wrote README

README.md

@@ -19,3 +19,15 @@ configs:
   - split: train
     path: data/train-*
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+This is a collection of ~1.5M chess puzzles from the [Lichess database](https://database.lichess.org/#puzzles) of ~3.9M puzzles (as of 2024-05-09).
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+The set of puzzles from ["Can You Learn an Algorithm? Generalizing from Easy to Hard Problems with Recurrent Networks"](https://github.com/aks2203/easy-to-hard-data/tree/main) is included, with the exception of 26,079 puzzles that are no longer in the Lichess database (on the assumption that they might have been removed for a good reason).
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+For each puzzle, `ctx` is a SAN transcript (with every half-move numbered) of an actual Lichess game, up to the puzzle position. Note that this includes the first move of the `Moves` column in the Lichess and Easy-to-Hard datasets.
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+`target` is the **best** next move, in SAN, with a leading space. This move (second move in `Moves` column) generally differs from the actual Lichess game, which may contain blunders.
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+Additional moves of the puzzle solution are not included.
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+This format matches that used in ["Weak-to-strong generalization"](https://openai.com/index/weak-to-strong-generalization/) and the set of puzzles is also intended to be as similar as possible (except for the 26k that Lichess removed).