2020 day2/haskell: add solution

2020/README.md

@@ -7,7 +7,7 @@ https://adventofcode.com/2020/
 |day| python | haskell | VHDL |
 |---|--------|---------|------|
 | 1 |  `**`  |  `**`   |      |
-| 2 |  `**`  |         | `**` |
+| 2 |  `**`  |  `**`   | `**` |
 | 3 |  `**`  |  `**`   |      |
 | 4 |  `**`  |  `**`   |      |
 | 5 |        |  `**`   |      |

2020/day2/day2.hs

@@ -0,0 +1,40 @@
+module Day2 where
+
+import AoC
+
+import Data.Char
+import Data.Maybe
+import Text.ParserCombinators.ReadP
+import Text.Read.Lex
+
+data Line = Line
+    { num1 :: Int
+    , num2 :: Int
+    , letter :: Char
+    , password :: String
+    }
+
+instance Show Line where
+    show (Line num1 num2 letter password) = show num1 ++ "-" ++ show num2 ++ " " ++ [letter] ++ ": " ++ password
+
+parseLine :: ReadP Line
+parseLine = do
+    num1 <- readDecP
+    string "-"
+    num2 <- readDecP
+    string " "
+    letter <- get
+    string ": "
+    password <- munch1 $ const True
+    eof
+    return (Line num1 num2 letter password)
+
+valid_1 :: Line -> Bool
+valid_1 (Line min max letter password) = within min max $ length $ filter (== letter) password
+    where within x y z = x <= z && z <= y
+
+valid_2 :: Line -> Bool
+valid_2 (Line pos1 pos2 letter password) = (matchesAt pos1) /= (matchesAt pos2)
+    where matchesAt pos = password !! (pos-1) == letter
+
+main = runAoC (map (fromJust . oneCompleteResult parseLine) . lines) (length . filter valid_1) (length . filter valid_2)