Add 2020 day 13

haskellAoC/inputs/2020/13

@@ -0,0 +1,2 @@
+1000510
+19,x,x,x,x,x,x,x,x,41,x,x,x,x,x,x,x,x,x,523,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,17,13,x,x,x,x,x,x,x,x,x,x,29,x,853,x,x,x,x,x,37,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,23

haskellAoC/inputs/2020/13_test

@@ -0,0 +1,2 @@
+939
+7,13,x,x,59,x,31,19

haskellAoC/src/Y2020/Day13.hs

@@ -0,0 +1,30 @@
+module Y2020.Day13 (y20day13) where
+
+import Data.List
+import Data.List.Split
+import Data.Function (on)
+
+timeToWait :: Int -> Int -> Int
+timeToWait mytime bustime = time_to_next_pass
+  where time_since_last_pass = mytime `mod` bustime
+        time_to_next_pass = bustime - time_since_last_pass
+
+-- timestamp, step -> offset, busid -> timestamp, step
+lowestTS :: (Int, Int) -> (Int, Int) -> (Int, Int)
+lowestTS (0, 0) (_, busid) = (busid, busid)
+lowestTS (curTS, step) (offset, busid)
+  | (curTS + offset) `mod` busid == 0 = (curTS, step * busid)
+  | otherwise = lowestTS (curTS + step, step) (offset, busid)
+
+y20day13 :: [String] -> (String, String)
+y20day13 [arrival', busids'] = (part1, part2)
+  where part1 = show $ next_bus_id * next_bus_time
+        part2 = show $ timestamp
+
+        arrival = read arrival' :: Int
+        (next_bus_id, next_bus_time) = minimumBy (compare `on` snd) $ map (\busid -> (busid, timeToWait arrival busid)) busids :: (Int, Int)
+
+        busids = map read $ filter (/="x") $ splitOn "," busids' :: [Int]
+
+        busIdWithOffset = map (\(a, b) -> (a, read b :: Int)) $ filter ((/= "x") . snd) $ zip (iterate (+1) 0) $ splitOn "," busids'
+        timestamp = foldl lowestTS (0, 0) busIdWithOffset

haskellAoC/src/Y2020/Days.hs

@@ -12,6 +12,7 @@ import Y2020.Day09
 import Y2020.Day10
 import Y2020.Day11
 import Y2020.Day12
+import Y2020.Day13
 
 year2020 :: String -> [String] -> (String, String)
 year2020 "01" = y20day01
@@ -26,3 +27,4 @@ year2020 "09" = y20day09
 year2020 "10" = y20day10
 year2020 "11" = y20day11
 year2020 "12" = y20day12
+year2020 "13" = y20day13