2021 Day 22 Part 2

2021/day22/main.go

@@ -10,6 +10,7 @@ func main() {
 	lines := aoc.Input().ReadLines("day22.txt")
 	steps := parseInput(lines)
 	fmt.Println(part1(steps))
+	fmt.Println(part2(steps))
 }
 
 func parseInput(lines []string) (steps []Step) {
@@ -25,6 +26,10 @@ func part1(steps []Step) int {
 	return r.CountOn()
 }
 
+func part2(steps []Step) int {
+	return reboot(steps)
+}
+
 type Step struct {
 	op     string
 	x1, x2 int
@@ -80,3 +85,146 @@ func (r Reactor) CountOn() (count int) {
 	}
 	return
 }
+
+func reboot(steps []Step) int {
+	var cells []Cuboid
+	for _, s := range steps {
+		c := Cuboid{
+			x: Interval{s.x1, s.x2},
+			y: Interval{s.y1, s.y2},
+			z: Interval{s.z1, s.z2},
+		}
+		cells = cut(cells, c)
+		if s.op == "on" {
+			cells = append(cells, c)
+		}
+	}
+	return countOn(cells)
+}
+
+func countOn(cells []Cuboid) (count int) {
+	for _, cell := range cells {
+		count += cell.Size()
+	}
+	return
+}
+
+func cut(cells []Cuboid, cutter Cuboid) (stillOn []Cuboid) {
+	for _, cell := range cells {
+		ix := cell.Intersect(cutter)
+		if ix.Empty() {
+			stillOn = append(stillOn, cell)
+			continue
+		}
+		mm := cell.Minus(*ix)
+		stillOn = append(stillOn, mm...)
+	}
+	return
+}
+
+type Cuboid struct {
+	x, y, z Interval
+}
+
+func (c Cuboid) String() string {
+	return fmt.Sprintf("x: %s, y: %s, z: %s", c.x, c.y, c.z)
+}
+
+func (c *Cuboid) Empty() bool {
+	return c == nil || c.x.Empty() || c.y.Empty() || c.z.Empty()
+}
+
+func (c Cuboid) Size() int {
+	return c.x.Size() * c.y.Size() * c.z.Size()
+}
+
+func (c Cuboid) Intersect(d Cuboid) (intersection *Cuboid) {
+	ix := c.x.Intersect(d.x)
+	if ix.Empty() {
+		return nil
+	}
+
+	iy := c.y.Intersect(d.y)
+	if iy.Empty() {
+		return nil
+	}
+
+	iz := c.z.Intersect(d.z)
+	if iz.Empty() {
+		return nil
+	}
+
+	return &Cuboid{ix, iy, iz}
+}
+
+func (c Cuboid) Minus(d Cuboid) []Cuboid {
+	x0, x1, x2 := c.x.Split(d.x)
+	y0, y1, y2 := c.y.Split(d.y)
+	z0, z1, z2 := c.z.Split(d.z)
+	return nonEmpty([]Cuboid{
+		{x0, y0, z0},
+		{x0, y0, z1},
+		{x0, y0, z2},
+		{x0, y1, z0},
+		{x0, y1, z1},
+		{x0, y1, z2},
+		{x0, y2, z0},
+		{x0, y2, z1},
+		{x0, y2, z2},
+		{x1, y0, z0},
+		{x1, y0, z1},
+		{x1, y0, z2},
+		{x1, y1, z0},
+		{x1, y1, z2},
+		{x1, y2, z0},
+		{x1, y2, z1},
+		{x1, y2, z2},
+		// {x1, y1, z1}, // Don't include the intersection block
+		{x2, y0, z0},
+		{x2, y0, z1},
+		{x2, y0, z2},
+		{x2, y1, z0},
+		{x2, y1, z1},
+		{x2, y1, z2},
+		{x2, y2, z0},
+		{x2, y2, z1},
+		{x2, y2, z2},
+	})
+}
+
+func nonEmpty(cs []Cuboid) (nonEmpty []Cuboid) {
+	for _, c := range cs {
+		if !c.Empty() {
+			nonEmpty = append(nonEmpty, c)
+		}
+	}
+	return
+}
+
+type Interval [2]int
+
+func (i Interval) String() string {
+	return fmt.Sprintf("[%d, %d]", i[0], i[1])
+}
+
+func (i Interval) Empty() bool {
+	return i.Size() <= 0
+}
+
+func (i Interval) Size() int {
+	return i[1] - i[0] + 1
+}
+
+func (i Interval) Intersect(j Interval) Interval {
+	return Interval{
+		max(i[0], j[0]),
+		min(i[1], j[1]),
+	}
+}
+
+func (i Interval) Split(j Interval) (prefix, inside, suffix Interval) {
+	prefix = Interval{i[0], j[0] - 1}
+	inside = Interval{j[0], j[1]}
+	suffix = Interval{j[1] + 1, i[1]}
+	return
+}