solve: day 2, part 1

2020/02/README.md

@@ -0,0 +1,5 @@
+# Day 2
+
+## Part 1
+
+Spent some time reading up on how to define a custom instance of `Read`. Pretty simple otherwise. Surprised `countElem` isn't in Prelude.

2020/02/part1.hs

@@ -0,0 +1,35 @@
+#!/usr/bin/env runghc
+
+import Data.Char
+
+data PasswordEntry = PasswordEntry { min :: Int, max :: Int, character :: Char, password :: String }
+  deriving (Show)
+
+-- This could be a lot better...
+instance Read PasswordEntry where
+  readsPrec _ input =
+    let (min', rest) = span isDigit input
+        min = read min' :: Int
+        (_, rest') = splitAt 1 rest
+        (max', rest'') = span isDigit rest'
+        max = read max' :: Int
+        (_:char:_:_:password) = rest''
+      in
+      [(PasswordEntry min max char password, "")]
+
+
+main = interact solve
+
+solve = show . length . filter (isValidPasswordEntry . readPasswordEntry) . lines
+
+readPasswordEntry :: String -> PasswordEntry
+readPasswordEntry = read
+
+isValidPasswordEntry (PasswordEntry min max char pass) =
+  let countedElem = countElem char pass
+   in min <= countedElem && max >= countedElem
+
+countElem _ [] = 0
+countElem a (x:xs)
+  | a == x = 1 + countElem a xs
+countElem a (_:xs) = countElem a xs

2020/README.md

@@ -14,7 +14,7 @@ asdf install
 
 |  S  |  M  |  T  |  W  |  T  |  F  |  S  |
 | :-: | :-: | :-: | :-: | :-: | :-: | :-: |
-|     |     | [1] | 2   | 3   | 4   | 5   |
+|     |     | [1] | [2] |  3  |  4  |  5  |
 |  6  |  7  |  8  |  9  | 10  | 11  | 12  |
 | 13  | 14  | 15  | 16  | 17  | 18  | 19  |
 | 21  | 22  | 23  | 24  | 25  |     |     |
@@ -22,3 +22,4 @@ asdf install
 [asdf]: https://asdf-vm.com/#/
 [haskell]: https://www.haskell.org
 [1]: ./01
+[2]: ./02