4 files changed, 33 insertions, 4 deletions

diff --git a/aoc.cabal b/aoc.cabal
index ad67f92..2d1d976 100644
--- a/aoc.cabal
+++ b/aoc.cabal
@@ -105,3 +105,9 @@ executable Day14
   build-depends:       base, aoc, containers, parsec, strings
   default-language:    Haskell2010
   hs-source-dirs:      execs
+
+executable Day15
+  main-is:             Day15.hs
+  build-depends:       base, aoc, containers, split
+  default-language:    Haskell2010
+  hs-source-dirs:      execs
diff --git a/execs/Day14.hs b/execs/Day14.hs
index d82424a..7f4368a 100644
--- a/execs/Day14.hs
+++ b/execs/Day14.hs
@@ -5,6 +5,7 @@ import           Text.ParserCombinators.Parsec
 import           Text.Parsec.Char
 import           Data.Either
 import           Data.Char
+import           Data.Bifunctor (bimap)
 import           Data.Strings
 import           Numeric (showIntAtBase, readInt)
 import           Data.Map (Map)
@@ -15,11 +16,11 @@ data Stmt = Mask String | Mem Int Int deriving Show
 parseMask :: Parser Stmt
 parseMask = string "mask = " >> Mask <$> many anyChar
 
+parseNumber :: Parser Int
+parseNumber = read <$> many1 digit
+
 parseMem :: Parser Stmt
-parseMem = do 
-    one <- string "mem[" >> many1 digit
-    two <- string "] = " >> many1 digit
-    return $ Mem (read one) (read two)
+parseMem = Mem <$ string "mem[" <*> parseNumber <* string "] = " <*> parseNumber
 
 parseLine :: Parser Stmt
 parseLine = try parseMask <|> parseMem
diff --git a/execs/Day15.hs b/execs/Day15.hs
new file mode 100644
index 0000000..cd498fb
--- /dev/null
+++ b/execs/Day15.hs
@@ -0,0 +1,21 @@
+module Main where
+
+import Data.Maybe
+import Data.List.Split
+import qualified Data.Map as M
+
+run :: [Int] -> Int -> [Int] -> Int
+run ls start input = fst $ foldl fn (start, startMap) ls
+    where startMap = M.fromList $ zip input [1..]
+          fn (last, seen) i = (i - last', seen')
+              where last' = fromMaybe i (M.lookup last seen)
+                    seen' = M.insert last i seen
+
+main :: IO ()
+main = do
+    n <- map read . splitOn "," <$> readFile "input/15"
+
+    -- holy off-by-one errors
+    print $ run [8..2020 - 1] 0 n
+    print $ run [8..30000000 - 1] 0 n
+
diff --git a/input/15 b/input/15
new file mode 100644
index 0000000..134afd4
--- /dev/null
+++ b/input/15
@@ -0,0 +1 @@
+0,20,7,16,1,18,15