Day 22

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

@@ -23,7 +23,8 @@ val solvers = Map[Int, Solver](
   18 -> Day18,
   19 -> Day19,
   20 -> Day20,
-  21 -> Day21
+  21 -> Day21,
+  22 -> Day22
 )
 
 def runSolver(solver: Solver, input: os.Path): Unit =

aoc/src/dev/ctsk/aoc/days/Day22.scala

@@ -0,0 +1,60 @@
+package dev.ctsk.aoc.days
+
+import dev.ctsk.aoc.*
+import scala.collection.mutable
+
+object Day22 extends Solver(22):
+  inline def hash(n: Long): Long =
+    var secret = n
+    secret ^= (secret << 6)
+    secret &= 0xffffff
+    secret ^= (secret >> 5)
+    secret &= 0xffffff
+    secret ^= (secret << 11)
+    secret &= 0xffffff
+    secret
+
+  def hash(n: Long, count: Int): Long =
+    Iterator.iterate(n)(hash).drop(count).next()
+
+  def part1(initial: Seq[Long]): Long = initial.map(hash(_, 2000)).sum
+
+  def part2(initial: Seq[Long]): Long =
+    inline def update(hash: Int, next: Int): Int =
+      ((hash << 5) & 0xfffff) | (next + 10)
+
+    val maxHash = Seq(9, 9, 9, 9).fold(0)(update)
+    val bananas = Array.fill(maxHash)(0)
+    val seen = mutable.BitSet(maxHash)
+
+    initial.foreach { value =>
+      seen.clear()
+      val it = Iterator.iterate(value)(hash).map(_.toInt % 10)
+      var last = it.next()
+      var window = 0
+
+      for v <- it.take(3) do
+        val diff = v - last
+        last = v
+        window = update(window, diff)
+
+      for v <- it.take(2000 - 3) do
+        val diff = v - last
+        last = v
+        window = update(window, diff)
+        if !seen(window) then
+          seen += window
+          bananas(window) += v
+    }
+
+    bananas.max
+
+  override def run(input: os.ReadablePath): (Timings, Solution) =
+    val initial = os.read.lines(input).map(_.toLong)
+    val (p1_time, p1_solution) = timed { part1(initial) }
+    val (p2_time, p2_solution) = timed { part2(initial) }
+
+    (
+      Timings(0, p1_time, p2_time),
+      Solution(Long.box(p1_solution), Long.box(p2_solution))
+    )

data/22.ans

@@ -0,0 +1,2 @@
+16894083306
+1925