Wordle Solver in Scala

With the basic Scala implementation of Wordle out of the way in my last post I thought it might be a fun activity to create a Wordle solver/helper in a similar way. I like to avoid using such things on real world puzzles as they suck the fun out of them but writing them and testing them is fun in and of itself, so lets get started.

First, just like Wordle, I need a word list to use as a dictionary of possible guesses. If possible this dictionary should match the word list used by the Wordle implementation I am testing against (which it happens to do since I made the list).

Next I need a way of representing the solution space I am searching through. Thankfully, Wordle is a pretty simple solution space as all I need to keep track of are:

I represent this with a simple set of case classes:

sealed abstract class Validity
case object Valid extends Validity
case object Invalid extends Validity
case object WrongPosition extends Validity

case class LetterValidity(
    letters: Set[Char] = 'a'.to('z').toSet
) {
  def remove(c: Char): LetterValidity = {
    copy(letters = letters.filterNot(_ == c))
  }

  def removeAll(chars: Set[Char]): LetterValidity = {
    copy(letters = letters.removedAll(chars))
  }
}

object LetterValidity {
  def apply(c: Char): LetterValidity = LetterValidity(Set(c))
}

case class WordValidity(
    guesses: Seq[String] = Seq.empty,
    letters: Seq[LetterValidity] = Seq.fill(6)(LetterValidity())
)

The LetterValidity class is mostly a wrapper around a set of characters with some convenience methods. The WordValidity encompasses a list of all previous guesses and a sequence of the LetterValidity objects for each letter/column in the word. I’ve initialised these all to sensible starting values.

Now I need to decide how the user will interact with this program. I’ve settled on them first typing in their guess, then typing in some symbols to represent how valid it was according to Wordle:

Symbol Validity
x Invalid
+ Valid
~ Wrong Position

I parse the input from a user into the above types using a simple method:

def parseLetters(
    word: String,
    validityString: String
): Seq[(Char, Validity)] = {
  validityString.zip(word).map { case (c, letter) =>
    (
      letter,
      c match {
        case 'x' => Invalid
        case '+' => Valid
        case '~' => WrongPosition
      }
    )
  }
}

This returns a simple sequence of tuples containing the letter and how valid it is.

Now comes the tedious bit, solving the problem. I will paste the code and explain what it is doing after. I’ve again opted to use a tail recursive function since Scala excels at these and it makes programming easier:

def main(args: Array[String]): Unit = {
  if (args.length < 1) {
    println("arg 1: should be word list file")
    return
  }

  val words =
    mutable.ListBuffer.from(Random.shuffle(File(args(0)).lines.toSeq))
  println(s"Word list of ${words.size} loaded")
  val state = WordValidity()
  guess(words, state)
}

@tailrec
def guess(words: mutable.ListBuffer[String], state: WordValidity): Unit = {
  println("Enter guess:")
  val wordGuess = scala.io.StdIn.readLine()
  println(
    "Enter letter validity, x means not present, + means correct, ~ means present but wrong place:"
  )
  val letterValidity = parseLetters(wordGuess, scala.io.StdIn.readLine())

  val newGuesses: Seq[String] = state.guesses.appended(wordGuess)

  val initialUpdated =
    letterValidity.zip(state.letters).map { case ((c, validity), letter) =>
      validity match {
        case Valid         => LetterValidity(c)
        case Invalid       => letter.remove(c)
        case WrongPosition => letter.remove(c)
      }
    }

  val invalidChars = letterValidity
    .filter { case (_, validity) => validity == Invalid }
    .map(_._1)
    .toSet

  val wrongPositionChars = letterValidity
    .filter { case (_, validity) => validity == WrongPosition }
    .map(_._1)
    .toSet

  val updated = initialUpdated.map { validity =>
    validity.removeAll(invalidChars)
  }

  // remove the guess
  words -= wordGuess
  alignWords(words, updated, wrongPositionChars)

  println(s"${words.size} words remain")
  words.take(10).foreach(println)

  if (words.size == 1) {
    println(s"word is: ${words.head}")
  } else if (words.isEmpty) {
    println("No matching word")
  } else {
    guess(words, WordValidity(newGuesses, updated))
  }
}

def alignWords(
    words: mutable.ListBuffer[String],
    valids: Seq[WordleHelper.LetterValidity],
    wrongPositionChars: Set[Char]
): Unit = {

  words
    .filterInPlace { word =>
      word
        .zip(valids)
        .count { case (c, validity) =>
          validity.letters.contains(c)
        } == word.length
    }
    .filterInPlace { word =>
      wrongPositionChars.intersect(word.toSet).size == wrongPositionChars.size
    }
}

This is a long bit of code so let’s go through it from top to bottom:

In the main method I am reading the whole word list into memory and shuffling it about, initialising the state and calling the guess function. I put the words into memory since they will all be looked up and manipulated to some degree.

Within the guess method I read in both the guess and how valid it was. I add the guess to a list of previous guesses if there were any. Then I do an initial pass on the sequence of LetterValidity objects I have and adjust them in accordance to the user’s input. I then do 2 further passes on the user’s validity information and create 2 sets, one containing all the invalid characters and another containing all the characters in the wrong positions. I use this first set of invalid characters to further adjust the LetterValidity objects I will be using in the next iteration of this algorithm. Next I remove the guess from my word list and execute the alignWords function, providing it with the words list, the updated LetterValidity objects and the list of wrong positioned characters.

In the alignWords method I perform 2 filters in place on the list of words. First I remove all words that do not match the LetterValidity objects, and finally I remove all words that do not contain the wrongly positioned characters.

Returning to the guess method I output the amount of remaining words along with 10 of them to aid in guessing. If there is 1 word left then I exit since that must be the answer. If there’s no words left then either the user has put some bad input in or the word list I am using is incomplete. And finally if there are still many more guesses to be had I run the guess function again with some updated state.

Let’s see how this code fairs on Wordle!

My first guess will be the word “adapt”:

Wordle guess

The first 4 letters are invalid and the 5th is in the wrong place so I enter this into the solver:

Enter guess:
adapt
Enter letter validity, x means not present, + means correct, ~ means present but wrong place:
xxxx~
1129 words remain
newts
timer
north
turms
metol
twine
motts
broth
gytes
visto

I choose to use the word “timer” next and enter the results into my solver:

Wordle guess

Enter guess:
timer
Enter letter validity, x means not present, + means correct, ~ means present but wrong place:
~xx~x
148 words remain
newts
kythe
ctene
kents
suety
ketol
coste
bents
fytte
butle

Not bad, I try “suety”:

Wordle guess

Enter guess:
suety
Enter letter validity, x means not present, + means correct, ~ means present but wrong place:
+x~~x
5 words remain
stole
seton
stone
stove
stoke

Fantastic! I am down to 5 words left with 3 guesses to go. I try “stone”:

Wordle guess

Enter guess:
stone
Enter letter validity, x means not present, + means correct, ~ means present but wrong place:
+++x+
3 words remain
stole
stove
stoke

That left me with 3 words left and only 1 character to guess. I try “stove” and win with 1 guess left to spare!

Wordle guess

This is obviously not a foolproof solver since in the last stage I could have still failed since all 3 words differ but the solver was able to get me to them!

There’s room for further enhancement, especially around suggestions of what word to guess next. For example, if desired I could scan the remaining dictionary/word list and suggest the word likely to eliminate the most words. This would be the word with the most letters in common with all other words, and would help cut down on the amount of guesses needed.

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