[hs] Solve 2020_10

hs/src/Aoc/Y2020.hs

@@ -12,6 +12,7 @@ import qualified Aoc.Y2020.D06 as D06
 import qualified Aoc.Y2020.D07 as D07
 import qualified Aoc.Y2020.D08 as D08
 import qualified Aoc.Y2020.D09 as D09
+import qualified Aoc.Y2020.D10 as D10
 
 year :: Year
 year = Year 2020
@@ -24,4 +25,5 @@ year = Year 2020
   , ( 7, D07.day)
   , ( 8, D08.day)
   , ( 9, D09.day)
+  , (10, D10.day)
   ]

hs/src/Aoc/Y2020/D10.hs

@@ -0,0 +1,33 @@
+module Aoc.Y2020.D10
+  ( day
+  ) where
+
+import           Data.List
+
+import           Aoc.Day
+import           Aoc.Parse
+
+parser :: Parser [Integer]
+parser = manyLines decimal
+
+possibilities :: Integer -> Integer -> Integer -> [Integer] -> Integer
+possibilities _ _ c []     = c
+possibilities a b c (1:xs) = possibilities b c (a + b + c) [x - 1 | x <- xs]
+possibilities _ b c (2:xs) = possibilities c 0 (b + c)     [x - 2 | x <- xs]
+possibilities _ _ c (3:xs) = possibilities 0 0 c           [x - 3 | x <- xs]
+possibilities _ _ _ (_:_)  = 0
+
+solver :: [Integer] -> IO ()
+solver nums = do
+  let snums = sort nums ++ [maximum nums + 3]
+
+  putStrLn ">> Part 1"
+  let diffs = zipWith (-) snums (0 : snums)
+  print $ length (filter (==1) diffs) * length (filter (==3) diffs)
+
+  putStrLn ""
+  putStrLn ">> Part 2"
+  print $ possibilities 0 0 1 snums
+
+day :: Day
+day = dayParse parser solver