the dictionary that the solver will leverage in the second post.

## Quartiles: A fully reconstructible perfect information game

Broadly speaking, Quartiles comprises three elements:

1. A game board constructed by chopping up 5 long English words into smaller word fragments, each comprising 2 to 4 letters, such that there are no duplicate word fragments. The word fragments obtained from the 5 words are all scrambled together and then arranged to form a 4×5 game board of 20 cells. For example, the word algorithms might be carved up into al, gor, it, and hms.
2. A solution tracker that enumerates valid English words discovered by the player. Each word uses 1 to 4 cells from the game board. Words using 4 cells are called quartiles. Cells may be reused, so multiple words may contain the same word fragment, but complete words may not be reused. The game board is constructed such that only 5 quartiles are present.
3. A dictionary of English words that encompasses a suitably large extract of the English lexicon. The game itself furnishes the game board and updates the solution tracker, but the player brings the dictionary, typically in the form of previously learned knowledge of the English language.

The game is won when the solution tracker contains 5 quartiles. Because of how the game board was constructed, these are guaranteed to be the 5 original quartiles; there will never be "extra" quartiles. To make the game competitive among friends, words are scored based on the number of word fragments that they contain, with quartiles earning the most points. The highest possible score arises from finding not only the 5 quartiles, but from discovering every word accepted by Apple’s official Quartiles dictionary (which does not appear to be available to the public). But we will not concern ourselves here with scoring mechanics, only with fully populating the solution tracker with respect to some dictionary.

So why build a solver for the relatively new Quartiles rather than the more famous and better established Wordle? Well, that’s easy — it’s because building an effective solver for Quartiles is possible whereas building one for Wordle is not!

## Perfect vs imperfect information

In order to ensure that we can build an effective solver, i.e., one that is guaranteed to arrive at a correct solution by executing some algorithm, the target game must provide perfect information to its players. A player has perfect information about a game if and only if:

1. The complete state of the game is visible whenever the player is permitted to make a move. A move mutates the state of the game in some way, so a sequence of moves causes the game to evolve through its state space.
2. Making a move does not entail any chance events. By definition, a chance event has an unknown outcome, even if the player knows the instantaneous probabilities of all possible outcomes.

Taken together, we arrive at a natural conclusion: a player cannot have perfect information if there exists any secret information, whether such information is kept secret by the game itself, by probability, or by another player. If any information is secret to a player, then that player has only imperfect information. If any information would be secret to a solver, then, in the general case, we cannot necessarily build an effective solver. Virtually all playing card games — poker, bridge, rummy, blackjack, spades, go fish, … — give players only imperfect information, whereas famous strategy games like chess and go give players perfect information.

But let’s talk about word games. We’ll take Wordle for a simple example. The whole point of the game is to unmask a secret 5-letter word in 6 or fewer moves, where each move is the proposal of a complete 5-letter English word. It therefore directly follows that the player only has imperfect information — knowledge of their own guesses and feedback from each guess. Analysis of the English language, especially letter frequencies and permissible juxtapositions, permits codification of an optimal strategy, and it so happens that this strategy can always solve the game in 5 moves on easy and 6 moves on hard. But of course, the correct answer has to be spoon-fed into the algorithm, or the correct feedback must be provided after each guess; without these concessions, there is no way to uncover the secret, because there is no way to ascertain whether an algorithm is moving toward the answer or away from it.

By contrast, Quartiles is more like Boggle. Assuming that the player is suitably versed in the English lexicon, both games are perfect information games. The player can see the whole board, the player knows which words have been played and accepted, the player knows (by construction) whether a word is a quartile, and consequently the player knows when all quartiles have been found. The player theoretically "knows" when every word has been found, because they can theoretically make an exhaustive search of the board, comparing each allowed combination of word fragments against their dictionary. (Besides, the online Quartiles is nice: it tells you when no words remain undiscovered.)

ℹ️ Fun facts

Quartiles is not only a perfect information game, but also a fully reconstructible game. This means that its complete instantaneous state is equivalent to its observable state, and may be correctly ascertained simply from examining its visible components — the game board, solution tracker, and dictionary. Most importantly for play, the legal next moves are fully determined by the observable game state.

Quartiles is also memoryless, meaning that the player’s moves need not be recorded in historical order to ensure adherence to the rules of play. In fact, the k moves played so far are mutually commutative, which is why play history not need be maintained. The solution tracker represents a set (rather than a sequence) of moves. Most importantly for play, the legal next moves are fully determinable without appeal to the play history.

