From 687cba82ddcb83855a5c5dd869ba81052f63ec4b Mon Sep 17 00:00:00 2001
From: kageru <kageru@encode.moe>
Date: Fri, 6 Dec 2019 11:44:33 +0100
Subject: [PATCH] Simplify D6P2 Kotlin further

---
 2019/06/day06.kt | 15 ++-------------
 1 file changed, 2 insertions(+), 13 deletions(-)

diff --git a/2019/06/day06.kt b/2019/06/day06.kt
index 48d4ad6..1309b4d 100644
--- a/2019/06/day06.kt
+++ b/2019/06/day06.kt
@@ -1,12 +1,12 @@
 fun main() {
     val input = generateSequence(::readLine).map { it.split(")") }.toList()
     val parentToChildren = input.groupBy { it[0] }.mapValues { it.value.map { it[1] } }
-    val childToParent = input.associateBy { it[1] }.mapValues { it.value[0] }
     val rootNode = (parentToChildren.keys - parentToChildren.values.flatten().toSet()).first()
     println("Part 1: ${countOrbiters(parentToChildren, rootNode, 0)}")
+    val childToParent = input.associateBy { it[1] }.mapValues { it.value[0] }
     val santaParents = getParents(childToParent, "SAN")
     val myParents = getParents(childToParent, "YOU")
-    val commonParent = firstCommon(santaParents, myParents)
+    val commonParent = santaParents.first { it in myParents }
     println("Part 2: ${santaParents.indexOf(commonParent) + myParents.indexOf(commonParent)}")
 }
 
@@ -15,14 +15,3 @@ fun <T> countOrbiters(graph: Map<T, List<T>>, key: T, acc: Int): Int =
 
 fun <T> getParents(graph: Map<T, T>, key: T): List<T> =
     graph[key]?.let { curr -> listOf(curr) + getParents(graph, curr) } ?: emptyList()
-
-fun <T> firstCommon(first: List<T>, second: List<T>): T {
-    val seen = mutableSetOf<T>()
-    for (i in 0..first.size) {
-        if (first[i] in seen) return first[i]
-        seen.add(first[i])
-        if (second[i] in seen) return second[i]
-        seen.add(second[i])
-    }
-    error("should be unreachable")
-}