Day 21

1 files changed, 48 insertions, 0 deletions

diff --git a/day21/part2.hs b/day21/part2.hs
new file mode 100644
index 0000000..926e1e5
--- /dev/null
+++ b/day21/part2.hs
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+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