Solution for 2024, day 14

2024/14/Makefile

@@ -0,0 +1,19 @@
+input:
+	http "https://adventofcode.com/2024/day/14/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

2024/14/common.go

@@ -0,0 +1,24 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+var max = P{101, 103}
+
+type P struct{ x, y int }
+type Robot struct{ Pos, Vel P }
+
+func parseInput() []Robot {
+	robots := []Robot{}
+	scanner := bufio.NewScanner(os.Stdin)
+	for scanner.Scan() {
+		line := scanner.Text()
+		var x, y, vx, vy int
+		fmt.Sscanf(line, "p=%d,%d v=%d,%d", &x, &y, &vx, &vy)
+		robots = append(robots, Robot{P{x, y}, P{vx, vy}})
+	}
+	return robots
+}

2024/14/main1.go

@@ -0,0 +1,36 @@
+package main
+
+import "fmt"
+
+func main() {
+	robots := parseInput()
+
+	for t := 0; t < 100; t++ {
+		for i := 0; i < len(robots); i++ {
+			robots[i].Pos.x = (robots[i].Pos.x + robots[i].Vel.x + max.x) % max.x
+			robots[i].Pos.y = (robots[i].Pos.y + robots[i].Vel.y + max.y) % max.y
+		}
+	}
+
+	q := [4]int{}
+	for _, r := range robots {
+		if r.Pos.x < max.x/2 {
+			if r.Pos.y < max.y/2 {
+				q[0]++
+			}
+			if r.Pos.y > max.y/2 {
+				q[1]++
+			}
+		}
+		if r.Pos.x > max.x/2 {
+			if r.Pos.y < max.y/2 {
+				q[2]++
+			}
+			if r.Pos.y > max.y/2 {
+				q[3]++
+			}
+		}
+	}
+
+	fmt.Println(q[0] * q[1] * q[2] * q[3])
+}

2024/14/main2.go

@@ -0,0 +1,41 @@
+package main
+
+import "fmt"
+
+func main() {
+	robots := parseInput()
+
+	for t := 0; ; t++ {
+		for i := 0; i < len(robots); i++ {
+			robots[i].Pos.x = (robots[i].Pos.x + robots[i].Vel.x + max.x) % max.x
+			robots[i].Pos.y = (robots[i].Pos.y + robots[i].Vel.y + max.y) % max.y
+		}
+
+		// robots gather vertically at t=3
+		// robots gather horizontally at t=75
+		// period is known
+
+		if ((t+max.x-3)%max.x == 0) && (t+max.y-75)%max.y == 0 {
+			Print(robots)
+			fmt.Println(t + 1)
+			return
+		}
+	}
+}
+
+func Print(robots []Robot) {
+	m := map[P]struct{}{}
+	for _, r := range robots {
+		m[r.Pos] = struct{}{}
+	}
+	for y := 50; y < max.y-16; y++ {
+		for x := 25; x < max.x-39; x++ {
+			if _, ok := m[P{x, y}]; ok {
+				fmt.Print("#")
+			} else {
+				fmt.Print(".")
+			}
+		}
+		fmt.Println()
+	}
+}