Day 2.

src/aoc2025/Day02.kt

@@ -0,0 +1,86 @@
+package aoc2025
+
+import println
+import readInput
+import kotlin.math.log10
+
+/*
+--- Day 2: Gift Shop ---
+https://adventofcode.com/2025/day/2
+*/
+fun main() {
+
+    val inlineTestInput = """
+11-22,95-115,998-1012,1188511880-1188511890,222220-222224,1698522-1698528,446443-446449,38593856-38593862,565653-565659,824824821-824824827,2121212118-2121212124
+"""
+
+    fun part1(input: List<String>): Long {
+        val ranges = input[0].split(",").map { it.split("-").map { it.toLong() } }
+        val powTenTable = generateSequence(1L) { it * 10L }.take(20).toList().toLongArray()
+        var sum = 0L
+        for ((low, high) in ranges) {
+            var i = low
+            while (i <= high) {
+                val numDigits = log10(i.toDouble()).toInt() + 1
+                if (numDigits % 2 == 1) {
+                    i = powTenTable[numDigits]
+                } else {
+                    val firstHalf = i / powTenTable[numDigits / 2]
+                    val matchNum = firstHalf * powTenTable[numDigits / 2] + firstHalf
+                    if (matchNum in low..high) {
+                        sum += matchNum
+                    }
+                    i = (firstHalf + 1) * powTenTable[numDigits / 2]
+                }
+            }
+        }
+        return sum
+    }
+
+    fun part2(input: List<String>): Long {
+        val ranges = input[0].split(",").map { it.split("-").map { it.toLong() } }
+        val powTenTable = generateSequence(1L) { it * 10L }.take(20).toList().toLongArray()
+        val mulElevenArray = arrayOf(
+            generateSequence(1L) { it * 10L + 1L }.take(20).toList().toLongArray(),
+            generateSequence(1L) { it * 100L + 1L }.take(10).toList().toLongArray(),
+            generateSequence(1L) { it * 1000L + 1L }.take(7).toList().toLongArray(),
+            generateSequence(1L) { it * 10000L + 1L }.take(5).toList().toLongArray(),
+            generateSequence(1L) { it * 100000L + 1L }.take(4).toList().toLongArray(),
+            generateSequence(1L) { it * 1000000L + 1L }.take(4).toList().toLongArray(),
+            generateSequence(1L) { it * 10000000L + 1L }.take(3).toList().toLongArray(),
+            generateSequence(1L) { it * 100000000L + 1L }.take(3).toList().toLongArray(),
+        )
+
+        var sum = 0L
+        for ((low, high) in ranges) {
+            var i = low
+            val illegalSet = mutableSetOf<Long>()
+            while (i <= high) {
+                val numDigits = log10(i.toDouble()).toInt() + 1
+                for (n in 1..numDigits / 2) {
+                    val firstBit = i / powTenTable[numDigits - n]
+                    val matchNum = firstBit * mulElevenArray[n - 1][(numDigits - 1) / n]
+                    illegalSet.add(matchNum)
+                }
+                val firstHalf = i / powTenTable[numDigits / 2]
+                i = (firstHalf + 1) * powTenTable[numDigits / 2]
+            }
+            sum += illegalSet.filter { it in low..high }.sum()
+        }
+        return sum
+    }
+
+    // test if implementation meets criteria from the description, like:
+    val testInput = inlineTestInput.trim().reader().readLines()
+    //val testInput = readInput("aoc2025/Day02_test")
+    val testInputPart1Result = part1(testInput)
+    println("Part 1 Test: $testInputPart1Result")
+    val testInputPart2Result = part2(testInput)
+    println("Part 2 Test: $testInputPart2Result")
+    check(testInputPart1Result == 1227775554L)
+    check(testInputPart2Result == 4174379265L)
+
+    val input = readInput("aoc2025/Day02")
+    part1(input).println()
+    part2(input).println()
+}