day(17): add solution

Signed-off-by: Matej Focko <mfocko@redhat.com>

src/year2021/day17/Day17.kt

@@ -0,0 +1,80 @@
+package year2021.day17
+
+import readInput
+import kotlin.math.max
+import kotlin.math.sign
+
+data class Vector(val x: Int, val y: Int) {
+    operator fun plus(other: Vector): Vector = Vector(x + other.x, y + other.y)
+}
+
+data class Area(val min: Vector, val max: Vector) {
+    fun has(z: Vector): Boolean = (z.x in min.x..max.x) && (z.y in min.y..max.y)
+}
+
+fun getRange(range: String): Pair<Int, Int> = range.split("=")[1].let {
+    val (minValue, maxValue) = it.split("..").map(String::toInt)
+    minValue to maxValue
+}
+
+fun String.toArea(): Area {
+    val (_, values) = this.split(": ")
+    val (xRange, yRange) = values.split(", ")
+
+    val (minX, maxX) = getRange(xRange)
+    val (minY, maxY) = getRange(yRange)
+
+    return Area(Vector(minX, minY), Vector(maxX, maxY))
+}
+
+fun findPeak(input: Area, v0: Vector): Int? {
+    fun updateVelocity(v: Vector): Vector = Vector(v.x - v.x.sign, v.y - 1)
+
+    var v = v0
+    var p = Vector(0, 0)
+
+    var maxY = 0
+    var hitTarget = false
+
+    while (p.x <= input.max.x && p.y >= input.min.y) {
+        p += v
+        v = updateVelocity(v)
+
+        hitTarget = hitTarget || input.has(p)
+        maxY = max(p.y, maxY)
+    }
+
+    return if (hitTarget) maxY else null
+}
+
+fun part1(input: Area): Int =
+    (0..input.max.x)
+        .flatMap { x -> (input.min.y..-input.min.y).map { y -> x to y } }
+        .maxOf { (x, y) ->
+            findPeak(input, Vector(x, y)) ?: 0
+        }
+
+// x = v_0x * t + 1/2 * a_x * t^2
+// y = v_0y * t + 1/2 * -1 * t^2
+// v_fy = v_0y + a_y * t => t_up = v_0y / a_y
+
+fun part2(input: Area): Int =
+    (0..input.max.x)
+        .flatMap { x -> (input.min.y..-input.min.y).map { y -> x to y } }
+        .count { (x, y) ->
+            findPeak(input, Vector(x, y)) != null
+        }
+
+fun main() {
+    val sample = readInput(17, "sample").first().toArea()
+    val input = readInput(17, "input").first().toArea()
+
+    println("x: 0..${sample.max.x}, y: ${sample.min.y}..${-sample.min.y}")
+    println("x: 0..${input.max.x}, y: ${input.min.y}..${-input.min.y}")
+
+    check(part1(sample) == 45)
+    println(part1(input))
+
+    check(part2(sample) == 112)
+    println(part2(input))
+}

src/year2021/day17/sample.txt

@@ -0,0 +1 @@
+target area: x=20..30, y=-10..-5