adventofcode/2025/day9/day9.go

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

type Pos struct {
	x int
	y int
}

func parse(input string) []Pos {
	lines := []Pos{}
	sc := bufio.NewScanner(strings.NewReader(input))
	for sc.Scan() {
		line := sc.Text()
		if !strings.Contains(line, ",") {
			continue
		}
		i := strings.Index(line, ",")
		x, _ := strconv.Atoi(line[:i])
		y, _ := strconv.Atoi(line[i+1:])
		lines = append(lines, Pos{x, y})
	}
	return lines
}

func part1(input string) int {
	lines := parse(input)
	maxArea := 0
	for i := range lines {
		x1, y1 := lines[i].x, lines[i].y
		for j := 0; j < i; j++ {
			x2, y2 := lines[j].x, lines[j].y
			area := (abs(x1-x2) + 1) * (abs(y1-y2) + 1)
			if area > maxArea {
				maxArea = area
			}
		}
	}
	return maxArea
}

func part2(input string) int {
	lines := parse(input)
	n := len(lines)
	maxArea := 0
	if n == 0 {
		return 0
	}

	for i := range lines {
		x1, y1 := lines[i].x, lines[i].y
		for j := 0; j < i; j++ {
			x2, y2 := lines[j].x, lines[j].y

			valid := true
			for k := 0; k < n; k++ {
				x3, y3 := lines[k].x, lines[k].y
				x4, y4 := lines[(k+1)%n].x, lines[(k+1)%n].y

				// Non-overlap check
				if !(maxi(x3, x4) <= mini(x1, x2) ||
					maxi(x1, x2) <= mini(x3, x4) ||
					maxi(y3, y4) <= mini(y1, y2) ||
					maxi(y1, y2) <= mini(y3, y4)) {

					valid = false
					break
				}
			}

			if valid {
				area := (abs(x1-x2) + 1) * (abs(y1-y2) + 1)
				if area > maxArea {
					maxArea = area
				}
			}
		}
	}

	return maxArea
}

func abs(a int) int {
	if a < 0 {
		return -a
	}
	return a
}
func mini(a, b int) int {
	if a < b {
		return a
	}
	return b
}
func maxi(a, b int) int {
	if a > b {
		return a
	}
	return b
}

func main() {
	data, err := os.ReadFile("input.txt")
	if err != nil {
		panic(err)
	}
	input := string(data)

	fmt.Println("Part 1:", part1(input))
	fmt.Println("Part 2:", part2(input))
}