Day 10

10/answers.txt

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+263
+1110

10/input.txt

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+.#..#..##.#...###.#............#.
+.....#..........##..#..#####.#..#
+#....#...#..#.......#...........#
+.#....#....#....#.#...#.#.#.#....
+..#..#.....#.......###.#.#.##....
+...#.##.###..#....#........#..#.#
+..#.##..#.#.#...##..........#...#
+..#..#.......................#..#
+...#..#.#...##.#...#.#..#.#......
+......#......#.....#.............
+.###..#.#..#...#..#.#.......##..#
+.#...#.................###......#
+#.#.......#..####.#..##.###.....#
+.#.#..#.#...##.#.#..#..##.#.#.#..
+##...#....#...#....##....#.#....#
+......#..#......#.#.....##..#.#..
+##.###.....#.#.###.#..#..#..###..
+#...........#.#..#..#..#....#....
+..........#.#.#..#.###...#.....#.
+...#.###........##..#..##........
+.###.....#.#.###...##.........#..
+#.#...##.....#.#.........#..#.###
+..##..##........#........#......#
+..####......#...#..........#.#...
+......##...##.#........#...##.##.
+.#..###...#.......#........#....#
+...##...#..#...#..#..#.#.#...#...
+....#......#.#............##.....
+#......####...#.....#...#......#.
+...#............#...#..#.#.#..#.#
+.#...#....###.####....#.#........
+#.#...##...#.##...#....#.#..##.#.
+.#....#.###..#..##.#.##...#.#..##

10/main.go

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+package main
+
+import (
+	"github.com/csmith/aoc-2019/common"
+	"math"
+	"sort"
+)
+
+type asteroid struct {
+	x, y, visible int
+}
+
+func buildMap(input []string) []*asteroid {
+	var res []*asteroid
+	for y, line := range input {
+		for x, r := range line {
+			if r == '#' {
+				res = append(res, &asteroid{x: x, y: y})
+			}
+		}
+	}
+	return res
+}
+
+func angleBetween(asteroid1, asteroid2 *asteroid) float64 {
+	if asteroid1.y == asteroid2.y {
+		if asteroid2.x > asteroid1.x {
+			return math.Pi / 2
+		} else {
+			return 3 * math.Pi / 2
+		}
+	} else {
+		angle := math.Atan(float64(asteroid2.x-asteroid1.x) / float64(asteroid1.y-asteroid2.y))
+		if asteroid1.y < asteroid2.y {
+			angle += math.Pi
+		}
+		if angle < 0 {
+			angle += math.Pi * 2
+		}
+		return angle
+	}
+}
+
+func checkAngles(asteroid1 *asteroid, others []*asteroid, countVisible bool) map[float64][]*asteroid {
+	angles := make(map[float64][]*asteroid)
+	for _, asteroid2 := range others {
+		if asteroid2 == asteroid1 {
+			continue
+		}
+
+		angle := angleBetween(asteroid1, asteroid2)
+
+		if len(angles[angle]) == 0 && countVisible {
+			asteroid1.visible++
+			asteroid2.visible++
+		}
+
+		angles[angle] = append(angles[angle], asteroid2)
+	}
+	return angles
+}
+
+func main() {
+	var (
+		input     = common.ReadFileAsStrings("10/input.txt")
+		asteroids = buildMap(input)
+		best      *asteroid
+	)
+
+	for i, asteroid1 := range asteroids {
+		checkAngles(asteroid1, asteroids[i+1:], true)
+		if best == nil || asteroid1.visible > best.visible {
+			best = asteroid1
+		}
+	}
+
+	if best == nil {
+		panic("No asteroids found?")
+	}
+
+	println(best.visible)
+
+	targets := checkAngles(best, asteroids, false)
+	angles := make([]float64, 0, len(targets))
+	for k := range targets {
+		angles = append(angles, k)
+	}
+	sort.Float64s(angles)
+
+	var destroyed *asteroid
+	var i = 0
+	for n := 0; n < 200; n++ {
+		if len(targets[angles[i]]) == 1 {
+			// There's a single target at this angle, skip the angle in the future
+			destroyed = targets[angles[i]][0]
+			angles = append(angles[:i], angles[i+1:]...)
+		} else {
+			// Multiple targets exists at this angle, remove the closest and move on to the next angle
+			var bestDistance = math.MaxFloat64
+			var bestTarget = 0
+			for j, target := range targets[angles[i]] {
+				distance := math.Abs(float64(target.x-best.x)) + math.Abs(float64(target.y-best.y))
+				if distance < bestDistance {
+					bestDistance = distance
+					bestTarget = j
+				}
+			}
+
+			destroyed = targets[angles[i]][bestTarget]
+			targets[angles[i]] = append(targets[angles[i]][:bestTarget], targets[angles[i]][bestTarget+1:]...)
+			i++
+		}
+
+		i = i % len(angles)
+	}
+
+	if destroyed == nil {
+		panic("Universe doesn't make sense. Reboot and try again?")
+	}
+
+	println(destroyed.x*100 + destroyed.y)
+}

10/main_test.go

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+package main
+
+import (
+	"math"
+	"testing"
+)
+
+func Test_angleBetween(t *testing.T) {
+	type args struct {
+		asteroid1 *asteroid
+		asteroid2 *asteroid
+	}
+	tests := []struct {
+		name string
+		args args
+		want float64
+	}{
+		{"above", args{&asteroid{x: 5, y: 5}, &asteroid{x: 5, y: 0}}, 0},
+		{"below", args{&asteroid{x: 5, y: 5}, &asteroid{x: 5, y: 10}}, math.Pi},
+		{"right", args{&asteroid{x: 5, y: 5}, &asteroid{x: 10, y: 5}}, math.Pi / 2},
+		{"left", args{&asteroid{x: 5, y: 5}, &asteroid{x: 0, y: 5}}, 3 * math.Pi / 2},
+		{"quadrant1", args{&asteroid{x: 5, y: 5}, &asteroid{x: 10, y: 0}}, math.Pi / 4},
+		{"quadrant2", args{&asteroid{x: 5, y: 5}, &asteroid{x: 10, y: 10}}, 3 * math.Pi / 4},
+		{"quadrant3", args{&asteroid{x: 5, y: 5}, &asteroid{x: 0, y: 10}}, 5 * math.Pi / 4},
+		{"quadrant4", args{&asteroid{x: 5, y: 5}, &asteroid{x: 0, y: 0}}, 7 * math.Pi / 4},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := angleBetween(tt.args.asteroid1, tt.args.asteroid2); got != tt.want {
+				t.Errorf("angleBetween() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}