Interestingly, chess and go are neither fully reconstructible nor memoryless. Examination of an arbitrary chess board does not reveal castling rights, whether en passant capture is possible, or when the 50 move rule or threefold repetition rule would be violated. Examination of an arbitrary go board does not reveal which moves would violate the rule of ko. So both games require maintaining a play history, i.e., a score sheet, to determine the legality of upcoming moves. As recording this (visible) score sheet is mandated by official rules, at least in formal settings, both games become fully reconstructible perfect information games, though neither are memoryless.

## Tractability

We have established that it is theoretically possible to build an effective solver for Quartiles, but before committing to code anything we should at least have a rough sense of whether execution of our algorithm will be tractable. In other words, will the program arrive at a solution in a reasonable amount of time?

Reasonableness is relative to the problem description, of course. If an algorithm takes a week to decide whether dog is correctly spelled, then it is intractable (for basically any problem); but if an algorithm takes (only) a week to complete a brute force attack against SHA-3, then someone would be absolutely delighted (and most other someones very, very upset).

😅 Don’t do the impossible

Before coming to Xebia, employers requested at least twice that I perform a task which, practically speaking, would entail winning a Millennium Prize first. Both times it would have been the prize for P versus NP, so at least there’s that. Moral of the story: always do a sanity check that your problem is both decidable and tractable before commencing work on the problem!

For a good rule of thumb about tractability of a mechanical problem like this, just ask yourself: is a human expected to do it well? Quartiles is a game for humans, so presumably humans ought to perform well at playing it. While our general intelligence, abstract reason, and world lore are still untouchable by machine learning algorithms and large language models for many critical tasks, our vertical scalability leaves much to be desired; most algorithms beat us handily at exploring combinatorial spaces quickly. But we tend to be very good at word games, so we expect mechanical solvers will be too.

But why take my word for it? Let’s take a short detour into Mathlandia.

### Combinatorics

Determination of tractability is essentially a counting problem, so we turn to combinatorics to establish the scale of our problem. Given that the player of Quartiles may construct a word from 1, 2, 3, or 4 tiles, we need to compute the numbers of permutations involving 1, 2, 3, and 4 tiles, then sum those counts together to establish the maximum extent of the search space. I say "maximum" because we can very effectively prune the search space to reduce the scale of the problem, which I will demonstrate below.

Going back to basics, the formula for k-permutations of n is:

Where n is the total number of elements in the set (of tiles in Quartiles) and k is the number of those elements to arrange into a sequence. n is 20, the number of tiles on the game board. We restrict k to [1,4] and calculate the sum of the four pertinent applications. The individual equations give us:

And drumroll:

There are "just" 123520 ways to arrange up to 4 tiles from our game board of 20. We wouldn’t want to search this space manually, but computers are fast, making this number small and our problem highly tractable — just as we expected.

## Dictionary representation

As I hinted above, we can further reduce this complexity. The key insight is this: most permutations of tiles will not produce an English word. To decide whether we’ve found a valid English word, we need to consult an English dictionary. But if we have to consult the dictionary for 123520 possible permutations, we haven’t saved any effort. We want to look at fewer permutations.

For that, we use a new key insight: most permutations of tiles will not produce even the beginning of an English word. This sounds more promising.

Let’s chop permutation up into 4 fragments, thus: pe, rmut, at, ion. If we know that rmut doesn’t begin any English word — and we do know this — then there’s no sense in consulting the dictionary about rmutpeation, rmutatpeion, rmutationpe, and so forth. Assuming that these are but 4 of 20 tiles of a Quartiles board, we can completely eliminate P(20,3) = 6840 permutations from the search space whenever the first tile isn’t a viable English prefix. We don’t have to consult the dictionary about them. Heck, we don’t even have to visit them during the search. If only there were some data structure to support this kind of search…

### Prefix trees

Prefix trees to the rescue! A prefix tree (or trie) is a search tree that distributes its keys over the entire data structure. A node’s address in the tree represents its key; specifically, the path followed from the root to the node encodes the key. Not every node need represent a valid key, so a prefix tree distinguishes its valid keys by attaching a payload to the terminal node of the key; for our purposes, a simple flag that says "I’m a key!" is good enough.

