advent2022/solutions/day15/day15.go

package day15

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

type Point struct {
	x int
	y int
}

func pointDistance(a Point, b Point) int {
	return diff(a.x, b.x) + diff(a.y, b.y)
}

func diff(a int, b int) int {
	if a > b {
		return a - b
	} else {
		return b - a
	}
}

type Sensor struct {
	location Point
	distance int
}

func parseLine(line string) (sensor Sensor, beacon Point) {
	var sensorX int
	var sensorY int
	var beaconX int
	var beaconY int

	fmt.Sscanf(line, "Sensor at x=%d, y=%d: closest beacon is at x=%d, y=%d", &sensorX, &sensorY, &beaconX, &beaconY)
	sensorLoc := Point{sensorX, sensorY}
	beacon = Point{beaconX, beaconY}
	distance := pointDistance(sensorLoc, beacon)
	sensor = Sensor{sensorLoc, distance}
	return
}

func isBeacon(x int, y int, beacons []Point) bool {
	for _, beacon := range beacons {
		if beacon.x == x && beacon.y == y {
			return true
		}
	}
	return false
}

func inSensorRange(x int, y int, sensors []Sensor) *Sensor {
	point := Point{x, y}
	for _, sensor := range sensors {
		distance := pointDistance(point, sensor.location)
		if distance <= sensor.distance {
			return &sensor
		}
	}
	return nil
}

func leftmostPoint(sensors []Sensor) int {
	leftmost := 0

	for _, sensor := range sensors {
		candidate := sensor.location.x - sensor.distance
		if candidate < leftmost {
			leftmost = candidate
		}
	}

	return leftmost
}

func rightmostPoint(sensors []Sensor) int {
	rightmost := 0

	for _, sensor := range sensors {
		candidate := sensor.location.x + sensor.distance
		if candidate > rightmost {
			rightmost = candidate
		}
	}

	return rightmost
}

func addLinePoints(pointSet *[]Point, start Point, end Point) {
	offsetX := 1
	offsetY := 1

	if end.x < start.x {
		offsetX = -1
	}
	if end.y < start.y {
		offsetY = -1
	}

	for x, y := start.x, start.y; y != end.y; x, y = x+offsetX, y+offsetY {
		*pointSet = append(*pointSet, Point{x, y})
	}
}

func addEdgePoints(pointSet *[]Point, sensor Sensor) {
	distance := sensor.distance + 1
	top := Point{sensor.location.x, sensor.location.y - distance}
	right := Point{sensor.location.x + distance, sensor.location.y}
	bottom := Point{sensor.location.x, sensor.location.y + distance}
	left := Point{sensor.location.x - distance, sensor.location.y}

	addLinePoints(pointSet, top, right)
	addLinePoints(pointSet, right, bottom)
	addLinePoints(pointSet, bottom, left)
	addLinePoints(pointSet, left, top)
}

func Run(filename string, yStr string, maxCoordStr string) {
	y, err := strconv.Atoi(yStr)
	if err != nil {
		fmt.Println("PANIC: bad y argument", yStr)
		return
	}

	maxCoord, err := strconv.Atoi(maxCoordStr)
	if err != nil {
		fmt.Println("PANIC: bad y argument", maxCoordStr)
		return
	}

	file, err := os.Open(filename)

	if err != nil {
		fmt.Println("Error opening file:", err)
		return
	}

	scanner := bufio.NewScanner(file)
	scanner.Split(bufio.ScanLines)
	sensors := []Sensor{}
	beacons := []Point{}

	for scanner.Scan() {
		line := scanner.Text()
		sensor, beacon := parseLine(line)
		sensors = append(sensors, sensor)
		beacons = append(beacons, beacon)
	}

	file.Close()

	fmt.Println(sensors)

	count := 0
	for x := leftmostPoint(sensors); x <= rightmostPoint(sensors); x++ {
		if isBeacon(x, y, beacons) {
			continue
		}

		if inSensorRange(x, y, sensors) != nil {
			count += 1
		}
	}
	fmt.Println("Beacons couldn't be in", count, "places on line", y)

	candidates := []Point{}
	for _, sensor := range sensors {
		addEdgePoints(&candidates, sensor)
	}

	for _, candidate := range candidates {
		if candidate.x >= 0 && candidate.y >= 0 && candidate.x <= maxCoord && candidate.y <= maxCoord {
			closestSensor := inSensorRange(candidate.x, candidate.y, sensors)
			if closestSensor == nil {
				fmt.Printf("Found possible beacon location at (%d, %d). Frequency value is %d\n", candidate.x, candidate.y, candidate.x*4000000+candidate.y)
			}
		}
	}
}