Day 21

Day21.scala

@@ -0,0 +1,194 @@
+package aoc.day21
+
+import aoc._
+import aoc.direction._
+import scala.collection.mutable
+
+val board =
+  lines
+    .map(_.toArray)
+    .toArray
+
+val startPoint =
+  board.toSeq.zipWithIndex.findMap { (row, idx) =>
+    row.toSeq.zipWithIndex.findMap: (cell, col) =>
+      if cell == 'S' then Some((idx, col)) else None
+  }.get
+
+def inside(x: Int, y: Int) =
+  x >= 0 && x < board.length && y >= 0 && y < board(0).length
+
+def walkable(x: Int, y: Int) = inside(x, y) && board(x)(y) != '#'
+
+def getInf(x: Int, y: Int) =
+  def mod(x: Int, m: Int): Int = if x < 0 then mod(x % m + m, m) else x % m
+  board(mod(x, board.size))(mod(y, board(0).size))
+def walkableInf(x: Int, y: Int) = getInf(x, y) != '#'
+
+// part 1
+
+def walk(steps: Int, starting: Set[(Int, Int)]): Set[(Int, Int)] =
+  if steps == 0 then starting
+  else
+    // println(s"$steps => ${starting.size}")
+    val ns =
+      starting
+        .flatMap((a, b) => Dir.all.map(_(a, b)))
+        .filter(walkableInf(_, _))
+    walk(steps - 1, ns)
+
+def part1 =
+  val possible = walk(300, Set(startPoint))
+  println(possible.size)
+
+// part 2
+
+object shortestPathsFrom:
+  val cache = mutable.Map.empty[(Int, Int), Array[Array[Int]]]
+
+  def apply(x: Int, y: Int) =
+    cache.getOrElseUpdate(
+      (x, y), {
+        val mp = mutable.Map((x, y) -> 0)
+        val queue = mutable.Queue((x, y))
+        while !queue.isEmpty do
+          val (x, y) = queue.dequeue()
+          val dist = mp((x, y))
+          Dir.all
+            .map(_(x, y))
+            .filter(walkable(_, _))
+            .foreach { pt =>
+              if !mp.contains(pt) then
+                mp += pt -> (dist + 1)
+                queue += pt
+            }
+        val (a, b) = mp.toMap.partition((_, dist) => dist % 2 == 0)
+        Iterator(a, b)
+          .map(v => v.values.toSeq.sorted.toArray)
+          .toArray
+      }
+    )
+
+// input is funni
+
+val _ =
+  assert(board.head.forall(_ == '.'))
+  assert(board.last.forall(_ == '.'))
+  assert(board.forall(_.head == '.'))
+  assert(board.forall(_.last == '.'))
+
+  val (x, y) = startPoint
+  assert((0 until board.length).forall(board(_)(y) != '#'))
+  assert((0 until board(0).length).forall(board(x)(_) != '#'))
+
+def pointsReachable(steps: Int): Long =
+  val (sx, sy) = startPoint
+  def distToRow(delta: Int) =
+    if delta == 0 then 0L
+    else if delta < 0 then
+      // go up
+      1L + sx + (delta + 1L).abs * board.size
+    else 0L + (board.size - sx) + (delta - 1L) * board.size
+  def distToCol(delta: Int) =
+    if delta == 0 then 0L
+    else if delta < 0 then
+      // go left
+      1L + sy + (delta + 1L).abs * board(0).size
+    else 0L + (board(0).size - sy) + (delta - 1L) * board(0).size
+
+  def startWhere(dx: Int, dy: Int) =
+    val x = if dx == 0 then sx else if dx < 0 then board.size - 1 else 0
+    val y = if dy == 0 then sy else if dy < 0 then board(0).size - 1 else 0
+    (x, y)
+
+  def countWhere(steps: Int, dx: Int, dy: Int) =
+    val dToBoard = distToRow(dx) + distToCol(dy)
+    if dToBoard > steps then 0L
+    else
+      val (stx, sty) = startWhere(dx, dy)
+      val dists = shortestPathsFrom(stx, sty)((steps - dToBoard).toInt % 2)
+      val res = dists.sortedCountUntil(steps - dToBoard.toInt).toLong
+      // println(s"got $dx $dy $dToBoard $stx $sty => $res ${shortestPathsFrom(stx, sty).map(_.size).toSeq}")
+      res
+
+  (-1 to 1).zip((-1 to 1)).foreach { (dx, dy) =>
+    val (stx, sty) = startWhere(dx, dy)
+    // println(s"$dx $dy ${shortestPathsFrom(stx, sty).map(_.size).toSeq}")
