Data commit

Task/Partition-an-integer-x-into-n-primes/Haskell/partition-an-integer-x-into-n-primes.hs

@@ -0,0 +1,45 @@
+import Data.List (delete, intercalate)
+import Data.Numbers.Primes (primes)
+import Data.Bool (bool)
+
+-------------------- PRIME PARTITIONS ---------------------
+partitions :: Int -> Int -> [Int]
+partitions x n
+  | n <= 1 =
+    [ x
+    | x == last ps ]
+  | otherwise = go ps x n
+  where
+    ps = takeWhile (<= x) primes
+    go ps_ x 1 =
+      [ x
+      | x `elem` ps_ ]
+    go ps_ x n = ((flip bool [] . head) <*> null) (ps_ >>= found)
+      where
+        found p =
+          ((flip bool [] . return . (p :)) <*> null)
+            ((go =<< delete p . flip takeWhile ps_ . (>=)) (x - p) (pred n))
+
+-------------------------- TEST ---------------------------
+main :: IO ()
+main =
+  mapM_ putStrLn $
+  (\(x, n) ->
+      intercalate
+        " -> "
+        [ justifyLeft 9 ' ' (show (x, n))
+        , let xs = partitions x n
+          in bool
+               (tail $ concatMap (('+' :) . show) xs)
+               "(no solution)"
+               (null xs)
+        ]) <$>
+  concat
+    [ [(99809, 1), (18, 2), (19, 3), (20, 4), (2017, 24)]
+    , (,) 22699 <$> [1 .. 4]
+    , [(40355, 3)]
+    ]
+
+------------------------- GENERIC -------------------------
+justifyLeft :: Int -> Char -> String -> String
+justifyLeft n c s = take n (s ++ replicate n c)