Pretty naive memoization for turn outcomes to solve part 2

2021/nim/day21.nim

@@ -1,29 +1,50 @@
 import ./common, std/[sequtils, algorithm, sugar, sets, strformat, strutils, tables, options, json, hashes, random]
 
-proc p1(input: Lines): uint64 =
-  const DIE_FACES = 100
-  const TARGET_SCORE = 1000
-  const BOARD_POSITIONS = 10
+const BOARD_POSITIONS = 10
 
+proc p1(input: Lines): uint64 =
   var scores = [0, 0]
   var pos = [input[0].split(": ")[1].parseInt - 1, input[1].split(": ")[1].parseInt - 1]
   var rollNum = 1
   var turn = 0
-  while scores[0] < TARGET_SCORE and scores[1] < TARGET_SCORE:
+  while scores[0] < 1000 and scores[1] < 1000:
     var move = 0
     for _ in 1..3:
-      move += rollNum mod DIE_FACES
+      move += rollNum mod 100
       inc rollNum
     pos[turn] = (pos[turn] + move) mod BOARD_POSITIONS
     scores[turn] += pos[turn] + 1
-    echo &"{move}, {pos[turn]}, {scores}"
     turn = (turn + 1) mod 2
-
-  echo rollNum, scores
   uint64(scores[turn mod 2] * (rollNum - 1))
 
+type DiracGamestate = tuple
+  scores: array[2, int]
+  positions: array[2, int]
+  turn: int
+
+var turnOutcomes = newTable[DiracGamestate, array[2, uint64]]()
+proc turn(g: DiracGamestate): array[2, uint64] =
+  if turnOutcomes.hasKey g: return turnOutcomes[g]
+  if g.scores[0] >= 21:
+    return [1'u64, 0]
+  elif g.scores[1] >= 21:
+    return [0'u64, 1]
+  for r in product(@[@[1,2,3],@[1,2,3],@[1,2,3]]):
+    let move = r.foldl(a + b)
+    var ng = g
+    ng.positions[ng.turn] = (ng.positions[ng.turn] + move) mod BOARD_POSITIONS
+    ng.scores[ng.turn] += ng.positions[ng.turn] + 1
+    ng.turn = (ng.turn + 1) mod 2
+    let r = turn(ng)
+    turnOutcomes[ng] = r
+    result[0] += r[0]
+    result[1] += r[1]
+
 proc p2(input: Lines): uint64 =
-  0
+  var pos = [input[0].split(": ")[1].parseInt - 1, input[1].split(": ")[1].parseInt - 1]
+  var g: DiracGamestate = (scores: [0, 0], positions: pos, turn: 0)
+  let wins = turn(g)
+  wins.max
 
 const input = """
 Player 1 starting position: 4