Day 5 Part 2

2024/src/main/kotlin/day5.kt

@@ -4,12 +4,12 @@ fun main() {
     var completedRules = false
 
     var part1 = 0
+    var part2 = 0
 
     for (line in readInput("day5.txt")) {
         if (completedRules) {
             val job = line.split(",").map { it.toInt() }
             assert(job.count() % 2 != 0)
-            val middlePage = job[job.count() / 2]
 
             var matches = true
             for ((first, second) in rules) {
@@ -23,7 +23,23 @@ fun main() {
             }
 
             if (matches) {
+                val middlePage = job[job.count() / 2]
                 part1 += middlePage
+            } else {
+                val usedRules = rules.filter { (first, second) -> job.contains(first) && job.contains(second) }
+
+                val pages = mutableMapOf<Int, Page>()
+                for (rule in usedRules) {
+                    val first = pages.getOrPut(rule.first) { Page(rule.first) }
+                    val second = pages.getOrPut(rule.second) { Page(rule.second) }
+
+                    second.previous.add(first)
+                }
+
+                val orderedJob = pages.values.sortedBy { it.previous.count() }.map { it.number }
+
+                val middlePage = orderedJob[orderedJob.count() / 2]
+                part2 += middlePage
             }
         } else if (line.isEmpty()) {
             completedRules = true
@@ -34,5 +50,9 @@ fun main() {
     }
 
     println("Part 1: $part1")
+    println("Part 2: $part2")
+}
 
+data class Page(val number: Int) {
+    val previous = mutableSetOf<Page>()
 }