Bitset is even faster (<1 sec).

src/aoc2024/Day22.kt

@@ -6,6 +6,7 @@ import kotlinx.coroutines.awaitAll
 import kotlinx.coroutines.runBlocking
 import println
 import readInput
+import java.util.*
 
 /*
 --- Day 22: Monkey Market ---
@@ -76,7 +77,7 @@ fun main() {
             var lastd = 0
             var code = 0
             var countDown = 4
-            val seenArray = BooleanArray(19 * 19 * 19 * 19)
+            val seenArray = BitSet(19 * 19 * 19 * 19)
             for (v in priceSequence(it).take(2001)) {
                 val d = v % 10
                 val delta = d - lastd