From 8d492486fb2aedf75a599c45eebc513635135b9d Mon Sep 17 00:00:00 2001
From: chrisly42 <chrisly@platon42.de>
Date: Sat, 7 Dec 2024 07:13:32 +0100
Subject: [PATCH] Day 7 Part 2 optimized to 666ms (instead of around 3000ms).

---
 src/aoc2024/Day07.kt | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/src/aoc2024/Day07.kt b/src/aoc2024/Day07.kt
index 8680eda..4ff38d8 100644
--- a/src/aoc2024/Day07.kt
+++ b/src/aoc2024/Day07.kt
@@ -4,7 +4,6 @@ import println
 import readInput
 import splitLongs
 import kotlin.math.pow
-import kotlin.math.roundToLong
 
 /*
 --- Day 7: Bridge Repair ---
@@ -66,6 +65,16 @@ fun main() {
         return sum
     }
 
+    val powTenTable = generateSequence(1L) { it * 10L }.take(10).toList().toLongArray()
+    val powThreeTable = generateSequence(1) { it * 3 }.take(12).toList().toIntArray()
+
+    fun fastCeilLog10(n: Long): Int {
+        for (i in 1 until 9) {
+            if (powTenTable[i] > n) return i
+        }
+        return -1
+    }
+
     fun part2(input: List<String>): Long {
         var sum = 0L
         for (i in input) {
@@ -75,10 +84,10 @@ fun main() {
             val combs = 3.0.pow(ops.size - 1).toInt()
             for (t in 0 until combs) {
                 val res = ops.reduceIndexed { index, acc, l ->
-                    when ((t / (3.0.pow(index - 1).toInt())) % 3) {
+                    when ((t / powThreeTable[index - 1]) % 3) {
                         0 -> acc + l
                         1 -> acc * l
-                        2 -> acc * (10.0.pow(l.toString().length.toDouble()).roundToLong()) + l
+                        2 -> acc * powTenTable[fastCeilLog10(l)] + l
                         else -> throw IllegalArgumentException()
                     }
                 }