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README.md

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 # Wordle Solver
 
-Cleanroom implementation of a solver for Wordle
+This is a toy project to implement a [Wordle](https://www.powerlanguage.co.uk/wordle/) solver. This project provides no packaging but has no dependencies on libraries that aren't builtin. The main function selects a random word, then simulates the game with the solver and prints the output.
+
+I didn't look at any other solutions before writing this, since that would ruin the fun.
+
+## Solver
+
+The solver is mostly brute-force. It takes every possible "guess" word from the dictionary (assumed to be in `/etc/share/dict/words`), and plays it to every possible "answer" in the dictionary, and determines the top moves. The ranking is based on a heuristic using the average number of remaining playable words, where fewer is better.
+
+Performance is reasonably optimized but the runtime of the overall algorithm is `O(n)^3`, where `n` is the size of the dictionary. Using 16 threads I was able to compute the first guess (the slowest) in about 6 hours using a dictionary of 3,000 5-letter words. The subsequent guesses are typically subsecond, and the first guess could be  trivially cached since the initial board state is always the same.
+
+## Performance Notes
+
+I learned from this project that `deepcopy.copy` is outrageously slow, even for simple data classes. Initially, it dominated runtime when it was used to copy the board state for each evaluation. I replaced it with a hand-written copy, assigning immutable values and shallow copying dictionaries, and this allowed meeting sufficient performance for a toy.