From ddb57cb49a60b6173712341940195e0275ef1c9d Mon Sep 17 00:00:00 2001
From: Tom Smeding <tom.smeding@gmail.com>
Date: Wed, 8 Aug 2018 22:58:13 +0200
Subject: Haskell solver that uses rules

---
 hsolve/FSu.hs | 152 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 152 insertions(+)
 create mode 100644 hsolve/FSu.hs

(limited to 'hsolve/FSu.hs')

diff --git a/hsolve/FSu.hs b/hsolve/FSu.hs
new file mode 100644
index 0000000..1bd250a
--- /dev/null
+++ b/hsolve/FSu.hs
@@ -0,0 +1,152 @@
+{-# LANGUAGE BangPatterns, ScopedTypeVariables, RankNTypes #-}
+module FSu(solve) where
+
+import Control.Monad
+import Control.Monad.ST
+import Data.Array.ST
+import Data.STRef
+
+
+type Value = Int  -- Sudoku value
+type Index = Int  -- Sudoku index
+
+type Arr s a = STArray s Int a
+
+data State s = State { stateMark :: Arr s Bool, stateResults :: STRef s [[Maybe Value]] }
+
+solve :: [Maybe Value] -> [[Maybe Value]]
+solve input = runST $ do
+    arr <- newListArray (0,80) $ map (maybe (-1) id) input :: ST s (Arr s Value)
+    mark <- newArray (0,8) False :: ST s (Arr s Bool)
+    results <- newSTRef [] :: ST s (STRef s [[Maybe Value]])
+    solveAt arr 0 (State mark results)
+    readSTRef results
+
+obtainResult :: Arr s Value -> ST s [Maybe Value]
+obtainResult arr = do
+    elems <- getElems arr
+    return $ [if v == (-1) then Nothing else Just v | v <- elems]
+
+solveAt :: Arr s Value -> Index -> State s -> ST s ()
+solveAt arr !i st = do
+    valid <- isValid arr st
+    when valid $ do
+        if i == 81
+        then do
+            res <- obtainResult arr
+            modifySTRef' (stateResults st) (res:)
+        else do
+            v <- readArray arr i
+            if v /= (-1)
+            then solveAt arr (i+1) st
+            else do
+                poss <- getPoss arr i st
+                tryAll arr i poss st
+
+tryAll :: Arr s Value -> Index -> [Value] -> State s -> ST s ()
+tryAll _ _ [] _ = return ()
+tryAll arr !i (v:vs) st = do
+    writeArray arr i v
+    solveAt arr (i+1) st
+    writeArray arr i (-1)
+    tryAll arr i vs st
+
+-- assumes the considered position is empty
+getPoss :: forall s. Arr s Value -> Index -> State s -> ST s [Value]
+getPoss arr i st = do
+    fillArray mark 0 8 True
+    goRow (rowOf i) 0
+    goCol (colOf i) 0
+    goBlock (blockOrigin (blockOf i)) 0
+    bs <- liftM (zip [0..8]) (getElems mark)
+    return $ map fst $ filter snd bs
+  where
+    mark = stateMark st
+
+    goRow :: Int -> Int -> ST s ()
+    goRow _ 9 = return ()
+    goRow r j = readArray arr (9 * r + j) >>= \v ->
+        when (v /= (-1)) (writeArray mark v False) >> goRow r (j+1)
+
+    goCol :: Int -> Int -> ST s ()
+    goCol _ 9 = return ()
+    goCol c j = readArray arr (9 * j + c) >>= \v ->
+        when (v /= (-1)) (writeArray mark v False) >> goCol c (j+1)
+
+    goBlock :: Int -> Int -> ST s ()
+    goBlock _ 9 = return ()
+    goBlock b j = readArray arr (b + 9 * (j `quot` 3) + j `rem` 3) >>= \v ->
+        when (v /= (-1)) (writeArray mark v False) >> goBlock b (j+1)
+
+isValid :: forall s. Arr s Value -> State s -> ST s Bool
+isValid arr st = do
+    goRows 0 >>= \r1 -> if r1
+        then goCols 0 >>= \r2 -> if r2
+            then goBlocks 0
+            else return False
+        else return False
+  where
+    goRows, goCols, goBlocks :: Int -> ST s Bool
+    goRows 9 = return True
+    goRows i = isValidRow arr i st >>= \r -> if r then goRows (i+1) else return False
+
+    goCols 9 = return True
+    goCols i = isValidCol arr i st >>= \r -> if r then goCols (i+1) else return False
+
+    goBlocks 9 = return True
+    goBlocks i = isValidBlock arr i st >>= \r -> if r then goBlocks (i+1) else return False
+
+isValidRow :: Arr s Value -> Index -> State s -> ST s Bool
+isValidRow arr r st = indexSetNoDups arr [9 * r + i | i <- [0..8]] st
+
+isValidCol :: Arr s Value -> Index -> State s -> ST s Bool
+isValidCol arr c st = indexSetNoDups arr [9 * i + c | i <- [0..8]] st
+
+isValidBlock :: Arr s Value -> Index -> State s -> ST s Bool
+isValidBlock arr b st = indexSetNoDups arr [blockOrigin b + 9 * y + x| y <- [0..2], x <- [0..2]] st
+
+indexSetNoDups :: forall s. Arr s Value -> [Index] -> State s -> ST s Bool
+indexSetNoDups arr set st = do
+    fillArray mark 0 8 False
+    applyInMark set
+  where
+    mark = stateMark st
+
+    applyInMark :: [Int] -> ST s Bool
+    applyInMark [] = return True
+    applyInMark (i:is) =
+        readArray arr i >>= \v ->
+            if v == (-1)
+            then applyInMark is
+            else do
+                b <- readArray mark v
+                if b
+                then return False
+                else writeArray mark v True >> applyInMark is
+
+fillArray :: Arr s a -> Int -> Int -> a -> ST s ()
+fillArray arr !i1 !i2 v
+    | i1 <= i2 = do
+        writeArray arr i1 v
+        fillArray arr (i1 + 1) i2 v
+    | otherwise = return ()
+
+rowOf :: Index -> Index
+rowOf i = i `quot` 9
+
+colOf :: Index -> Index
+colOf i = i `rem` 9
+
+blockOf :: Index -> Index
+blockOf i = 3 * (i `quot` 27) + (i `rem` 9) `quot` 3
+
+blockOrigin :: Index -> Index
+blockOrigin 0 = 0
+blockOrigin 1 = 3
+blockOrigin 2 = 6
+blockOrigin 3 = 27
+blockOrigin 4 = 30
+blockOrigin 5 = 33
+blockOrigin 6 = 54
+blockOrigin 7 = 57
+blockOrigin 8 = 60
-- 
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