Day 10

aoc/src/dev/ctsk/aoc/Main.scala

@@ -11,7 +11,8 @@ val solvers = Map[Int, Solver](
   6 -> Day06,
   7 -> Day07,
   8 -> Day08,
-  9 -> Day09
+  9 -> Day09,
+  10 -> Day10
 )
 
 def runSolver(solver: Solver, input: os.Path): Unit =

aoc/src/dev/ctsk/aoc/days/Day07.scala

@@ -8,33 +8,47 @@ object Day07 extends Solver(7):
     while m * 10 <= b do m *= 10
     a * (m * 10) + b
 
+  def deconcat(a: Long, b: Long): Option[Long] =
+    val b_len = Math.log10(b).floor.toLong + 1
+    if (a % Math.pow(10, b_len).toLong == b)
+    then Some(a / Math.pow(10, b_len).toLong)
+    else None
+
   class Searcher(target: Long, nums: Vector[Long], elephants: Boolean):
     def search(acc: Long = nums(0), pos: Int = 1): Boolean =
       if (pos >= nums.length) return acc == target
-      if (acc > target) return false
       search(acc + nums(pos), pos + 1)
-        || search(acc * nums(pos), pos + 1)
-        || (elephants && search(concat(acc, nums(pos)), pos + 1))
+      || search(acc * nums(pos), pos + 1)
+      || (elephants && search(concat(acc, nums(pos)), pos + 1))
+
+  class Tester(nums: Vector[Long], elephants: Boolean):
+    def test(target: Long, pos: Int = nums.length - 1): Boolean =
+      if pos == 0 then return target == nums(0)
+      if target <= 0 then return false
+      test(target - nums(pos), pos - 1)
+      || (target % nums(pos) == 0 && test(target / nums(pos), pos - 1))
+      || (elephants && deconcat(target, nums(pos)).exists(test(_, pos - 1)))
+
+  def check(target: Long, nums: Vector[Long], thirdElephant: Boolean): Boolean =
+    if nums.contains(0)
+    then Searcher(target, nums, thirdElephant).search()
+    else Tester(nums, thirdElephant).test(target)
+
   def run(input: os.ReadablePath): (Timings, Solution) =
     val REGEX = """(\d+): (.*)""".r
     val (pre_time, in) = timed {
-      os.read.lines(input).map {
-        case REGEX(value, rest) => (value.toLong, longs(rest))
-      }.toVector
+      os.read
+        .lines(input)
+        .map { case REGEX(value, rest) => (value.toLong, longs(rest)) }
+        .toVector
     }
 
     val (p1_time, p1_solution) = timed {
-      in
-        .filter((target, nums) => Searcher(target, nums, false).search())
-        .map(_.head)
-        .sum
+      in.filter(p => check(p._1, p._2, false)).map(_.head).sum
     }
 
     val (p2_time, p2_solution) = timed {
-      in
-        .filter((target, nums) => Searcher(target, nums, true).search())
-        .map(_.head)
-        .sum
+      in.filter(p => check(p._1, p._2, true)).map(_.head).sum
     }
 
     (

aoc/src/dev/ctsk/aoc/days/Day09.scala

@@ -1,42 +1,39 @@
 package dev.ctsk.aoc.days
 
-import dev.ctsk.aoc._
-import scala.collection.mutable.PriorityQueue
-import scala.math.Ordered.orderingToOrdered
+import dev.ctsk.aoc.*
 import scala.annotation.tailrec
+import scala.collection.mutable
 
 object Day09 extends Solver(9):
-  implicit def ord: Ordering[Record] = Ordering.by(-_.offset)
-
+  implicit def ord: Ordering[Record] = Ordering.by(-1 * _.offset)
   case class Record(offset: Int, size: Int, id: Int):
     def checksum: Long = id.toLong * (size * offset + size * (size - 1) / 2)
 
