From 9396c143d3588894d3b484ae90a3335602ac6080 Mon Sep 17 00:00:00 2001
From: kageru <kageru@encode.moe>
Date: Fri, 6 Dec 2019 11:06:35 +0100
Subject: [PATCH] Add D6P2 in Kotlin

---
 2019/06/day06.kt | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/2019/06/day06.kt b/2019/06/day06.kt
index 38e5c9b..48d4ad6 100644
--- a/2019/06/day06.kt
+++ b/2019/06/day06.kt
@@ -1,8 +1,28 @@
 fun main() {
-    val input = generateSequence(::readLine).map { it.split(")") }.groupBy { it[0] }.mapValues{ it.value.map { it[1] } }
-    val rootNode = (input.keys - input.values.flatten().toSet()).first()
-    println("Part 1: ${countOrbiters(input, rootNode, 0)}")
+    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 santaParents = getParents(childToParent, "SAN")
+    val myParents = getParents(childToParent, "YOU")
+    val commonParent = firstCommon(santaParents, myParents)
+    println("Part 2: ${santaParents.indexOf(commonParent) + myParents.indexOf(commonParent)}")
 }
 
-fun <T>countOrbiters(graph: Map<T, List<T>>, key: T, acc: Int): Int =
+fun <T> countOrbiters(graph: Map<T, List<T>>, key: T, acc: Int): Int =
     acc + (graph[key]?.map { countOrbiters(graph, it, acc+1) }?.sum() ?: 0)
+
+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")
+}