From 48be4bf4bc8bdaa1233f0d6509402fd4ff15044d Mon Sep 17 00:00:00 2001
From: tiennm99 <tiennm99@outlook.com>
Date: Tue, 9 Dec 2025 21:46:02 +0700
Subject: [PATCH] feat(2025): add day9

---
 2025/day9/day9.go | 117 ++++++++++++++++++++++++++++++++++++++++++++++
 2025/day9/go.mod  |   3 ++
 README.md         |   2 +
 3 files changed, 122 insertions(+)
 create mode 100644 2025/day9/day9.go
 create mode 100644 2025/day9/go.mod

diff --git a/2025/day9/day9.go b/2025/day9/day9.go
new file mode 100644
index 0000000..90853b0
--- /dev/null
+++ b/2025/day9/day9.go
@@ -0,0 +1,117 @@
+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))
+}
diff --git a/2025/day9/go.mod b/2025/day9/go.mod
new file mode 100644
index 0000000..1dc6f79
--- /dev/null
+++ b/2025/day9/go.mod
@@ -0,0 +1,3 @@
+module day9
+
+go 1.23.4
diff --git a/README.md b/README.md
index 34da63d..ed26592 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,4 @@
 # adventofcode
 [Advent of Code](https://adventofcode.com/2025) solutions
+
+**Note**: *I use LLM to generate solutions since day 8*.