Solve day 2, part 1

AdventOfCode2020.cabal

@@ -17,11 +17,13 @@ maintainer:          gael@depreeuw.dev
 extra-source-files:  CHANGELOG.md, README.md
 
 library
-  exposed-modules:     Day1
+  exposed-modules:     Day1, Day2
   -- other-modules:
   -- other-extensions:
   build-depends:       base ^>=4.13.0.0
                      , containers ^>=0.6.4.1
+                     , text ^>=1.2.4.0
+                     , attoparsec ^>=0.13.2.4
   hs-source-dirs:      src
   default-language:    Haskell2010
 

Main.hs

@@ -1,16 +1,9 @@
 module Main where
 
-import qualified Day1 (fixExpenseReport)
-
-day1 :: IO ()
-day1 = do
-  r <- readFile "./input/day1"
-  putStr "[Day 1-1] fix: "
-  print . Day1.fixExpenseReport 2020 2 . fmap read . lines $ r
-  putStr "[Day 1-2] fix: "
-  print . Day1.fixExpenseReport 2020 3 . fmap read . lines $ r
-
+import Day1 (day1)
+import Day2 (day2)
 
 main :: IO ()
 main = do
   day1
+  day2

src/Day1.hs

@@ -1,4 +1,4 @@
-module Day1 (fixExpenseReport) where
+module Day1 (day1) where
 
 import qualified Data.IntSet as DIS
 import Data.Maybe (mapMaybe)
@@ -7,6 +7,14 @@ type Target = Int
 type Expenses = [Int]
 type Expenses' = DIS.IntSet
 
+day1 :: IO ()
+day1 = do
+  r <- readFile "./input/day1"
+  putStr "[Day 1-1] fix: "
+  print . fixExpenseReport 2020 2 . fmap read . lines $ r
+  putStr "[Day 1-2] fix: "
+  print . fixExpenseReport 2020 3 . fmap read . lines $ r
+
 -- Turn the list into an IntSet. For each element in the set, look up the
 -- number in the set to make it 2020. If found, return the multiplication of
 -- the two numbers.

src/Day2.hs

@@ -0,0 +1,43 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+module Day2 (day2) where
+
+import qualified Data.Text as T
+import qualified Data.Attoparsec.Text as P
+
+type Max = Integer
+type Min = Integer
+type Letter = Char
+
+data Policy = Policy { lBound :: Min
+                     , uBound :: Max
+                     , letter :: Letter
+                     }
+
+parserPolicy :: P.Parser Policy
+parserPolicy = do
+    lB <- P.decimal
+    _ <- P.char '-'
+    uB <- P.decimal
+    _ <- P.char ' '
+    le <- P.letter
+    _ <- P.string ": "
+    return $ Policy lB uB le
+
+validatePolicy :: Policy -> String -> Bool
+validatePolicy p s =
+    let l = toInteger . length $ filter (== letter p) s in
+    l >= lBound p && l <= uBound p
+
+validateLine :: String -> Bool
+validateLine s =
+    case P.parse parserPolicy (T.pack s) of
+        P.Fail {} -> error "day2 - parser error"
+        P.Partial _ -> error "day2 - input error"
+        P.Done i r -> validatePolicy r (T.unpack i)
+
+day2 :: IO ()
+day2 = do
+  r <- readFile "./input/day2"
+  putStr "[Day 1-1] # valid passwords: "
+  print . length . filter (==True) . fmap validateLine . lines $ r