day 15

15/main.go

@@ -0,0 +1,161 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+	"time"
+)
+
+func mustAtoi(line string) int {
+	i, _ := strconv.Atoi(line)
+	return i
+}
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+func makeScanner(test bool) *bufio.Scanner {
+	var f *os.File
+	if test {
+		f, _ = os.Open("inputs/testinput")
+	} else {
+		f, _ = os.Open("inputs/input")
+	}
+	reader := bufio.NewReader(f)
+	return bufio.NewScanner(reader)
+}
+
+type coord struct {
+	x int
+	y int
+}
+
+func abs(i, j int) int {
+	if i < j {
+		return j - i
+	}
+	return i - j
+}
+
+func getCoords(x, y string, _ bool) coord {
+	return coord{x: mustAtoi(x), y: mustAtoi(y)}
+}
+
+func distance(sensor, beacon coord) int {
+	return abs(sensor.x, beacon.x) + abs(sensor.y, beacon.y)
+}
+
+func partOne() {
+	test := false
+	scanner := makeScanner(test)
+	row := 2000000
+	if test {
+		row = 10
+	}
+	cave := map[coord]int{} // 0 for unknown space, 1 for sensor, 2 for beacon, 3 for known empty
+
+	for scanner.Scan() {
+		line := scanner.Text()
+		s, b, _ := strings.Cut(line, ": ")
+		sensor := getCoords(strings.Cut(strings.Trim(s, "Sensor at x="), ", y="))
+		beacon := getCoords(strings.Cut(strings.Trim(b, "closest beacon is at x="), ", y="))
+		dist := distance(sensor, beacon)
+		cave[sensor] = 1
+		cave[beacon] = 2
+		rel := distance(sensor, coord{x: sensor.x, y: row})
+		if rel <= dist {
+			c := dist - rel
+			for i := sensor.x - c; i <= sensor.x+c; i++ {
+				if _, ok := cave[coord{x: i, y: row}]; !ok {
+					cave[coord{x: i, y: row}] = 3
+				}
+			}
+		}
+	}
+	sum := 0
+	for _, i := range cave {
+		if i == 3 {
+			sum++
+		}
+	}
+	fmt.Println(sum)
+}
+
+func sensorHalo(sensor coord, distance int) []coord {
+	halo := []coord{}
+	corners := []coord{
+		{x: sensor.x, y: sensor.y + distance + 1},
+		{x: sensor.x + distance + 1, y: sensor.y},
+		{x: sensor.x, y: sensor.y - distance - 1},
+		{x: sensor.x - distance - 1, y: sensor.y},
+	}
+	y := corners[0].y
+	for i := corners[0].x; i < corners[1].x; i++ {
+		halo = append(halo, coord{i, y})
+		y--
+	}
+	for i := corners[1].x; i > corners[2].x; i-- {
+		halo = append(halo, coord{i, y})
+		y--
+	}
+	for i := corners[2].x; i > corners[3].x; i-- {
+		halo = append(halo, coord{i, y})
+		y++
+	}
+	for i := corners[3].x; i > corners[0].x; i++ {
+		halo = append(halo, coord{i, y})
+		y++
+	}
+	halo = append(halo, corners[3])
+	return halo
+}
+
+func partTwo() {
+	test := false
+	scanner := makeScanner(test)
+	max := 4000000
+	if test {
+		max = 20
+	}
+	cave := map[coord]int{} // sensor: distance to beacon
+
+	for scanner.Scan() {
+		line := scanner.Text()
+		s, b, _ := strings.Cut(line, ": ")
+		sensor := getCoords(strings.Cut(strings.Trim(s, "Sensor at x="), ", y="))
+		beacon := getCoords(strings.Cut(strings.Trim(b, "closest beacon is at x="), ", y="))
+		dist := distance(sensor, beacon)
+		cave[sensor] = dist
+	}
+	// since there's only one of these holes in the search space,
+	// it must be just outside the maximum distance from a sensor
+	// it'll take far too long to search the entire cavern, after all
+	possibleCoords := []coord{}
+	for sensor, dist := range cave {
+		possibleCoords = append(possibleCoords, sensorHalo(sensor, dist)...)
+	}
+	for _, poss := range possibleCoords {
+		if poss.x < 0 || poss.y < 0 || poss.x > max || poss.y > max {
+			continue
+		}
+		hit := false
+		for sensor, dist := range cave {
+			if distance(sensor, poss) < dist {
+				hit = true
+			}
+		}
+		if !hit {
+			fmt.Println(poss.x*4000000 + poss.y)
+			return
+		}
+	}
+}