Revisited Day 22 for faster algorithm

src/aoc2024/Day22.kt

@@ -36,7 +36,7 @@ fun main() {
     fun part1(input: List<String>) =
         input.map(String::toInt).sumOf { priceSequence(it).elementAt(2000).toLong() }
 
-    fun part2(input: List<String>): Int {
+    fun part2bruteforce(input: List<String>): Int {
         val strings = input.map(String::toInt).map {
             val normalString = StringBuffer(2024)
             val deltaString = StringBuffer(2024)
@@ -70,6 +70,27 @@ fun main() {
         }
     }
 
+    fun part2(input: List<String>): Int {
+        val map = HashMap<Int, MutableList<Int>>()
+        input.map(String::toInt).forEach {
+            var lastd = 0
+            var code = 0
+            var countDown = 4
+            val seenArray = BooleanArray(19 * 19 * 19 * 19)
+            for (v in priceSequence(it).take(2001)) {
+                val d = v % 10
+                val delta = d - lastd
+                code = ((code * 19) % (19 * 19 * 19 * 19)) + (delta + 9)
+                if (--countDown < 0 && !seenArray[code]) {
+                    map.getOrPut(code) { ArrayList() }.add(d)
+                    seenArray[code] = true
+                }
+                lastd = d
+            }
+        }
+        return map.maxOf { it.value.sum() }
+    }
+
     // test if implementation meets criteria from the description, like:
     val testInput = inlineTestInput.trim().reader().readLines()
     val testInput2 = inlineTestInput2.trim().reader().readLines()