From b476cb67744c79c606101017421ba7cff7bf16db Mon Sep 17 00:00:00 2001
From: Xavier Morel <xavier.morel@cybelangel.com>
Date: Tue, 15 Dec 2020 14:32:57 +0100
Subject: [PATCH] Add 2020 day 15

---
 haskellAoC/inputs/2020/15     |  1 +
 haskellAoC/src/Y2020/Day15.hs | 41 +++++++++++++++++++++++++++++++++++
 haskellAoC/src/Y2020/Days.hs  |  2 ++
 3 files changed, 44 insertions(+)
 create mode 100644 haskellAoC/inputs/2020/15
 create mode 100644 haskellAoC/src/Y2020/Day15.hs

diff --git a/haskellAoC/inputs/2020/15 b/haskellAoC/inputs/2020/15
new file mode 100644
index 0000000..fe5b9c2
--- /dev/null
+++ b/haskellAoC/inputs/2020/15
@@ -0,0 +1 @@
+9,6,0,10,18,2,1
\ No newline at end of file
diff --git a/haskellAoC/src/Y2020/Day15.hs b/haskellAoC/src/Y2020/Day15.hs
new file mode 100644
index 0000000..be008a5
--- /dev/null
+++ b/haskellAoC/src/Y2020/Day15.hs
@@ -0,0 +1,41 @@
+module Y2020.Day15 (y20day15) where
+
+import qualified Data.IntMap as IM
+import           Data.List.Split
+
+data State = State {
+  turn :: Int
+  , prevOffset :: IM.IntMap Int
+  , memory :: [Int]
+  } deriving (Show)
+
+playGame :: State -> State
+playGame (State t prev xs@(x:_)) =
+  let newOffsets = IM.insert x (t - 1) prev
+      newTurn = (t + 1)
+      newState mem = State newTurn newOffsets mem
+  in
+    case x `IM.lookup` prev of
+      Nothing -> newState (0:xs)
+      Just n  -> newState (y:xs)
+        where y = (t - 1) - n
+
+playNtimes :: Int -> State -> State
+playNtimes n input
+  | (turn input) == n = input
+  | otherwise = playNtimes n $ playGame input
+
+memToOffsets :: Int -> [Int] -> [(Int, Int)]
+memToOffsets _ [] = []
+memToOffsets ofs (h:t) = (h,ofs):(memToOffsets (ofs+1) t)
+
+y20day15 :: [String] -> (String, String)
+y20day15 (input:_) = (part1, part2)
+  where part1 = show $ head $ memory $ playNtimes (2020 + 1) initialState
+        part2 = show $ head $ memory $ playNtimes (30000000 + 1) initialState
+
+        initialState = State initialTurn initialPrevious initialMemory
+
+        initialTurn = (1 + length initialMemory)
+        initialPrevious = (IM.fromList $ memToOffsets 1 $ reverse $ tail $ initialMemory)
+        initialMemory = reverse $ map read $ splitOn "," input :: [Int]
diff --git a/haskellAoC/src/Y2020/Days.hs b/haskellAoC/src/Y2020/Days.hs
index e14b565..af5b65d 100644
--- a/haskellAoC/src/Y2020/Days.hs
+++ b/haskellAoC/src/Y2020/Days.hs
@@ -14,6 +14,7 @@ import Y2020.Day11
 import Y2020.Day12
 import Y2020.Day13
 import Y2020.Day14
+import Y2020.Day15
 
 year2020 :: String -> [String] -> (String, String)
 year2020 "01" = y20day01
@@ -30,3 +31,4 @@ year2020 "11" = y20day11
 year2020 "12" = y20day12
 year2020 "13" = y20day13
 year2020 "14" = y20day14
+year2020 "15" = y20day15