From 7a58627132f6a1c8b283f9cd0c7f80e14215dc5d Mon Sep 17 00:00:00 2001
From: chrisly42 <chrisly@platon42.de>
Date: Mon, 8 Dec 2025 06:58:11 +0100
Subject: [PATCH] Day 8.

---
 src/aoc2025/Day08.kt | 123 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 123 insertions(+)
 create mode 100644 src/aoc2025/Day08.kt

diff --git a/src/aoc2025/Day08.kt b/src/aoc2025/Day08.kt
new file mode 100644
index 0000000..736c671
--- /dev/null
+++ b/src/aoc2025/Day08.kt
@@ -0,0 +1,123 @@
+package aoc2025
+
+import println
+import readInput
+import splitInts
+import kotlin.math.sqrt
+
+/*
+--- Day 8: Playground ---
+https://adventofcode.com/2025/day/8
+*/
+fun main() {
+
+    val inlineTestInput = """
+162,817,812
+57,618,57
+906,360,560
+592,479,940
+352,342,300
+466,668,158
+542,29,236
+431,825,988
+739,650,466
+52,470,668
+216,146,977
+819,987,18
+117,168,530
+805,96,715
+346,949,466
+970,615,88
+941,993,340
+862,61,35
+984,92,344
+425,690,689
+"""
+
+    fun dist(n1: IntArray, n2: IntArray) =
+        sqrt(((n1[0] - n2[0]).toFloat() * (n1[0] - n2[0]) + (n1[1] - n2[1]).toFloat() * (n1[1] - n2[1]) + (n1[2] - n2[2]).toFloat() * (n1[2] - n2[2])))
+
+    fun find(parent: IntArray, n: Int): Int {
+        if (parent[n] != n) {
+            parent[n] = find(parent, parent[n])
+        }
+        return parent[n]
+    }
+
+    fun union(parent: IntArray, n1: Int, n2: Int): Boolean {
+        val r1 = find(parent, n1)
+        val r2 = find(parent, n2)
+        if (r1 != r2) {
+            parent[r1] = r2
+            return true
+        }
+        return false
+    }
+
+    // priority queue would have been more efficient... who cares
+    fun createDistances(nodes: Array<IntArray>): Iterable<Pair<Pair<Int, Int>, Float>> {
+        val distances = ArrayList<Pair<Pair<Int, Int>, Float>>()
+        for (i1 in 0 until nodes.size) {
+            for (i2 in i1 + 1 until nodes.size) {
+                val d = dist(nodes[i1], nodes[i2])
+                distances.add((i1 to i2) to d)
+            }
+        }
+        distances.sortBy { it.second }
+        return distances
+    }
+
+    fun part1(input: List<String>, numConnections: Int): Long {
+        val nodes = input.map { it.splitInts(",").toIntArray() }.toTypedArray()
+        val distances = createDistances(nodes)
+
+        val n = nodes.size
+        var numCircuits = 0
+        val unionFindParentArray = IntArray(n) { it }
+        for (d in distances) {
+            val (n1, n2) = d.first
+            union(unionFindParentArray, n1, n2)
+            numCircuits++
+            if (numCircuits == numConnections) break
+        }
+
+        val circuitSizes = LongArray(n)
+        for (i in 0 until n) {
+            circuitSizes[find(unionFindParentArray, i)]++
+        }
+        return circuitSizes.sortedDescending().take(3).fold(1L) { acc, l -> acc * l }
+    }
+
+    fun part2(input: List<String>): Int {
+        val nodes = input.map { it.splitInts(",").toIntArray() }.toTypedArray()
+        val distances = createDistances(nodes)
+
+        val n = nodes.size
+        var numCircuits = 1
+        val unionFindParentArray = IntArray(n) { it }
+        for (d in distances) {
+            val (n1, n2) = d.first
+            if (union(unionFindParentArray, n1, n2)) {
+                numCircuits++
+                if (numCircuits == n) {
+                    return nodes[n1][0] * nodes[n2][0]
+                }
+            }
+        }
+        return -1
+    }
+
+    // test if implementation meets criteria from the description, like:
+    val testInput = inlineTestInput.trim().reader().readLines()
+    //val testInput = readInput("aoc2025/Day08_test")
+    val testInputPart1Result = part1(testInput, 10)
+    println("Part 1 Test: $testInputPart1Result")
+    val testInputPart2Result = part2(testInput)
+    println("Part 2 Test: $testInputPart2Result")
+    check(testInputPart1Result == 40L)
+    check(testInputPart2Result == 25272)
+
+    val input = readInput("aoc2025/Day08")
+    part1(input, 1000).println()
+    part2(input).println()
+}