Day 11.

src/aoc2024/Day11.kt

@@ -0,0 +1,71 @@
+package aoc2024
+
+import println
+import readInput
+import splitLongs
+
+/*
+--- Day 11: Plutonian Pebbles ---
+https://adventofcode.com/2024/day/11
+*/
+fun main() {
+
+    val inlineTestInput = """
+        125 17
+"""
+
+    val powTenTable = generateSequence(1L) { it * 10L }.take(20).toList().toLongArray()
+    val lookupTable = HashMap<Pair<Long, Int>, Long>()
+
+    fun fastCeilLog10(n: Long): Int {
+        for (i in 1 until powTenTable.size) {
+            if (powTenTable[i] > n) return i
+        }
+        return -1
+    }
+
+    fun rec(n: Long, i: Int): Long {
+        //println("$i $n")
+        if (i == 0) return 1
+        val ll = lookupTable[n to i]
+        if (ll != null) return ll
+        val result = if (n == 0L) {
+            rec(1L, i - 1)
+        } else {
+            val size = fastCeilLog10(n)
+            if (size and 1 == 0) {
+                val left = n / powTenTable[size / 2]
+                val right = n % powTenTable[size / 2]
+                rec(left, i - 1) + rec(right, i - 1)
+            } else {
+                rec(n * 2024L, i - 1)
+            }
+        }
+        lookupTable[n to i] = result
+        return result
+    }
+
+    fun part1(input: List<String>): Long {
+        val numbers = input[0].splitLongs()
+        return numbers.sumOf { rec(it, 25) }
+    }
+
+    fun part2(input: List<String>): Long {
+        val numbers = input[0].splitLongs()
+        return numbers.sumOf { rec(it, 75) }
+    }
+
+    // test if implementation meets criteria from the description, like:
+    val testInput = inlineTestInput.trim().reader().readLines()
+    //val testInput = readInput("aoc2024/Day11_test")
+    val testInputPart1Result = part1(testInput)
+    println("Part 1 Test: $testInputPart1Result")
+    val testInputPart2Result = part2(testInput)
+    println("Part 2 Test: $testInputPart2Result")
+    check(testInputPart1Result == 55312L)
+    //check(testInputPart2Result == 0L)
+
+    val input = readInput("aoc2024/Day11")
+    part1(input).println()
+    part2(input).println()
+}