Representing an English dictionary as a prefix tree is straightforward:

1. Each node comprises only the marker flag mentioned above. Let’s say it’s a boolean called endsWord, meaning that the node serves as the terminus for a word in the dictionary. endsWord is true if and only if the path traversed to reach the node spells an English word contained within the dictionary.
2. Each edge is annotated with a Roman letter.
3. Each node has up to 26 out-edges, one for each possible Roman letter.
4. The root node is a sentinel — the empty "word" comprising no letters.

Here’s a simple example that illustrates the 6-word dictionary , where the leftmost node is the root node:

The label on each node is the value of endsWord, which we write briefly as either true or false. The left-to-right orientation of the diagram, chosen to mirror the left-to-right text direction of English, nicely visualizes how English words are encoded in the edges of the prefix tree.

Using the prefix tree representation, there are two ways to conclude that some word isn’t present in this dictionary:

1. Left-to-right traversal of the word’s constituent letters completes but arrives at a node whose endsWord marker is false. For example, this case eliminates wh.
2. Left-to-right traversal of the word’s constituent letters must be abandoned because of a missing edge. For example, this case eliminates mook (which is valid North American slang, but not present in the tiny example dictionary).

The second case is much more interesting than the first, because it provides the basis for eliminating fruitless prefixes. Given that the node corresponding to moo does not have an out-edge on j, we conclude that no words known to this dictionary begin with mooj. Now we have a nice way to prune the search space, statistically ensuring that we won’t need to exhaustively check all 123520 possible candidates.

## Praxis

Enough theory! Let’s write some Rust! Remember to check out the completed project if you want to follow along. The project source also includes the Rustdoc that I elide, for brevity, from the code excerpts below.

### Project setup

Some of the project components should be reusable, like the English dictionary and even the Quartiles solver, so we want to separate these out from the text-based user interface that we will write in part three of this blog series.

We want to end up with two crates, one binary crate and one library crate, each with the name quartiles-solver. So let’s set up our project thus:

quartiles-solver/
src/
lib.rs
main.rs
Cargo.toml

We don’t want to write our own prefix tree implementation, because someone has already written a nice one called pfx. To avoid embedding the English dictionary into our program or rebuilding the prefix tree from scratch whenever the application starts, we decide that our prefix trees should support bincode serialization via Serde. We also want to make execution traceable via logging, so we choose log as our veneer and env_logger as our specific provider. We use this Cargo.toml to get started:

[package]
name = "quartiles-solver"
version = "0.1.0"
edition = "2021"
authors = ["Todd L Smith <todd.smith@xebia.com>"]

[dependencies]
bincode = "1.3"
env_logger = "0.11"
log = "0.4"
pfx = { version = "0.4", features = ["serde"] }
serde = { version = "1.0", features = ["derive"] }

We’ll expand upon this incrementally, but this is a good beginning. Note that we will commit our Cargo.lock because one of our crates is executable.

### Implementing the dictionary

Let’s drop a new Rust file into the project: src/dictionary.rs. As you might expect, we’ll implement the dictionary herein.

For our dictionary, we employ the newtype pattern from functional programming. We wrap pfx::PrefixTreeSet and derive some relevant traits for it, including serde::Deserialize and serde::Serialize.

#[derive(Clone, Debug, Default, Eq, PartialEq, Serialize, Deserialize)]
#[must_use]
pub struct Dictionary(PrefixTreeSet<String>);

We expect instances of Dictionary to be expensive, as each potentially contains an entire English dictionary (of ≤70,000 words, if using the word list included in the project). To protect against accidental discard of a Dictionary at a function call site, we apply the must_use attribute. Now the compiler will object whenever a Dictionary returned by a function is unused.

Now we make a nice big impl Dictionary block to put our logic inside. We lead with a simple constructor that behaves identically to Default::default but which can be inlined.

#[inline]
pub fn new() -> Self
{
Self(Default::default())
}

Now we add some simple but important delegation methods for interrogating the content of a Dictionary:

#[inline]
#[must_use]
pub fn is_empty(&self) -> bool
{
self.0.is_empty()
}

#[inline]
#[must_use]
pub fn contains(&self, word: &str) -> bool
{
self.0.contains(word)
}

#[inline]
#[must_use]
pub fn contains_prefix(&self, prefix: &str) -> bool
{
self.0.contains_prefix(prefix)
}

The last one will be especially important when we write the solver, as this achieves the prefix-based pruning that I described above.

