Solution 2017-24 (Electromagnetic Moat)

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/year2017/day24/ElectromagneticMoat.kt

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+package de.ronny_h.aoc.year2017.day24
+
+import de.ronny_h.aoc.AdventOfCode
+import de.ronny_h.aoc.extensions.collections.filterMaxBy
+
+fun main() = ElectromagneticMoat().run(1906, 1824)
+
+class ElectromagneticMoat : AdventOfCode<Int>(2017, 24) {
+    override fun part1(input: List<String>): Int {
+        return buildStrongestBridge(0, 0, input.parseComponents())
+    }
+
+    override fun part2(input: List<String>): Int {
+        return buildLongestBridge(0, emptyList(), input.parseComponents()).strength()
+    }
+}
+
+fun List<String>.parseComponents() = map {
+    val (port1, port2) = it.split("/")
+    Component(port1.toInt(), port2.toInt())
+}
+
+data class Component(private val port1: Int, private val port2: Int) {
+    fun hasPort(port: Int): Boolean = port1 == port || port2 == port
+    fun other(port: Int) = if (port1 != port) port1 else port2
+    fun strength() = port1 + port2
+}
+
+private fun List<Component>.strength() = sumOf(Component::strength)
+
+fun buildStrongestBridge(lastPort: Int, strength: Int, remaining: List<Component>): Int {
+    if (remaining.isEmpty()) {
+        return strength
+    }
+
+    val matching = remaining.filter { it.hasPort(lastPort) }
+    if (matching.isEmpty()) {
+        return strength
+    }
+
+    return matching.maxOf { buildStrongestBridge(it.other(lastPort), strength + it.strength(), remaining - it) }
+}
+
+fun buildLongestBridge(lastPort: Int, bridge: List<Component>, remaining: List<Component>): List<Component> {
+    if (remaining.isEmpty()) {
+        return bridge
+    }
+
+    val matching = remaining.filter { it.hasPort(lastPort) }
+    if (matching.isEmpty()) {
+        return bridge
+    }
+
+    return matching
+        .map { buildLongestBridge(it.other(lastPort), bridge + it, remaining - it) }
+        .filterMaxBy(List<Component>::size)
+        .maxBy(List<Component>::strength)
+}

src/test/kotlin/de/ronny_h/aoc/year2017/day24/ElectromagneticMoatTest.kt

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+package de.ronny_h.aoc.year2017.day24
+
+import de.ronny_h.aoc.extensions.asList
+import io.kotest.core.spec.style.StringSpec
+import io.kotest.matchers.shouldBe
+
+class ElectromagneticMoatTest : StringSpec({
+
+    val input = """
+        0/2
+        2/2
+        2/3
+        3/4
+        3/5
+        0/1
+        10/1
+        9/10
+    """.asList()
+
+    "components can be parsed" {
+        input.parseComponents() shouldBe listOf(
+            Component(0, 2),
+            Component(2, 2),
+            Component(2, 3),
+            Component(3, 4),
+            Component(3, 5),
+            Component(0, 1),
+            Component(10, 1),
+            Component(9, 10),
+        )
+    }
+
+    "part 1: the strongest bridge's strength is 31" {
+        ElectromagneticMoat().part1(input) shouldBe 31
+    }
+
+    "part 2: the longest bridge's strength is 19" {
+        ElectromagneticMoat().part2(input) shouldBe 19
+    }
+})