AoC 2025 Day1
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🐙PiperYxzzy
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## \-\-\- Day 24: Never Tell Me The Odds ---
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It seems like something is going wrong with the snow-making process. Instead of forming snow, the water that's been absorbed into the air seems to be forming [hail](https://en.wikipedia.org/wiki/Hail)!
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Maybe there's something you can do to break up the hailstones?
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Due to strong, probably-magical winds, the hailstones are all flying through the air in perfectly linear trajectories. You make a note of each hailstone's _position_ and _velocity_ (your puzzle input). For example:
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```
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19, 13, 30 @ -2, 1, -2
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18, 19, 22 @ -1, -1, -2
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20, 25, 34 @ -2, -2, -4
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12, 31, 28 @ -1, -2, -1
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20, 19, 15 @ 1, -5, -3
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```
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Each line of text corresponds to the position and velocity of a single hailstone. The positions indicate where the hailstones are _right now_ (at time `0`). The velocities are constant and indicate exactly how far each hailstone will move in _one nanosecond_.
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Each line of text uses the format `px py pz @ vx vy vz`. For instance, the hailstone specified by `20, 19, 15 @ 1, -5, -3` has initial X position `20`, Y position `19`, Z position `15`, X velocity `1`, Y velocity `-5`, and Z velocity `-3`. After one nanosecond, the hailstone would be at `21, 14, 12`.
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Perhaps you won't have to do anything. How likely are the hailstones to collide with each other and smash into tiny ice crystals?
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To estimate this, consider only the X and Y axes; _ignore the Z axis_. Looking _forward in time_, how many of the hailstones' _paths_ will intersect within a test area? (The hailstones themselves don't have to collide, just test for intersections between the paths they will trace.)
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In this example, look for intersections that happen with an X and Y position each at least `7` and at most `27`; in your actual data, you'll need to check a much larger test area. Comparing all pairs of hailstones' future paths produces the following results:
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```
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Hailstone A: 19, 13, 30 @ -2, 1, -2
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Hailstone B: 18, 19, 22 @ -1, -1, -2
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Hailstones' paths will cross inside the test area (at x=14.333, y=15.333).
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Hailstone A: 19, 13, 30 @ -2, 1, -2
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Hailstone B: 20, 25, 34 @ -2, -2, -4
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Hailstones' paths will cross inside the test area (at x=11.667, y=16.667).
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Hailstone A: 19, 13, 30 @ -2, 1, -2
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Hailstone B: 12, 31, 28 @ -1, -2, -1
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Hailstones' paths will cross outside the test area (at x=6.2, y=19.4).
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Hailstone A: 19, 13, 30 @ -2, 1, -2
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Hailstone B: 20, 19, 15 @ 1, -5, -3
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Hailstones' paths crossed in the past for hailstone A.
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Hailstone A: 18, 19, 22 @ -1, -1, -2
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Hailstone B: 20, 25, 34 @ -2, -2, -4
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Hailstones' paths are parallel; they never intersect.
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Hailstone A: 18, 19, 22 @ -1, -1, -2
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Hailstone B: 12, 31, 28 @ -1, -2, -1
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Hailstones' paths will cross outside the test area (at x=-6, y=-5).
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Hailstone A: 18, 19, 22 @ -1, -1, -2
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Hailstone B: 20, 19, 15 @ 1, -5, -3
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Hailstones' paths crossed in the past for both hailstones.
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Hailstone A: 20, 25, 34 @ -2, -2, -4
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Hailstone B: 12, 31, 28 @ -1, -2, -1
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Hailstones' paths will cross outside the test area (at x=-2, y=3).
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Hailstone A: 20, 25, 34 @ -2, -2, -4
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Hailstone B: 20, 19, 15 @ 1, -5, -3
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Hailstones' paths crossed in the past for hailstone B.
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Hailstone A: 12, 31, 28 @ -1, -2, -1
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Hailstone B: 20, 19, 15 @ 1, -5, -3
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Hailstones' paths crossed in the past for both hailstones.
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```
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So, in this example, `2` hailstones' future paths cross inside the boundaries of the test area.
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However, you'll need to search a much larger test area if you want to see if any hailstones might collide. Look for intersections that happen with an X and Y position each at least `200000000000000` and at most `400000000000000`. Disregard the Z axis entirely.
