adventofcode/2023/day24/main.go

package main

import (
	"fmt"

	h "git.bullercodeworks.com/brian/adventofcode/helpers"
	"golang.org/x/exp/slices"
)

func main() {
	inp := h.StdinToStringSlice()
	part1(inp)
	fmt.Println()
	part2(inp)
}

func part1(inp []string) {
	//testMin, testMax := float64(7), float64(27) // Test Input
	testMin, testMax := float64(200000000000000), float64(400000000000000)
	var all []*hailstone
	for _, i := range inp {
		all = append(all, NewHailstone(i))
	}
	var count int
	for i := 0; i < len(all)-1; i++ {
		for j := i + 1; j < len(all); j++ {
			x, y := findIntersection(all[i], all[j])
			if x >= testMin && x <= testMax && y >= testMin && y <= testMax {
				count++
			}
		}
	}

	fmt.Println("# Part 1")
	fmt.Println(count)
}

func part2(inp []string) {
	var all []*hailstone
	for _, i := range inp {
		all = append(all, NewHailstone(i))
	}
	fmt.Println("# Part 2")
	fmt.Println(findThrow(all))
}

func findThrow(all []*hailstone) int {
	maybeX, maybeY, maybeZ := []int{}, []int{}, []int{}
	for i := 0; i < len(all)-1; i++ {
		for j := i + 1; j < len(all); j++ {
			a, b := all[i], all[j]
			if a.DX == b.DX {
				nextMaybe := findMatchingVelocity(int(b.X-a.X), int(a.DX))
				if len(maybeX) == 0 {
					maybeX = nextMaybe
				} else {
					maybeX = getIntersect(maybeX, nextMaybe)
				}
			}
			if a.DY == b.DY {
				nextMaybe := findMatchingVelocity(int(b.Y-a.Y), int(a.DY))
				if len(maybeY) == 0 {
					maybeY = nextMaybe
				} else {
					maybeY = getIntersect(maybeY, nextMaybe)
				}
			}
			if a.DZ == b.DZ {
				nextMaybe := findMatchingVelocity(int(b.Z-a.Z), int(a.DZ))
				if len(maybeZ) == 0 {
					maybeZ = nextMaybe
				} else {
					maybeZ = getIntersect(maybeZ, nextMaybe)
				}
			}
		}
	}

	if len(maybeX) == len(maybeY) && len(maybeY) == len(maybeZ) && len(maybeZ) == 1 {
		rockVel := hailstone{
			X: float64(maybeX[0]), Y: float64(maybeY[0]), Z: float64(maybeZ[0]),
		}
		hailA, hailB := all[0], all[1]
		mA := (hailA.DY - rockVel.Y) / (hailA.DX - rockVel.X)
		mB := (hailB.DY - rockVel.Y) / (hailB.DX - rockVel.X)
		cA := hailA.Y - (mA * hailA.X)
		cB := hailB.Y - (mB * hailB.X)
		xPos := (cB - cA) / (mA - mB)
		yPos := mA*xPos + cA
		time := (xPos - hailA.X) / (hailA.DX - rockVel.X)
		zPos := hailA.Z + (hailA.DZ-rockVel.Z)*time
		return int(xPos + yPos + zPos)
	}
	return h.MIN_INT
}

func getIntersect(a, b []int) []int {
	result := []int{}
	for _, val := range a {
		if slices.Contains(b, val) {
			result = append(result, val)
		}
	}
	return result
}

func findMatchingVelocity(diff, vel int) []int {
	match := []int{}
	for v := -1000; v < 1000; v++ {
		if v != vel && diff%(v-vel) == 0 {
			match = append(match, v)
		}
	}
	return match
}

func slope(h *hailstone) float64 {
	return h.DY / h.DX
}

func findIntersection(i, j *hailstone) (float64, float64) {
	if slope(i) == slope(j) {
		return -1, -1
	}
	cx := (j.Y - (j.DY/j.DX)*j.X - (i.Y - (i.DY/i.DX)*i.X)) / (i.DY/i.DX - j.DY/j.DX)
	perp := !((cx > i.X) == (i.DX > 0) && (cx > j.X) == (j.DX > 0))
	if perp {
		return -1, -1
	}

	y := ((-i.DY/j.DY)*(j.X*j.DY-j.Y*j.DX) + i.X*i.DY - i.Y*i.DX) / ((j.DX * i.DY / j.DY) - i.DX)
	x := y*j.DX/j.DY + (j.X*j.DY-j.Y*j.DX)/j.DY
	return x, y
}

type hailstone struct {
	X, Y, Z    float64
	DX, DY, DZ float64
}

func NewHailstone(inp string) *hailstone {
	h := hailstone{}
	fmt.Sscanf(inp, "%f, %f, %f @ %f, %f, %f",
		&h.X, &h.Y, &h.Z, &h.DX, &h.DY, &h.DZ)
	return &h
}

func (h hailstone) String() string {
	return fmt.Sprintf("%f, %f, %f @ %f, %f, %f",
		h.X, h.Y, h.Z, h.DX, h.DY, h.DZ)
}