day 3

day3/first.hs

@@ -0,0 +1,28 @@
+import Data.List
+
+binaryToInt :: String -> Integer
+binaryToInt bits = aux 0 0 $ reverse bits
+  where aux expt acc []         = acc
+        aux expt acc ('1':bits) = aux (expt + 1) (acc + 2^expt) bits
+        aux expt acc ('0':bits)    = aux (expt + 1) acc bits
+
+mostCommon :: Eq a => a -> a -> [a] -> a
+mostCommon x y elems
+  | xs <= ys = y
+  | ys < xs = x
+  where xs = lenMap x
+        ys = lenMap y
+        lenMap a = length $ filter id $ map (== a) elems
+
+leastCommon x y elems
+  | mostCommon x y elems == x = y
+  | mostCommon x y elems == y = x
+
+gamma :: [String] -> Integer
+gamma = binaryToInt . map (mostCommon '1' '0') . transpose
+
+epsilon :: [String] -> Integer
+epsilon = binaryToInt . map (leastCommon '1' '0') . transpose
+
+main = interact $ show . solution . words
+  where solution bitstrings = gamma bitstrings * epsilon bitstrings

day3/second.hs

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+import Data.List
+
+binaryToInt :: String -> Integer
+binaryToInt bits = aux 0 0 $ reverse bits
+  where aux expt acc []         = acc
+        aux expt acc ('1':bits) = aux (expt + 1) (acc + 2^expt) bits
+        aux expt acc ('0':bits)    = aux (expt + 1) acc bits
+
+mostCommon :: Eq a => a -> a -> [a] -> a
+mostCommon x y elems
+  | xs < ys = y
+  | ys <= xs = x
+  where xs = lenMap x
+        ys = lenMap y
+        lenMap a = length $ filter id $ map (== a) elems
+
+leastCommon x y elems
+  | mostCommon x y elems == x = y
+  | mostCommon x y elems == y = x
+
+filterNums :: Eq a => ([a] -> a) -> [[a]] -> [a]
+filterNums test ns = aux 0 ns
+  where aux _ [final] = final
+        aux pos ns =
+          aux (pos + 1) $ filter ((==) needle . head . drop pos) ns
+          where needle = test $ map head $ map (drop pos) ns
+
+oxygen :: [String] -> Integer
+oxygen = binaryToInt . filterNums (mostCommon '1' '0')
+
+co2 :: [String] -> Integer
+co2 = binaryToInt . filterNums (leastCommon '1' '0')
+
+main = interact $ show . solution . words
+  where solution bitstrings = oxygen bitstrings * co2 bitstrings