Solution 2015-13 (Knights of the Dinner Table)

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/year2015/day13/KnightsOfTheDinnerTable.kt

@@ -0,0 +1,54 @@
+package de.ronny_h.aoc.year2015.day13
+
+import de.ronny_h.aoc.AdventOfCode
+import de.ronny_h.aoc.extensions.permutationsOf
+
+fun main() = KnightsOfTheDinnerTable().run(664, 640)
+
+class KnightsOfTheDinnerTable : AdventOfCode<Int>(2015, 13) {
+
+    override fun part1(input: List<String>): Int {
+        return input.parse().maxHappiness()
+    }
+
+    private fun List<HappinessPotential>.maxHappiness(): Int {
+        val happinessLookup = associate { Pair(it.person, it.nextTo) to it.happiness }
+        fun happinessChange(a: String, b: String) = happinessLookup[a to b]!! + happinessLookup[b to a]!!
+
+        val allGuests = map { it.person }.toSet().toList()
+        return permutationsOf(allGuests).maxOf { guests ->
+            var happiness = 0
+            guests.windowed(2) { (person, nextTo) ->
+                happiness += happinessChange(person, nextTo)
+            }
+            happiness += happinessChange(guests.first(), guests.last())
+            happiness
+        }
+    }
+
+    override fun part2(input: List<String>): Int {
+        val happinessPotentials = input.parse()
+        val allGuests = happinessPotentials.map { it.person }.toSet().toList()
+
+        val myHappinessPotential = allGuests.flatMap {
+            listOf(
+                HappinessPotential("me", it, 0),
+                HappinessPotential(it, "me", 0)
+            )
+        }
+        return (happinessPotentials + myHappinessPotential).maxHappiness()
+    }
+}
+
+data class HappinessPotential(val person: String, val nextTo: String, val happiness: Int)
+
+fun List<String>.parse() = this.map {
+    val (person, remainder) = it.split(" would ")
+    val sign = when {
+        remainder.startsWith("gain") -> 1
+        else -> -1
+    }
+    val (happiness, nextTo) = remainder.substring(5, remainder.lastIndex)
+        .split(" happiness units by sitting next to ")
+    HappinessPotential(person, nextTo, sign * happiness.toInt())
+}

src/test/kotlin/de/ronny_h/aoc/year2015/day13/KnightsOfTheDinnerTableTest.kt

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+package de.ronny_h.aoc.year2015.day13
+
+import io.kotest.core.spec.style.StringSpec
+import io.kotest.data.forAll
+import io.kotest.data.row
+import io.kotest.matchers.shouldBe
+
+class KnightsOfTheDinnerTableTest : StringSpec({
+
+    val input = listOf(
+        "Alice would gain 54 happiness units by sitting next to Bob.",
+        "Alice would lose 79 happiness units by sitting next to Carol.",
+        "Alice would lose 2 happiness units by sitting next to David.",
+        "Bob would gain 83 happiness units by sitting next to Alice.",
+        "Bob would lose 7 happiness units by sitting next to Carol.",
+        "Bob would lose 63 happiness units by sitting next to David.",
+        "Carol would lose 62 happiness units by sitting next to Alice.",
+        "Carol would gain 60 happiness units by sitting next to Bob.",
+        "Carol would gain 55 happiness units by sitting next to David.",
+        "David would gain 46 happiness units by sitting next to Alice.",
+        "David would lose 7 happiness units by sitting next to Bob.",
+        "David would gain 41 happiness units by sitting next to Carol.",
+    )
+
+    "input can be parsed" {
+        forAll(
+            row(
+                "Alice would gain 54 happiness units by sitting next to Bob.",
+                HappinessPotential("Alice", "Bob", 54)
+            ),
+            row(
+                "Alice would lose 79 happiness units by sitting next to Carol.",
+                HappinessPotential("Alice", "Carol", -79)
+            ),
+        ) { sentence, expected ->
+            listOf(sentence).parse() shouldBe listOf(expected)
+        }
+    }
+
+    "part 1: the optimal total change in happiness" {
+        KnightsOfTheDinnerTable().part1(input) shouldBe 330
+    }
+
+    "part 2: the optimal total change in happiness, myself included" {
+        KnightsOfTheDinnerTable().part2(input) shouldBe 286
+    }
+})