-  case class GapMap(
-      data: Map[Int, PriorityQueue[Record]]
-  ):
+  case class GapMap(data: Map[Int, mutable.PriorityQueue[Record]]):
     def relocate(record: Record): Record =
       val bucket = data.view
         .filter((gapSize, gaps) => gapSize >= record.size && gaps.nonEmpty)
-        .minByOption(_._2.head.offset)
+        .maxByOption(_._2.head)
 
       bucket match
         case None => record
         case Some((gapSize, gaps)) =>
           val gap = gaps.dequeue()
           if gap.offset >= record.offset then return record
-          val remaining = gapSize - record.size
-          if remaining > 0 then
-            data(remaining)
-              .enqueue(Record(gap.offset + record.size, remaining, gap.id))
+          val rest = gapSize - record.size
+          if rest > 0 then
+            data(rest).enqueue(Record(gap.offset + record.size, rest, -1))
           Record(gap.offset, record.size, record.id)
 
   def part1(input: Array[Int]): Long =
     val materialized = input.zipWithIndex.flatMap((x, i) =>
-      if i % 2 == 0 then Iterator.fill(x.toInt)(i / 2)
-      else Iterator.fill(x.toInt)(-1)
+      if i % 2 == 0
+      then Iterator.fill(x)(i / 2)
+      else Iterator.fill(x)(-1)
     )
 
-    @tailrec def compact(l: Int = 0, r: Int = materialized.length - 1): Unit =
+    @tailrec
+    def compact(l: Int = 0, r: Int = materialized.length - 1): Unit =
       if l >= r then return ()
       if materialized(l) != -1 then return compact(l + 1, r)
       if materialized(r) == -1 then return compact(l, r - 1)
@@ -48,19 +45,19 @@ object Day09 extends Solver(9):
 
   def part2(input: Array[Int]): Long =
     val offsets = input.scanLeft(0)(_ + _).toArray
-    val records =
-      input.zipWithIndex.zip(offsets).map { case ((value, index), offset) =>
-        if index % 2 == 0
-        then Record(offset, value, index / 2)
-        else Record(offset, value, -1)
-      }
 
-    val (gaps, files) = records.partition(_.id == -1)
+    val (gaps, files) = input.zip(offsets).zipWithIndex.partitionMap {
+      case ((value, offset), index) =>
+        if index % 2 == 0
+        then Right(Record(offset, value, index / 2))
+        else Left(Record(offset, value, -1))
+    }
+
     val gapMap = GapMap(
-      gaps.groupBy(_.size).mapValues(PriorityQueue.from(_)).toMap
+      gaps.groupBy(_.size).view.mapValues(mutable.PriorityQueue.from).toMap
     )
 
-    files.reverse.map(gapMap.relocate(_)).map(_.checksum).sum
+    files.reverse.map(gapMap.relocate).map(_.checksum).sum
 
   def run(input: os.ReadablePath): (Timings, Solution) =
     val disk = os.read.lines(input)(0).map(_.asDigit).toArray

aoc/src/dev/ctsk/aoc/days/Day10.scala

@@ -0,0 +1,46 @@
+package dev.ctsk.aoc.days
+
+import dev.ctsk.aoc._
+import dev.ctsk.aoc.Direction._
+
+object Day10 extends Solver(10):
+  type Input = (Grid[Char], Seq[Point])
+
+  def pre(input: os.ReadablePath): Input =
+    val grid = Grid(os.read.lines(input).map(_.toArray).toArray)
+    val zero = grid.find(_ == '0').toSeq
+    (grid, zero)
+
+  def step(
+      grid: Grid[Char],
+      start: Seq[(Point, Point)],
+      height: Int
+  ): Seq[(Point, Point)] =
+    (for
+      (p, origin) <- start
+      a <- Seq(Up, Down, Left, Right)
+      if grid(a(p)).exists(_.asDigit == height)
+    yield (a(p), origin))
+
+  def part1(input: Input): Int =
+    val (grid, zero) = input
+    (1 to 9)
+      .foldLeft(zero.zip(zero)) { case (acc, h) => step(grid, acc, h) }
+      .distinct
+      .size
+
+  def part2(input: Input): Int =
+    val (grid, zero) = input
+    (1 to 9)
+      .foldLeft(zero.zip(zero)) { case (acc, h) => step(grid, acc, h) }
+      .size
+
+  def run(input: os.ReadablePath): (Timings, Solution) =
+    val (pre_time, (grid, zero)) = timed { pre(input) }
+    val (p1_time, p1_solution) = timed { part1((grid, zero)) }
+    val (p2_time, p2_solution) = timed { part2((grid, zero)) }
+
+    (
+      Timings(pre_time, p1_time, p2_time),
+      Solution(Int.box(p1_solution), Int.box(p2_solution))
+    )

data/10.ans

@@ -0,0 +1,2 @@
+638
+1289

data/10.in

@@ -0,0 +1,50 @@
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