From 13592a60ee30e1505f135e4fc6fc9d5337d9ef70 Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Mon, 8 Dec 2025 21:21:24 +0100
Subject: [PATCH] 2025 day 8 in Go

---
 2025/day08/.gitignore |   1 +
 2025/day08/README.md  |  11 +++
 2025/day08/go.mod     |   3 +
 2025/day08/sample.txt |  20 +++++
 2025/day08/solve.go   | 166 ++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 201 insertions(+)
 create mode 100644 2025/day08/.gitignore
 create mode 100644 2025/day08/README.md
 create mode 100644 2025/day08/go.mod
 create mode 100644 2025/day08/sample.txt
 create mode 100644 2025/day08/solve.go

diff --git a/2025/day08/.gitignore b/2025/day08/.gitignore
new file mode 100644
index 0000000..b199abc
--- /dev/null
+++ b/2025/day08/.gitignore
@@ -0,0 +1 @@
+solve
diff --git a/2025/day08/README.md b/2025/day08/README.md
new file mode 100644
index 0000000..30bceb6
--- /dev/null
+++ b/2025/day08/README.md
@@ -0,0 +1,11 @@
+# Day 08: Go
+
+Nice puzzle, got to use an obscure data structure. And implement it, because Go doesn't have it in
+its standard library.
+
+```console
+$ go build
+$ ./solve sample.txt 10
+Part 1: 40
+Part 2: 25272
+```
diff --git a/2025/day08/go.mod b/2025/day08/go.mod
new file mode 100644
index 0000000..2be72c9
--- /dev/null
+++ b/2025/day08/go.mod
@@ -0,0 +1,3 @@
+module bertptrs/solve/v2
+
+go 1.25.5
diff --git a/2025/day08/sample.txt b/2025/day08/sample.txt
new file mode 100644
index 0000000..e98a3b6
--- /dev/null
+++ b/2025/day08/sample.txt
@@ -0,0 +1,20 @@
+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
diff --git a/2025/day08/solve.go b/2025/day08/solve.go
new file mode 100644
index 0000000..66b0904
--- /dev/null
+++ b/2025/day08/solve.go
@@ -0,0 +1,166 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"sort"
+	"strconv"
+)
+
+type SetEntry struct {
+	parent, size int
+}
+
+type DisjointSet struct {
+	set []SetEntry
+}
+
+func NewDisjointSet(size int) *DisjointSet {
+	set := make([]SetEntry, size)
+
+	for i := range len(set) {
+		set[i].parent = i
+		set[i].size = 1
+	}
+
+	instance := new(DisjointSet)
+	instance.set = set
+
+	return instance
+}
+
+func (d *DisjointSet) Find(item int) int {
+	for d.set[item].parent != item {
+		d.set[item].parent = d.set[d.set[item].parent].parent
+		item = d.set[item].parent
+	}
+
+	return item
+}
+
+func (d *DisjointSet) Union(x, y int) bool {
+	xp := d.Find(x)
+	yp := d.Find(y)
+
+	if xp == yp {
+		return false
+	}
+
+	if yp < xp {
+		xp, yp = yp, xp
+	}
+
+	d.set[xp].size += d.set[yp].size
+	d.set[yp].parent = xp
+
+	return true
+}
+
+func (d *DisjointSet) Size(item int) int {
+	if d.set[item].parent == item {
+		return d.set[item].size
+	} else {
+		return 0
+	}
+}
+
+func read_input(filename string) [][3]int {
+	var points [][3]int
+	var point [3]int
+
+	file, err := os.Open(filename)
+
+	if err != nil {
+		panic(err)
+	}
+
+	for {
+		parsed, err := fmt.Fscanf(file, "%d,%d,%d\n", &point[0], &point[1], &point[2])
+		if err != nil || parsed != 3 {
+			break
+		}
+		points = append(points, point)
+	}
+
+	file.Close()
+
+	return points
+}
+
+func usage() {
+	fmt.Printf("Usage: %v INPUT_FILE [connections]\n", os.Args[0])
+	os.Exit(1)
+}
+
+func compute_group_sizes(groups *DisjointSet, len int) []int {
+	var sizes []int
+	for i := range len {
+		size := groups.Size(i)
+		if size > 0 {
+			sizes = append(sizes, size)
+		}
+	}
+
+	sort.Ints(sizes)
+
+	return sizes
+}
+
+func main() {
+	if len(os.Args) < 2 {
+		usage()
+	}
+
+	connections := 1000
+	if len(os.Args) >= 3 {
+		parsed, err := strconv.Atoi(os.Args[2])
+		if err != nil {
+			usage()
+		}
+		connections = parsed
+	}
+
+	points := read_input(os.Args[1])
+
+	var distances [][3]int
+
+	for i, a := range points {
+		for j := i + 1; j < len(points); j += 1 {
+			b := points[j]
+
+			square_dist := (a[0]-b[0])*(a[0]-b[0]) + (a[1]-b[1])*(a[1]-b[1]) + (a[2]-b[2])*(a[2]-b[2])
+
+			distances = append(distances, [3]int{square_dist, i, j})
+		}
+	}
+
+	sort.Slice(distances, func(i, j int) bool { return distances[i][0] < distances[j][0] })
+
+	groups := NewDisjointSet(len(points))
+
+	for _, d := range distances[:connections] {
+		groups.Union(d[1], d[2])
+	}
+
+	sizes := compute_group_sizes(groups, len(points))
+
+	product := 1
+	for i := len(sizes) - 3; i < len(sizes); i += 1 {
+		product *= sizes[i]
+	}
+
+	fmt.Printf("Part 1: %v\n", product)
+
+	to_merge := len(sizes) - 1
+	for _, d := range distances[connections:] {
+		if groups.Union(d[1], d[2]) {
+			to_merge -= 1
+			if to_merge == 0 {
+				fmt.Printf("Part 2: %v\n", points[d[1]][0]*points[d[2]][0])
+				return
+			}
+		}
+	}
+
+	println("I should not get here")
+}