5 files changed, 126 insertions, 0 deletions
diff --git a/day21/input.txt b/day21/input.txt
new file mode 100644
index 0000000..bfb2937
--- /dev/null
+++ b/day21/input.txt
@@ -0,0 +1,2 @@
+Player 1 starting position: 7
+Player 2 starting position: 5
diff --git a/day21/main.hs b/day21/main.hs
new file mode 100644
index 0000000..fa6926b
--- /dev/null
+++ b/day21/main.hs
@@ -0,0 +1,20 @@
+module Main where
+
+import Debug.Trace (traceShow)
+import Part1
+import Part2
+import Utils
+
+main :: IO ()
+main = do
+  contents <- getContents
+  let initialPositions@[p1Pos, p2Pos] = contents $> lines .> map (drop 28) .> map read :: [Int]
+  let initialState = makeState1 initialPositions [0, 0] 1 0
+  print initialState
+  let (_, loserScore, dieRolledN) = simulate1 initialState 0
+  print $ loserScore * dieRolledN
+
+  let part2State = makeState2 (p1Pos, 0) (p2Pos, 0)
+
+  -- this has a 50% chance of giving the right answer :D
+  print $ simulate2 0 part2State
+\ No newline at end of file
diff --git a/day21/part1.hs b/day21/part1.hs
new file mode 100644
index 0000000..2c60e49
--- /dev/null
+++ b/day21/part1.hs
@@ -0,0 +1,54 @@
+module Part1 (simulate1, makeState1) where
+
+import Debug.Trace (traceShow)
+import Utils
+
+data State = State
+  { positions :: [Int],
+    scores :: [Int],
+    die :: Int,
+    dieRolledTotal :: Int
+  }
+  deriving (Show)
+
+makeState1 = State
+
+rollDie :: State -> (Int, State)
+rollDie prevState = (die prevState, prevState {die = die prevState `mod` 100 + 1})
+
+setAtIndex :: Int -> b -> [b] -> [b]
+setAtIndex i value lst = zip [0 ..] lst $> map (\(i', value') -> if i == i' then value else value')
+
+position player state = positions state !! player
+
+score player state = scores state !! player
+
+move player prevState =
+  let (dieRoll1, state1) = rollDie prevState
+      (dieRoll2, state2) = rollDie state1
+      (dieRoll3, state3) = rollDie state2
+      state = state3 {dieRolledTotal = dieRolledTotal state3 + 3}
+
+      newPos = (position player state + dieRoll1 + dieRoll2 + dieRoll3 - 1) `mod` 10 + 1
+      newScore = score player state + newPos
+   in state
+        { positions = setAtIndex player newPos (positions state),
+          scores = setAtIndex player newScore (scores state)
+        }
+
+checkWinCondition state
+  | (scores state !! 0) >= 1000 = Left (1, scores state !! 1, dieRolledTotal state)
+  | (scores state !! 1) >= 1000 = Left (0, scores state !! 0, dieRolledTotal state)
+  | otherwise = Right state
+
+takeTurn :: Int -> State -> Either (Int, Int, Int) State
+takeTurn player state = do
+  let movedState = move player state
+  checkWinCondition movedState
+
+simulate1 state player =
+  let turnTaken = takeTurn player state
+      nextPlayer = (player + 1) `mod` 2
+   in case turnTaken of
+        Right newState -> simulate1 newState nextPlayer
+        Left result -> result
diff --git a/day21/part2.hs b/day21/part2.hs
new file mode 100644
index 0000000..926e1e5
--- /dev/null
+++ b/day21/part2.hs
@@ -0,0 +1,48 @@
+module Part2 (makeState2, simulate2) where
+
+import Debug.Trace (traceShow)
+import Utils
+
+-- (position, score)
+type Player = (Int, Int)
+
+type State = (Player, Player)
+
+makeState2 :: Player -> Player -> State
+makeState2 p1 p2 = (p1, p2)
+
+win player state = snd (pick player state) >= 21
+
+pick :: Int -> State -> Player
+pick 0 = fst
+pick 1 = snd
+
+move :: Int -> Int -> (Player, Player) -> State
+move delta player state =
+  let (oldPos, oldScore) = pick player state
+      newPos = (oldPos + delta - 1) `mod` 10 + 1
+      newScore = oldScore + newPos
+      newState = updatePlayer player (newPos, newScore) state
+   in newState
+
+updatePlayer :: Int -> Player -> State -> State
+updatePlayer 0 value (_, other) = (value, other)
+updatePlayer 1 value (other, _) = (other, value)
+
+die = [1, 2, 3]
+
+possible3RollSums = [3 .. 9]
+
+coeffs = [1, 3, 6, 7, 6, 3, 1]
+
+simulate2 :: Int -> ((Int, Int), (Int, Int)) -> Integer
+simulate2 player state
+  | win 0 state = 1
+  | win 1 state = 0
+  | otherwise =
+    let (pos, score) = pick player state
+        nextStates = possible3RollSums $> map (\roll -> move roll player state)
+        nextStatesWithC = zip coeffs nextStates
+        nextPlayer = (player + 1) `mod` 2
+        wins = nextStatesWithC $> map (\(c, nextState) -> c * simulate2 nextPlayer nextState) .> sum
+     in wins
diff --git a/day21/test.txt b/day21/test.txt
new file mode 100644
index 0000000..3f69194
--- /dev/null
+++ b/day21/test.txt
@@ -0,0 +1,2 @@
+Player 1 starting position: 4
+Player 2 starting position: 8