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Considering only the X and Y axes, check all pairs of hailstones' future paths for intersections. _How many of these intersections occur within the test area?_
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170
2023/24/code.go
170
2023/24/code.go
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package main
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import (
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"errors"
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"fmt"
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"regexp"
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"strconv"
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"strings"
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"github.com/jpillora/puzzler/harness/aoc"
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)
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func main() {
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aoc.Harness(run)
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}
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// on code change, run will be executed 4 times:
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// 1. with: false (part1), and example input
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// 2. with: true (part2), and example input
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// 3. with: false (part1), and user input
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// 4. with: true (part2), and user input
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// the return value of each run is printed to stdout
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type Hailstone struct {
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X int
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Y int
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Z int
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Xv int
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Yv int
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Zv int
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}
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func (h *Hailstone) DoPathsCrossXY(o *Hailstone, boundMin, boundMax float64) (x, y float64, err error) {
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// Find which bounds this hailstone is going to
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hyBound, hxBound := float64(h.Y), float64(h.X)
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if h.Yv < 0 {
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hyBound = boundMin
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} else if h.Yv > 0 {
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hyBound = boundMax
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}
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if h.Xv < 0 {
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hxBound = boundMin
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} else if h.Xv > 0 {
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hxBound = boundMax
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}
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// which one reached first
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/*
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19, 13, _ @ -2, 1, _
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y=19 -2c
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x=13 + c
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*/
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}
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func (h *Hailstone) FindIntersectionXY(o *Hailstone) (ns, x, y float64, err error) {
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if h == o {
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return -1, -1, -1, errors.New("same hailstone")
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}
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yc := (float64(h.Y) - float64(o.Y)) / (float64(o.Yv) - float64(h.Yv))
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xc := (float64(h.X) - float64(o.X)) / (float64(o.Xv) - float64(h.Xv))
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// if xInt == yInt is the same, we have a match
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fmt.Printf("%v ; %v\n", xc, yc)
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if yc == xc && yc > 0 {
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return xc, float64(h.X) + float64(h.Xv)*xc, float64(h.Y) + float64(h.Yv)*yc, nil
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}
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return -1, -1, -1, errors.New("No intersect")
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// y = ax+b Y =
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/*
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19, 13, _ @ -2, 1, _
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18, 19, _ @ -1, -1, _
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1:
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y= 19 - 2c
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x= 13 + c
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hY = h.Y + c*h.Yv
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oY = o.Y + c.o.Yv
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if hY=oY:
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h.Y + c*h.Yv = o.Y + c*o.Yv
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h.Y - o.Y = c*o.Yv - c*h.Yv
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h.Y - o.Y = c(o.Yv - h.Yv)
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(h.Y - o.Y) / (o.Yv - h.Yv) =
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2:
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y= 18 - c
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x= 19 - c
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--- ?
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where does y1 = y2
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19 - 2c = 18 - c
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19 - 18 - 2c = -c
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1 = -c + 2c
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1 = c
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@ 1
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*/
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}
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func run(part2 bool, input string) any {
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hailRxp := regexp.MustCompile(`(?P<X>\d+), +(?P<Y>\d+), +(?P<Z>\d+) @ +(?P<VX>-?\d+), +(?P<VY>-?\d+), +(?P<VZ>-?\d+)`)
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stones := make([]Hailstone, 0)
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for _, l := range strings.Split(input, "\n") {
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matches := hailRxp.FindStringSubmatch(l)
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if len(matches) == 0 {
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continue
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}
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px, _ := strconv.Atoi(matches[1])
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py, _ := strconv.Atoi(matches[2])
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pz, _ := strconv.Atoi(matches[3])
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vx, _ := strconv.Atoi(matches[4])
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vy, _ := strconv.Atoi(matches[5])
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vz, _ := strconv.Atoi(matches[6])
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stones = append(stones, Hailstone{px, py, pz, vx, vy, vz})
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}
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// when you're ready to do part 2, remove this "not implemented" block
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if part2 {
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return "not implemented"
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}
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// solve part 1 here
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testMin, testMax := 7, 27
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insideRange := 0
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for _, h := range stones {
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for _, o := range stones {
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ns, x, y, err := h.FindIntersectionXY(&o)
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if err == nil {
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fmt.Printf("Intersection of %v:%v at %vns (%v, %v)\n", h, o, ns, x, y)
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if x > float64(testMin) && x < float64(testMax) && y > float64(testMin) && y < float64(testMax) {
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insideRange += 1
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}
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} else {
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fmt.Printf("ERROR %v\n", err)
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}
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}
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}
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return insideRange
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}
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@@ -1,5 +0,0 @@
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19, 13, 30 @ -2, 1, -2
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18, 19, 22 @ -1, -1, -2
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20, 25, 34 @ -2, -2, -4
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12, 31, 28 @ -1, -2, -1
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20, 19, 15 @ 1, -5, -3
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