aoc-2024/cmd/day_14/main.go

package main

import (
	"bufio"
	"errors"
	"fmt"
	"io"
	"log"
	"os"
	"strconv"
	"strings"
)

type Coord struct {
	x, y int
}

type Robots struct {
	P []Coord
	V []Coord
}

func main() {
	robots := Robots{
		P: make([]Coord, 0, 16),
		V: make([]Coord, 0, 16),
	}
	r := bufio.NewReader(os.Stdin)

	// Read input
	i := 0
	for {
		i++
		line, err := r.ReadString('\n')
		if errors.Is(err, io.EOF) {
			break
		}
		if err != nil {
			log.Fatalf("While reading input: %v", err)
		}
		if len(line) > 0 {
			line = line[:len(line)-1]
		}
		fs := strings.Fields(line)
		if len(fs) != 2 {
			log.Fatalf("Unexpected number of fields in line: %#v", line)
		}

		// Position
		parts := strings.Split(fs[0], "=")
		parts = strings.Split(parts[1], ",")
		x, err := strconv.ParseInt(parts[0], 10, 32)
		if err != nil {
			log.Fatalf("While parsing x coordinate of position at line %d: %q", i, line)
		}
		y, err := strconv.ParseInt(parts[1], 10, 32)
		if err != nil {
			log.Fatalf("While parsing y coordinate of position at line %d: %q", i, line)
		}
		robots.P = append(robots.P, Coord{int(x), int(y)})

		// Velocity
		parts = strings.Split(fs[1], "=")
		parts = strings.Split(parts[1], ",")
		x, err = strconv.ParseInt(parts[0], 10, 32)
		if err != nil {
			log.Fatalf("While parsing x component of velocity at line %d: %q", i, line)
		}
		y, err = strconv.ParseInt(parts[1], 10, 32)
		if err != nil {
			log.Fatalf("While parsing y component of velocity at line %d: %q", i, line)
		}
		robots.V = append(robots.V, Coord{int(x), int(y)})
	}

	width, height := 11, 7
	counts := make(map[Coord]int)

	// Calculate counts after 100 timesteps
	for i := range robots.P {
		v := robots.V[i]
		for j := 0; j < 100; j++ {
			robots.P[i].x += v.x
			robots.P[i].y += v.y
		}
		wrappedX := addIfNegative(robots.P[i].x%width, width)
		wrappedY := addIfNegative(robots.P[i].y%height, height)
		coord := Coord{wrappedX, wrappedY}
		if _, ok := counts[coord]; !ok {
			counts[coord] = 0
		}
		counts[coord]++
	}

	xmid := width / 2
	ymid := height / 2
	quadrants := make([]int, 4)

	for k, v := range counts {
		if k.x == xmid || k.y == ymid {
			continue
		}
		switch {
		case k.x < xmid && k.y < ymid:
			quadrants[0] += v
		case k.x > xmid && k.y < ymid:
			quadrants[1] += v
		case k.x < xmid && k.y > ymid:
			quadrants[2] += v
		case k.x > xmid && k.y > ymid:
			quadrants[3] += v
		}
	}
	fmt.Printf("%d\n", quadrants[0]*quadrants[1]*quadrants[2]*quadrants[3])
}

func addIfNegative(a, b int) int {
	if a < 0 {
		return a + b
	}
	return a
}