1 files changed, 73 insertions, 0 deletions
diff --git a/day07/day07.hs b/day07/day07.hs
new file mode 100644
index 0000000..1f3cce1
--- /dev/null
+++ b/day07/day07.hs
@@ -0,0 +1,73 @@
+import Data.Bits
+import Data.List
+import Data.List.Split
+
+type Eqn = (Int, [Int])
+data Op = Add | Mul
+data Op2 = Add2 | Mul2 | Cat2 deriving Show
+
+atoi :: String -> Int
+atoi = read
+
+process :: String -> Eqn
+process line = 
+  let [targs, inss] = splitOn ": " line
+      targ = atoi targs
+      ins = map atoi $ words inss in
+        (targ, ins)
+
+opList :: [Op] -> Int -> Int -> [Op]
+opList acc 0 _ = acc
+opList acc w n =
+  opList ((case bit of {0 -> Add; otherwise -> Mul}):acc) dw n
+  where dw = w - 1
+        bit = n .&. (1 `shift` dw)
+
+reifies :: Eqn -> [Op] -> Bool
+reifies (targ, (h:t)) ops =
+  foldl operate h tasks == targ
+  where tasks = zip ops t
+        operate = \acc (op, arg) -> case op of {Add -> acc + arg; Mul -> acc * arg}
+
+reifiable :: Eqn -> Bool
+reifiable eqn@(_, args) =
+  any (reifies eqn) $ map (opList [] l) [0..(2^l - 1)]
+  where l = (length args) - 1
+
+op2List :: [Op2] -> Int -> Int -> [Op2]
+op2List acc 0 _ = acc
+op2List acc w n =
+  op2List ((case trit of {0 -> Add2; 1 -> Mul2; 2 -> Cat2}):acc) dw n
+  where dw = w - 1
+        trit = (n `div` (3 ^ dw)) `mod` 3
+
+reifies2 :: Eqn -> [Op2] -> Bool
+reifies2 (targ, (h:t)) ops =
+  foldl operate h tasks == targ
+  where tasks = zip ops t
+        operate = \acc (op, arg) -> case op of
+                                      Add2 -> acc + arg
+                                      Mul2 -> acc * arg
+                                      Cat2 -> atoi (show acc ++ show arg)
+
+reifiable2 :: Eqn -> Bool
+reifiable2 eqn@(_, args) =
+  any (reifies2 eqn) $ map (op2List [] l) [0..(3^l - 1)]
+  where l = (length args) - 1
+
+part1 :: [Eqn] -> Int
+part1 eqns =
+  foldl sumCar 0 $ filter reifiable eqns
+  where sumCar = \acc (i, _) -> acc + i
+
+part2 :: [Eqn] -> Int
+part2 eqns =
+  foldl sumCar 0 $ filter reifiable2 eqns
+  where sumCar = \acc (i, _) -> acc + i
+
+main = do
+  file <- readFile "day07.input"
+  let lns = lines file
+      eqns = map process lns
+  putStrLn ("Part 1: " ++ (show $ part1 eqns))
+  putStrLn ("Part 2: " ++ (show $ part2 eqns))