+  }
+
+  extension (arr: Array[Int])
+    def sortedCountUntil(max: Int) =
+      var low = 0
+      var hi = arr.size
+      while low < hi do
+        val mid = (low + hi) / 2
+        // println(s"$mid => ${arr(mid)} ~ $max")
+        if arr(mid) <= max then low = mid + 1 else hi = mid
+      // println(s"found $low => ${arr(low - 1)}")
+      low
+
+  ((0 to steps).toIterator
+    .takeWhile(distToRow(_) <= steps)
+    ++ (-1 to -steps by -1).toIterator.takeWhile(distToRow(_) <= steps))
+    .map(dx =>
+      val dRow = distToRow(dx)
+      val zero = countWhere(steps, dx, 0)
+      val right =
+        val (stx, sty) = startWhere(dx, 1)
+        val baseDistance = dRow + distToCol(1) // dToBoard = baseDistance + (delta-1) * board.size
+        if baseDistance > steps then 0L
+        else
+          val maxRightAllBoard =
+            ((steps - baseDistance - shortestPathsFrom(stx, sty).map(_.last).max) / board.size).toInt.max(0) + 1
+          val odd = ((steps - baseDistance) % 2).toInt
+          val a = (1L until maxRightAllBoard by 2).size * shortestPathsFrom(stx, sty)(odd).size
+          val b = (2L until maxRightAllBoard by 2).size * shortestPathsFrom(stx, sty)((board.size + odd) % 2).size
+          // println(
+          //   s"$dx => $maxRightAllBoard [${distToRow(dx) + distToCol(maxRightAllBoard)} ${shortestPathsFrom(stx, sty)
+          //       .map(_.last)
+          //       .max}] ${a + b}"
+          // )
+          val rest = (maxRightAllBoard to steps).toIterator
+            .takeWhile(dRow + distToCol(_) <= steps)
+            .map(countWhere(steps, dx, _))
+            .sum
+          a + b + rest
+      val left =
+        val (stx, sty) = startWhere(dx, -1)
+        val baseDistance = dRow + distToCol(-1) // dToBoard = baseDistance + (delta-1) * board.size
+        if baseDistance > steps then 0L
+        else
+          val maxLeftAllBoard =
+            ((steps - baseDistance - shortestPathsFrom(stx, sty).map(_.last).max) / board.size).toInt.max(0) + 1
+          // println(s"$dx => -$maxLeftAllBoard")
+          val odd = ((steps - baseDistance) % 2).toInt
+          val a = (-1L until -maxLeftAllBoard by -2).size * shortestPathsFrom(stx, sty)(odd).size
+          val b = (-2L until -maxLeftAllBoard by -2).size * shortestPathsFrom(stx, sty)((board.size + odd) % 2).size
+          // println(s"$dx => -$maxLeftAllBoard [${distToRow(dx) + distToCol(-maxLeftAllBoard)}] ${a + b}")
+          val rest = (-maxLeftAllBoard to -steps by -1).toIterator
+            .takeWhile(dRow + distToCol(_) <= steps)
+            .map(countWhere(steps, dx, _))
+            .sum
+          a + b + rest
+      zero + right + left
+    )
+    .sum
+
+def part2 =
+  val inputs = Seq(26501365)
+  // val inputs = Seq(1, 131)
+  println(inputs.map(pointsReachable))
+  // val s = walk(200, Set(startPoint))
+  // // divide by quadrant
+  // val quads = s.toSeq
+  //   .groupBy((x, y) =>
+  //     val qx = if x < 0 then -(-x + board.size - 1) / board.size else x / board.size
+  //     val qy = if y < 0 then -(-y + board(0).size - 1) / board(0).size else y / board(0).size
+  //     (qx, qy)
+  //   )
+  //   .mapValues(_.size)
+  // println((quads.toSeq, quads.values.sum))
+
+@main def Day21(part: Int) = part match
+  case 1 => part1
+  case 2 => part2

Direction.scala

@@ -5,6 +5,8 @@ import aoc._
 object Dir:
   val dx = Array(-1, 0, 1, 0)
   val dy = Array(0, 1, 0, -1)
+
+  val all = Array(Up, Right, Down, Left)
 import Dir._
 enum Dir:
   case Up, Right, Down, Left

inputs/day21.input

@@ -0,0 +1,131 @@
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