Day 2

Author: WVerlaek

src/main/kotlin/y25/Day2.kt

@@ -0,0 +1,81 @@
+package y25
+
+import common.puzzle.solvePuzzle
+import common.puzzle.Input
+import common.puzzle.Puzzle
+import common.datastructures.*
+import common.ext.*
+import common.util.*
+import java.util.*
+import kotlin.math.*
+import kotlin.system.exitProcess
+
+
+fun main() = solvePuzzle(year = 2025, day = 2) { Day2(it) }
+
+class Day2(val input: Input) : Puzzle {
+    data class Range(val start: Long, val end: Long)
+
+    private val ranges = input.string.split(",").map { range ->
+        val (start, end) = range.split("-")
+        Range(start.toLong(), end.toLong())
+    }
+
+    private fun Range.invalidIds(): List<Long> {
+        val invalidIds = mutableListOf<Long>()
+        var half = start.toString(10).let { str ->
+            val halfLen = str.length / 2
+            if (halfLen > 0) {
+                str.take(halfLen)
+            } else {
+                "0"
+            }
+        }
+        while (true) {
+            val full = "$half$half".toLong()
+            if (full > end) {
+                return invalidIds
+            }
+            if (full >= start) {
+                invalidIds.add(full)
+            }
+            half = (half.toLong() + 1).toString()
+        }
+    }
+
+    private fun Range.invalidIds2(): Set<Long> {
+        val invalidIds = mutableSetOf<Long>()
+        var base = "1"
+        while (true) {
+            var sequence = "$base$base"
+            if (sequence.toLong() > end) {
+                return invalidIds
+            }
+
+            while (true) {
+                val longValue = sequence.toLong()
+                if (longValue > end) {
+                    break
+                }
+                if (longValue >= start) {
+                    invalidIds += longValue
+                }
+                sequence += base
+            }
+
+            base = (base.toLong() + 1).toString()
+        }
+    }
+
+    override fun solveLevel1(): Any {
+        return ranges.sumOf { range ->
+            range.invalidIds().sum()
+        }
+    }
+
+    override fun solveLevel2(): Any {
+        return ranges.sumOf { range ->
+            range.invalidIds2().sum()
+        }
+    }
+}

src/test/kotlin/y25/Day2Test.kt

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+package y25
+
+import common.puzzle.Input
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+
+internal class Day2Test {
+    private val sample = Input("""
+11-22,95-115,998-1012,1188511880-1188511890,222220-222224,1698522-1698528,446443-446449,38593856-38593862,565653-565659,824824821-824824827,2121212118-2121212124
+    """.trimIndent())
+
+    private val day = Day2(sample)
+
+    @Test
+    fun solveLevel1() {
+        assertEquals(1227775554L, day.solveLevel1())
+    }
+
+    @Test
+    fun solveLevel2() {
+         assertEquals(4174379265L, day.solveLevel2())
+    }
+}