| [Day 24: Never Tell Me The Odds](https://adventofcode.com/2023/day/24) | [Solution(Part 1 only)](/adventofcode2023/Day24.kt) | [Inputs](/resources/adventofcode2023/Day24.txt) |

package adventofcode2023

import java.io.File

object Day24 {

private val inputs = File("resources/adventofcode2023/Day24.txt")

.readLines()

.map { line ->

val (position, velocity) = line.split(" @ ")

val (posX, posY, posZ) = position.split(", ").map { it.toLong() }

val (velX, velY, velZ) = velocity.split(", ").map { it.toLong() }

Hailstone(Vector3(posX, posY, posZ), Vector3(velX, velY, velZ))

}

private data class Vector2(val x: Double, val y: Double)

private data class Vector3(val x: Long, val y: Long, val z: Long)

private data class Hailstone(val position: Vector3, val velocity: Vector3)

private fun intersection2d(first: Hailstone, second: Hailstone): Vector2? {

val pos1 = first.position

val pos2 = second.position

val slope1 = first.velocity.y.toDouble() / first.velocity.x

val slope2 = second.velocity.y.toDouble() / second.velocity.x

if (slope1 == slope2) return null

val xInterjection = (pos2.y - pos1.y + slope1 * pos1.x - slope2 * pos2.x) / (slope1 - slope2)

val yInterjection = slope1 * (xInterjection - pos1.x) + pos1.y

return Vector2(xInterjection, yInterjection)

}

private fun inputsPairPermutations(): List<Pair<Hailstone, Hailstone>> =

inputs.flatMapIndexed { i, element1 ->

inputs.subList(i + 1, inputs.size).mapNotNull { element2 ->

element1.takeUnless { it === element2 }?.let { it to element2 }

}

}

fun part1(min: Long = 200000000000000, max: Long = 400000000000000) = println(

inputsPairPermutations().count {

val intersection = intersection2d(it.first, it.second)

if (intersection == null) false

else {

val t1 = (intersection.x - it.first.position.x) / it.first.velocity.x

val t2 = (intersection.x - it.second.position.x) / it.second.velocity.x

if (t1 < 0 || t2 < 0) false

else intersection.x >= min && intersection.x <= max && intersection.y >= min && intersection.y <= max

}

}

)

}

fun main() {

Day24.part1()

}