Skip to content

Instantly share code, notes, and snippets.

@gigamonkey gigamonkey/easy.txt
Last active Sep 23, 2015

Embed
What would you like to do?
My first Haskell program -- it didn't work the first time it type checked but I got it there. Simple Sudoku solver based on the search part of Peter Norvig's essay about solving every Sudoku.
. 5 . | . . 1 | 4 7 9
. . 2 | 7 . . | . . 8
. . . | . 4 6 | 2 . .
------+-------+------
. 4 6 | . . 9 | 5 3 7
. . . | . 6 . | . . .
8 9 3 | 5 . . | 6 4 .
------+-------+------
. . 9 | 6 1 . | . . .
1 . . | . . 2 | 3 . .
3 2 7 | 4 . . | . 1 .
import Control.Applicative
import Control.Monad
import qualified Data.Vector as V
import Data.Vector (Vector, indexed, fromList, toList, (//), (!?), (!))
import Data.List
import Data.Maybe
import System.Environment
import System.IO
type Square = Int
type Digit = Char
type Board = Vector (Maybe Char)
-- Some useful sets of indices ---------------------------------------
digits = "123456789"
squares = [0..80]
rows = [ [ r*9 + c | c <- [0..8] ] | r <- [0..8] ]
cols = [ [ r*9 + c | r <- [0..8] ] | c <- [0..8] ]
boxes = [ [ r*9 + c | r <- [rs..rs+2], c <- [cs..cs+2] ] | rs <- [0,3,6], cs <- [0,3,6] ]
all_units = rows ++ cols ++ boxes
units = [ [ u | u <- all_units, elem s u ] | s <- squares ]
peers = [ delete s (foldl union [] (units !! s)) | s <- squares ]
-- To and from textual representation --------------------------------
givens :: [Char] -> Board
givens = fromList . fromText
board :: [Char] -> Maybe Board
board text = Just (givens text)
oneline :: Board -> [Char]
oneline = map unsquare . toList
square c = if c == '.' then Nothing else Just c
unsquare = fromMaybe '.'
fromText = map square . filter (`elem` ('.':digits))
-- Solving code ------------------------------------------------------
solve :: Maybe Board -> Maybe Board
solve board = do
b <- board
case emptySquare b of
Nothing -> board
Just s -> search b digits s
search :: Board -> [Digit] -> Square -> Maybe Board
search b digits s =
case digits of
d:ds ->
case assign b s d of
Nothing -> tryNextDigit
next -> solve next <|> tryNextDigit
where tryNextDigit = search b ds s
[] -> Nothing
emptySquare b = (V.map fst $ V.filter (isNothing . snd) $ indexed b) !? 0
assign b s d = if inPeers b s d then Nothing else Just (b // [(s, Just d)])
inPeers b s d = any inPeer (peers !! s)
where inPeer p = maybe False (d ==) (b ! p)
-- Main --------------------------------------------------------------
main = do
args <- getArgs
forM_ args $ \a -> do
puzzle <- readFile a
putStrLn $ oneline (givens puzzle)
putStrLn $ case solve (board puzzle) of
Nothing -> "No solution."
Just b -> oneline b
@EmilStenstrom

This comment has been minimized.

Copy link

EmilStenstrom commented Jul 10, 2014

Here's the essay for everyone curious: http://norvig.com/sudoku.html

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
You can’t perform that action at this time.