Programming Praxis – Squaring The Bishop

In today’s Programming Praxis exercise, our goal is to write a program that can create word squares. Let’s get started, shall we?

Some imports:

import qualified Data.ByteString.Char8 as B
import qualified Data.List.Key as K
import qualified Data.Map as M
import qualified Data.Trie as T

First we need to load the words into a practical data structure. The obvious one here is a trie. Rather than one big trie for the whole dictionary, we make group the words by length, making a trie for each different length.

loadWords :: IO (M.Map Int (T.Trie Int))
loadWords = fmap (M.fromList . map (\(w:ws) -> (snd w, T.fromList (w:ws))) .
         snd . K.sort snd . map (\w -> (w, B.length w)) .
                  B.words) $ B.readFile "words.txt"

Next, we need a function to find all the possible words of the correct length given a prefix.

findWords :: Int -> String -> M.Map Int (T.Trie a) -> [B.ByteString]
findWords l prefix = T.keys . T.submap (B.pack prefix) . (M.! l)

Finally, constructing the square is a matter recursively finding all the possible next words and keeping only the combinations that result in a full square.

square :: String -> M.Map Int (T.Trie a) -> [[B.ByteString]]
square word ds = f 1 [B.pack word] where
    f n ws = if n == length word then [ws] else 
             (\w -> f (n + 1) (ws ++ [w])) =<<
             findWords (length word) (map (`B.index` n) ws) ds

Some tests to see if everything is working properly:

main :: IO ()
main = do print . square "bonsai" =<< loadWords
          print . (== 122) . length . square "bishop" =<< loadWords

Looks like it. Interestingly, the word bonsai only has a single word square:

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2 Responses to “Programming Praxis – Squaring The Bishop”

  1. Tom R Says:

    Out of interest, why the use of ByteString.Char8 instead of Char?

  2. Remco Niemeijer Says:

    If you mean why I used ByteStrings instead of plain Strings, it’s because the Data.Trie package works with ByteStrings and not with Strings. If you mean why I used Data.ByteString.Char8 instead of Data.ByteString, it’s because the latter has no words function.

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