day(17): add solution
Signed-off-by: Matej Focko <mfocko@redhat.com>
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src/year2021/day17/Day17.kt
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src/year2021/day17/Day17.kt
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package year2021.day17
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import readInput
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import kotlin.math.max
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import kotlin.math.sign
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data class Vector(val x: Int, val y: Int) {
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operator fun plus(other: Vector): Vector = Vector(x + other.x, y + other.y)
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}
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data class Area(val min: Vector, val max: Vector) {
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fun has(z: Vector): Boolean = (z.x in min.x..max.x) && (z.y in min.y..max.y)
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}
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fun getRange(range: String): Pair<Int, Int> = range.split("=")[1].let {
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val (minValue, maxValue) = it.split("..").map(String::toInt)
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minValue to maxValue
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}
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fun String.toArea(): Area {
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val (_, values) = this.split(": ")
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val (xRange, yRange) = values.split(", ")
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val (minX, maxX) = getRange(xRange)
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val (minY, maxY) = getRange(yRange)
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return Area(Vector(minX, minY), Vector(maxX, maxY))
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}
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fun findPeak(input: Area, v0: Vector): Int? {
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fun updateVelocity(v: Vector): Vector = Vector(v.x - v.x.sign, v.y - 1)
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var v = v0
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var p = Vector(0, 0)
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var maxY = 0
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var hitTarget = false
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while (p.x <= input.max.x && p.y >= input.min.y) {
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p += v
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v = updateVelocity(v)
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hitTarget = hitTarget || input.has(p)
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maxY = max(p.y, maxY)
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}
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return if (hitTarget) maxY else null
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}
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fun part1(input: Area): Int =
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(0..input.max.x)
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.flatMap { x -> (input.min.y..-input.min.y).map { y -> x to y } }
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.maxOf { (x, y) ->
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findPeak(input, Vector(x, y)) ?: 0
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}
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// x = v_0x * t + 1/2 * a_x * t^2
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// y = v_0y * t + 1/2 * -1 * t^2
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// v_fy = v_0y + a_y * t => t_up = v_0y / a_y
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fun part2(input: Area): Int =
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(0..input.max.x)
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.flatMap { x -> (input.min.y..-input.min.y).map { y -> x to y } }
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.count { (x, y) ->
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findPeak(input, Vector(x, y)) != null
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}
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fun main() {
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val sample = readInput(17, "sample").first().toArea()
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val input = readInput(17, "input").first().toArea()
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println("x: 0..${sample.max.x}, y: ${sample.min.y}..${-sample.min.y}")
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println("x: 0..${input.max.x}, y: ${input.min.y}..${-input.min.y}")
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check(part1(sample) == 45)
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println(part1(input))
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check(part2(sample) == 112)
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println(part2(input))
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}
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1
src/year2021/day17/sample.txt
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src/year2021/day17/sample.txt
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target area: x=20..30, y=-10..-5
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