feat(2025): add day4

2025/day4/day4.go

@@ -0,0 +1,130 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+var arounds = []Vec2{
+	{-1, -1},
+	{-1, 0},
+	{-1, 1},
+	{0, -1},
+	{0, 1},
+	{1, -1},
+	{1, 0},
+	{1, 1},
+}
+
+type Vec2 struct {
+	X, Y int
+}
+
+func isValid(input []string, pos Vec2) bool {
+	if input[pos.X][pos.Y] != '@' {
+		return false
+	}
+	count := 0
+	for _, around := range arounds {
+		x := pos.X + around.X
+		y := pos.Y + around.Y
+		if x < 0 || x >= len(input) || y < 0 || y >= len(input[x]) {
+			continue
+		}
+		if input[x][y] == '@' {
+			count++
+		}
+	}
+	return count < 4
+}
+
+func count(input []string) int {
+	count := 0
+	for i, l := range input {
+		for j := range l {
+			if isValid(input, Vec2{i, j}) {
+				count++
+			}
+		}
+	}
+	return count
+}
+
+func part1() {
+	data, _ := os.ReadFile("input.txt")
+	content := strings.TrimSpace(string(data))
+	lines := strings.Split(content, "\n")
+	count := count(lines)
+	fmt.Printf("%d\n", count)
+}
+
+func isValid2(input [][]rune, pos Vec2) bool {
+	if input[pos.X][pos.Y] != '@' {
+		return false
+	}
+	count := 0
+	for _, around := range arounds {
+		x := pos.X + around.X
+		y := pos.Y + around.Y
+		if x < 0 || x >= len(input) || y < 0 || y >= len(input[x]) {
+			continue
+		}
+		if input[x][y] == '@' {
+			count++
+		}
+	}
+	return count < 4
+}
+
+func count2(input []string) int {
+	runes := make([][]rune, len(input))
+	for i, l := range input {
+		runes[i] = []rune(l)
+	}
+	queue := make(map[Vec2]bool)
+	for i, l := range runes {
+		for j := range l {
+			if runes[i][j] == '@' {
+				queue[Vec2{i, j}] = true
+			}
+		}
+	}
+	accepted := make(map[Vec2]bool)
+	for len(queue) > 0 {
+		for pos, _ := range queue {
+			delete(queue, pos)
+			if isValid2(runes, pos) {
+				accepted[pos] = true
+				runes[pos.X][pos.Y] = '.'
+				for _, around := range arounds {
+					x := pos.X + around.X
+					y := pos.Y + around.Y
+					if x < 0 || x >= len(runes) || y < 0 || y >= len(runes[x]) {
+						continue
+					}
+					if accepted[Vec2{x, y}] {
+						continue
+					}
+					if runes[x][y] == '@' {
+						queue[Vec2{x, y}] = true
+					}
+				}
+			}
+		}
+	}
+	return len(accepted)
+}
+
+func part2() {
+	data, _ := os.ReadFile("input.txt")
+	content := strings.TrimSpace(string(data))
+	lines := strings.Split(content, "\n")
+	count := count2(lines)
+	fmt.Printf("%d\n", count)
+}
+
+func main() {
+	part1()
+	part2()
+}

2025/day4/go.mod

@@ -0,0 +1,3 @@
+module day4
+
+go 1.23.4