2025 day 4

Author: lucianoq

2025/4/Makefile

@@ -0,0 +1,20 @@
+input:
+	http "https://adventofcode.com/2025/day/4/input" "Cookie:session=${AOC_SESSION};" >input
+
+main1:
+	go build -o main1 main1.go common.go
+
+main2:
+	go build -o main2 main2.go common.go
+
+.PHONY: run1 run2 clean
+
+run1: main1 input
+	./main1 <input
+
+run2: main2 input
+	./main2 <input
+
+clean:
+	rm -f main1 main2 input
+

2025/4/common.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"bufio"
+	"os"
+)
+
+const Size = 136
+
+type P struct{ x, y int }
+
+func parseMap() map[P]struct{} {
+	m := map[P]struct{}{}
+	scanner := bufio.NewScanner(os.Stdin)
+	for i := 0; scanner.Scan(); i++ {
+		for j, c := range scanner.Text() {
+			if c == '@' {
+				m[P{i, j}] = struct{}{}
+			}
+		}
+	}
+	return m
+}
+
+func adjacentRolls(m map[P]struct{}, x, y int) int {
+	count := 0
+	for dx := -1; dx <= 1; dx++ {
+		for dy := -1; dy <= 1; dy++ {
+			if dx == 0 && dy == 0 {
+				continue
+			}
+
+			nx, ny := x+dx, y+dy
+			if nx < 0 || nx >= Size ||
+				ny < 0 || ny >= Size {
+				continue
+			}
+
+			if _, ok := m[P{nx, ny}]; ok {
+				count++
+			}
+		}
+	}
+	return count
+}

2025/4/main1.go

@@ -0,0 +1,16 @@
+package main
+
+import "fmt"
+
+func main() {
+	m := parseMap()
+	count := 0
+	for i := 0; i < Size; i++ {
+		for j := 0; j < Size; j++ {
+			if _, ok := m[P{i, j}]; ok && adjacentRolls(m, i, j) < 4 {
+				count++
+			}
+		}
+	}
+	fmt.Println(count)
+}

2025/4/main2.go

@@ -0,0 +1,29 @@
+package main
+
+import "fmt"
+
+func main() {
+	m := parseMap()
+	count := 0
+	for {
+		toDelete := make([]P, 0)
+		for i := 0; i < Size; i++ {
+			for j := 0; j < Size; j++ {
+				if _, ok := m[P{i, j}]; ok && adjacentRolls(m, i, j) < 4 {
+					toDelete = append(toDelete, P{i, j})
+				}
+			}
+		}
+
+		if len(toDelete) == 0 {
+			break
+		}
+
+		for _, p := range toDelete {
+			delete(m, P{p.x, p.y})
+			count++
+		}
+	}
+
+	fmt.Println(count)
+}