It might be good to have the ability to insert some words into a Dictionary,
so let’s start simple:

pub fn populate<T: AsRef<str>>(&mut self, words: &[T])
{
for word in words
{
self.0.insert(word.as_ref().to_string());
}
}

This is a blanket solution that can populate a Dictionary from any slice whose elements can be converted into &str, so we don’t have to know up front exactly what types those will be. populate is useful for testing, but it’s not necessarily convenient if we want to read a dictionary from a text file. Let’s assume the most basic possible representation of a text file containing an English word list: ASCII charset, one word per line. Here’s a simple implementation:

pub fn read_from_file<T: AsRef<Path>>(path: T) -> Result<Self, io::Error>
{
let file = File::open(path)?;
let mut dictionary = Self::new();
dictionary.populate(&words);
Ok(dictionary)
}

Basically, read_from_file slurps the whole text file into memory, splitting at resident line delimiters, and then uses populate to build the Dictionary. populate incrementally populates the underlying PrefixTreeSet, one word at a time, so it must traverse the tree repeatedly. Honestly, this is fast enough even for 70,000 words, but we can do better. Not the first time, no, but on subsequent reads in subsequent runs of the application. How? By serializing, and later deserializing, the PrefixTreeSet.

pub fn serialize_to_file<T: AsRef<Path>>(
&self,
path: T
) -> Result<(), io::Error>
{
let mut file = File::create(path)?;
let content = bincode::serialize(self)
.map_err(|_e| ErrorKind::InvalidData)?;
file.write_all(&content)?;
Ok(())
}

pub fn deserialize_from_file<T: AsRef<Path>>(
path: T
) -> Result<Self, io::Error>
{
let file = File::open(path)?;
let mut content = Vec::new();
let dictionary = bincode::deserialize(&content)
.map_err(|_e| ErrorKind::InvalidData)?;
Ok(dictionary)
}

We have the whole gamut of endpoints for constructing a Dictionary now. We can create an empty Dictionary, we can populate one from an English word list already in memory, we can populate one from a text file, and we can populate one from a compact, no-fluff binary file. But we’ve introduced some complexity by providing all these strategies. So we introduce one more method to tame this complexity and streamline our API:

pub fn open<T: AsRef<Path>>(dir: T, name: &str) -> Result<Self, io::Error>
{
let dict_path = dir.as_ref().join(format!("{}.dict", name));
if dict_path.exists()
{
let dictionary = Self::deserialize_from_file(&dict_path);
dictionary
}
else
{
let txt_path = dir.as_ref().join(format!("{}.txt", name));
match dictionary.serialize_to_file(&dict_path)
{
Ok(_) => trace!(
"Wrote binary dictionary: {}",
dict_path.display()
),
Err(e) => warn!(
"Failed to write binary dictionary: {}: {}",
dict_path.display(),
e
)
}
Ok(dictionary)
}
}

The parameters to open are:

• dir: The Path to the filesystem directory that contains the desired English dictionary, whether text, binary, or both.
• name: The name of the dictionary on disk, sans any file extension.

Here’s an illustration of the algorithm, simplified to disregard failure modes:

In words, look for a file named {dir}/{name}.dict, treating it as a serialized dictionary if it exists. If it does, great, use it. If it doesn’t, look for a file named {dir}/{name}.txt, treating it as a plaintext English word list. Build the dictionary from the text file, then write out the serialized form to {dir}/{name}.dict for some future pass through this algorithm. The happy paths all lead to a ready-to-go Dictionary.

To make sure that everything works, we add some basic unit tests, which you can see at the bottom of the full src/dictionary.rs.

## Benchmarking

If you wondered whether deserializing a real Dictionary was really faster than just rebuilding it from a text file, well, so did I! I decided to test it, empirically, by establishing some benchmarks.

While nightly Rust supports benchmarking directly through the bench attribute, it’s nice to use the stable channel wherever possible, as this gives the warm fuzzies of writing the safest possible Rust code. Fortunately, we can still benchmark our code on stable, but we’ll need to bring in a benchmarking crate, like Criterion, to close the feature gap. We’ll need to tweak our Cargo.toml, of course. We’ll add this at the bottom:

[dev-dependencies]
const_format = "0.2"
criterion = { version = "0.4", features = ["html_reports"] }

[[bench]]
name = "benchmarks"
harness = false

The dev-dependencies section just pulls in some crates that we want to use for our benchmarks.

