Day 12 solution

Okay, I had to seek help on this one. The orbital period + least-common-multiple solution was not coming to me.

days/12.go

@@ -0,0 +1,156 @@
+package days
+
+import (
+	"fmt"
+	"math"
+	"strconv"
+	"strings"
+
+	u "parnic.com/aoc2019/utilities"
+)
+
+type moonData struct {
+	pos u.Vec3[int]
+	vel u.Vec3[int]
+}
+
+func (m *moonData) applyGravity(other *moonData) {
+	applyGravityAxis := func(pos1, pos2, vel1, vel2 *int) {
+		if *pos1 < *pos2 {
+			*vel1++
+			*vel2--
+		} else if *pos1 > *pos2 {
+			*vel1--
+			*vel2++
+		}
+	}
+
+	applyGravityAxis(&m.pos.X, &other.pos.X, &m.vel.X, &other.vel.X)
+	applyGravityAxis(&m.pos.Y, &other.pos.Y, &m.vel.Y, &other.vel.Y)
+	applyGravityAxis(&m.pos.Z, &other.pos.Z, &m.vel.Z, &other.vel.Z)
+}
+
+func (m *moonData) applyVelocity() {
+	m.pos.Add(m.vel)
+}
+
+func (m moonData) getPotentialEnergy() int {
+	return int(math.Abs(float64(m.pos.X))) +
+		int(math.Abs(float64(m.pos.Y))) +
+		int(math.Abs(float64(m.pos.Z)))
+}
+
+func (m moonData) getKineticEnergy() int {
+	return int(math.Abs(float64(m.vel.X))) +
+		int(math.Abs(float64(m.vel.Y))) +
+		int(math.Abs(float64(m.vel.Z)))
+}
+
+func (m moonData) getTotalEnergy() int {
+	return m.getPotentialEnergy() * m.getKineticEnergy()
+}
+
+type Day12 struct {
+	moons []*moonData
+}
+
+func (d *Day12) Parse() {
+	lines := u.GetStringLines("12p")
+	d.moons = make([]*moonData, len(lines))
+	for i, line := range lines {
+		trimmed := line[1 : len(line)-1]
+		vals := strings.Split(trimmed, ", ")
+		x, _ := strconv.Atoi(vals[0][2:])
+		y, _ := strconv.Atoi(vals[1][2:])
+		z, _ := strconv.Atoi(vals[2][2:])
+		d.moons[i] = &moonData{
+			pos: u.Vec3[int]{X: x, Y: y, Z: z},
+		}
+	}
+}
+
+func (d Day12) Num() int {
+	return 12
+}
+
+func (d Day12) copyMoons() []*moonData {
+	moons := make([]*moonData, len(d.moons))
+	for i, moon := range d.moons {
+		moonCopy := *moon
+		moons[i] = &moonCopy
+	}
+
+	return moons
+}
+
+func getAllEnergy(moons ...*moonData) int {
+	energy := 0
+	for _, moon := range moons {
+		energy += moon.getTotalEnergy()
+	}
+	return energy
+}
+
+func (d *Day12) Part1() string {
+	moons := d.copyMoons()
+
+	numSteps := 1000
+
+	for i := 0; i < numSteps; i++ {
+		for i, moon1 := range moons {
+			for _, moon2 := range moons[i+1:] {
+				moon1.applyGravity(moon2)
+			}
+
+			moon1.applyVelocity()
+		}
+	}
+
+	return fmt.Sprintf("Total energy after %d steps: %s%d%s", numSteps, u.TextBold, getAllEnergy(moons...), u.TextReset)
+}
+
+func (d *Day12) Part2() string {
+	moons := d.copyMoons()
+
+	orig := make([]u.Vec3[int], len(moons))
+	for i, moon := range moons {
+		orig[i] = moon.pos
+	}
+	period := u.Vec3[int]{}
+
+	for loops := 0; period.X == 0 || period.Y == 0 || period.Z == 0; loops++ {
+		for i, moon1 := range moons {
+			for _, moon2 := range moons[i+1:] {
+				moon1.applyGravity(moon2)
+			}
+			moon1.applyVelocity()
+		}
+
+		foundX := true
+		foundY := true
+		foundZ := true
+		for i, moon := range moons {
+			if moon.pos.X != orig[i].X || moon.vel.X != 0 {
+				foundX = false
+			}
+			if moon.pos.Y != orig[i].Y || moon.vel.Y != 0 {
+				foundY = false
+			}
+			if moon.pos.Z != orig[i].Z || moon.vel.Z != 0 {
+				foundZ = false
+			}
+		}
+		if foundX && period.X == 0 {
+			period.X = loops + 1
+		}
+		if foundY && period.Y == 0 {
+			period.Y = loops + 1
+		}
+		if foundZ && period.Z == 0 {
+			period.Z = loops + 1
+		}
+	}
+
+	stepsRequired := u.LCM(period.X, period.Y, period.Z)
+	return fmt.Sprintf("Iterations to reach a previous state: %s%d%s", u.TextBold, stepsRequired, u.TextReset)
+}