Solution 2025-02 (Gift Shop)

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/year2025/day02/GiftShop.kt

@@ -0,0 +1,59 @@
+package de.ronny_h.aoc.year2025.day02
+
+import de.ronny_h.aoc.AdventOfCode
+
+fun main() = GiftShop().run(32976912643, 54446379122)
+
+class GiftShop : AdventOfCode<Long>(2025, 2) {
+    override fun part1(input: List<String>): Long =
+        input.parseIdRanges()
+            .filterInvalidIds(Long::isSequenceRepeatedTwice)
+            .sum()
+
+    override fun part2(input: List<String>): Long =
+        input.parseIdRanges()
+            .filterInvalidIds(Long::isSequenceRepeatedAtLeastTwice)
+            .sum()
+}
+
+private fun List<LongRange>.filterInvalidIds(isInvalid: (Long) -> Boolean): List<Long> =
+    flatMap { range ->
+        range.filter { isInvalid(it) }
+    }
+
+fun Long.isSequenceRepeatedTwice(): Boolean {
+    val id = "$this"
+    if (id.length % 2 != 0) {
+        return false
+    }
+    val first = id.take(id.length / 2)
+    val second = id.substring(id.length / 2)
+    return first == second
+}
+
+fun Long.isSequenceRepeatedAtLeastTwice(): Boolean {
+    val id = "$this"
+    for (size in 1..id.length / 2) {
+        if (id.length % size != 0) {
+            continue
+        }
+        val subsequences = buildList {
+            id.windowed(size, size) {
+                add(it)
+            }
+        }
+        if (subsequences.all { it == subsequences.first() }) {
+            return true
+        }
+    }
+    return false
+}
+
+fun List<String>.parseIdRanges() = first()
+    .split(",")
+    .map { range ->
+        val (from, to) = range
+            .split("-")
+            .map(String::toLong)
+        from..to
+    }

src/test/kotlin/de/ronny_h/aoc/year2025/day02/GiftShopTest.kt

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+package de.ronny_h.aoc.year2025.day02
+
+import de.ronny_h.aoc.extensions.asList
+import io.kotest.core.spec.style.StringSpec
+import io.kotest.data.forAll
+import io.kotest.data.row
+import io.kotest.matchers.shouldBe
+
+class GiftShopTest : StringSpec({
+
+    val input = """
+        11-22,95-115,998-1012,1188511880-1188511890,222220-222224,1698522-1698528,446443-446449,38593856-38593862,565653-565659,824824821-824824827,2121212118-2121212124
+    """.asList()
+
+    "id ranges can be parsed" {
+        val input = listOf("11-22,95-115,998-1012,1188511880-1188511890")
+        input.parseIdRanges() shouldBe listOf(11L..22L, 95L..115L, 998L..1012L, 1188511880L..1188511890L)
+    }
+
+    "isSequenceRepeatedTwice" {
+        forAll(
+            row(11L, true),
+            row(22L, true),
+            row(123L, false),
+            row(1234L, false),
+            row(1188511885L, true),
+        ) { id, isInvalid ->
+            id.isSequenceRepeatedTwice() shouldBe isInvalid
+        }
+    }
+
+    "isSequenceRepeatedAtLeastTwice" {
+        forAll(
+            row(11L, true),
+            row(22L, true),
+            row(111L, true),
+            row(123L, false),
+            row(1234L, false),
+            row(1111111L, true),
+            row(123123123L, true),
+            row(1188511885L, true),
+        ) { id, isInvalid ->
+            id.isSequenceRepeatedAtLeastTwice() shouldBe isInvalid
+        }
+    }
+
+    "part 1: the sum of all invalid IDs with sequences repeated exactly twice" {
+        GiftShop().part1(input) shouldBe 1227775554L
+    }
+
+    "part 2: the sum of all invalid IDs with sequences repeated at least twice" {
+        GiftShop().part2(input) shouldBe 4174379265L
+    }
+})