The [[bench]] section is more important, as it tells cargo bench, the benchmark runner, where to find our benchmarks and how to run them. We use name to ensure that the runner looks for our benchmarks in benches/benchmarks.rs. We set harness to false to disable the libtest harness, which allows us to provide our own main function, thereby securing fine-grained control over how our benchmarks are organized, configured, and executed.

Everything is properly configured now, so let’s turn our attention to the benchmarks themselves.

### Text file benchmark

First, let’s create a benchmark for building a Dictionary from a text file. The test itself is quite small, but quite complex because of dependency injection:

fn bench_read_from_file<M: Measurement>(g: &mut BenchmarkGroup<M>)
{
});
}

criterion::measurement::Measurement abstracts over the conceivable metrics for benchmarking — wall time, memory utilization, energy consumption, etc. As of the time of writing, criterion::measurement::WallTime is the only supported benchmark, however; this is fine, because it’s what we want to measure. Related benchmarks are organized into instances of criterion::BenchmarkGroup, allowing configuration to be aggregated. bench_function accepts an identifier ("read_from_file") and an FnMut, injecting a criterion::Bencher that can run the actual benchmark repeatedly via iter. As expected, the actual function under test is Dictionary::read_from_file.

path_dict is just a trivial utility function that supplies the filesystem path to the English word list:

#[inline]
#[must_use]
const fn path_txt() -> &'static str
{
concatcp!(dir(), "/", name(), ".txt")
}

#[inline]
#[must_use]
const fn dir() -> &'static str
{
"dict"
}

/// The name of the dictionary file.
#[inline]
#[must_use]
const fn name() -> &'static str
{
"english"
}

const_format::concatcp performs compile-time string conversion and concatenation of primitive types. We leverage this to provide some abstraction without losing the benefit of hard coding string literals.

Note that bench_read_from_file does not run the benchmark. Rather, it defines it and installs it into the specified BenchmarkGroup. We have to tell the benchmark manager to run our groups, which we’ll do in main, below.

### Binary file benchmark

Having seen bench_read_from_file already, you can read the parallel benchmark like a champ:

fn bench_deserialize_from_file<M: Measurement>(g: &mut BenchmarkGroup<M>)
{
g.bench_function("deserialize_from_file", |b| {
b.iter(|| Dictionary::deserialize_from_file(path_dict()).unwrap());
});
}

#[inline]
#[must_use]
const fn path_dict() -> &'static str
{
concatcp!(dir(), "/", name(), ".dict")
}

The boilerplate is identical, but we’re testing Dictionary::deserialize_from_file instead.

### Wiring everything up

Because we asserted responsibility for main, defining the BenchmarkGroup is our responsibility. Our main function wires everything together and runs the benchmarks:

fn main()
{
// Ensure that both the text and binary files exist.
let _ = Dictionary::open(dir(), name()).unwrap();

// Run the benchmarks.
let mut criterion = Criterion::default().configure_from_args();
let mut group = criterion.benchmark_group("benchmarks");
group.measurement_time(Duration::from_secs(30));
bench_deserialize_from_file(&mut group);
bench_solver(&mut group);
group.finish();

// Generate the final summary.
criterion.final_summary();
}

criterion::Criterion is the benchmark manager. We create a basic one and use benchmark_group to create the BenchmarkGroup with which bench_read_from_file and bench_deserialize_from_file register themselves. We use measurement_time to run each test as many times as possible in 30 seconds. finish consumes the BenchmarkGroup, executes the constituent benchmarks, and attaches the summary reports to the benchmark manager that created it. Lastly, final_summary generates and emits the final summary.

### Verdict

Here’s the simplified output of cargo bench for my hardware:

     Running benches/benchmarks.rs
time:   [29.051 ms 29.092 ms 29.133 ms]

benchmarks/deserialize_from_file
time:   [24.011 ms 24.058 ms 24.105 ms]

Dictionary::deserialize_from_file is consistently about 5ms faster than Dictionary::read_from_file, which is significant in computer time. In wall time, it’s not much, but it’s definitely and reliably faster to deserialize a Dictionary than it is to construct one from a plaintext English word list. It’s a win, and I’ll take it.

### The English dictionary

One final note, about the English dictionary itself. If you’re interested in how I created and curated the English dictionary that I bundled with the project, check out dict/README.md`. Methodology, as well as relevant copyrights and attributions, are all contained therein.

That’s all for this installment. Next time we’ll build the solver itself. Stay tuned for the next blog post in this series!

Rust Solution Architect at Xebia Functional. Co-maintainer of the Avail programming language.
Questions?