2024 Day 09 Part 2

Author: fibsifan

src/main/kotlin/de/jball/aoc2024/day09/Day09.kt

@@ -2,33 +2,64 @@ package de.jball.aoc2024.day09
 
 import de.jball.AdventOfCodeDay
 
-class Day09(test: Boolean = false): AdventOfCodeDay<Long>(test, 1928, 0) {
-	val fileDb = input[0].windowed(2, 2, partialWindows = true).flatMapIndexed { index, blocks ->
-		val fileSize = blocks.first().toString().toInt()
-		val gapSize = if (blocks.length > 1) blocks.last().toString().toInt() else 0
-		val file = List<Long?>(fileSize) { index.toLong() }
-		val gap = List<Long?>(gapSize) { null }
-		file + gap
-	}.toMutableList()
+class Day09(test: Boolean = false): AdventOfCodeDay<Long>(test, 1928, 2858) {
+	val fileDb = input[0].windowed(2, 2, partialWindows = true)
+		.map { blocks ->
+			val fileSize = blocks.first().toString().toInt()
+			val gapSize = if (blocks.length > 1) blocks.last().toString().toInt() else 0
+			Pair(fileSize, gapSize)
+		}
 
 	override fun part1(): Long {
-		var currentGap = fileDb.indexOfFirst { it == null }
-		var lastValue = fileDb.indexOfLast { it != null }
+		val defragmented = fileDb.flatMapIndexed { index, (fileSize, gapSize) ->
+			val file = List<Long?>(fileSize) { index.toLong() }
+			val gap = List<Long?>(gapSize) { null }
+			file + gap
+		}.toMutableList()
+
+		var currentGap = defragmented.indexOfFirst { it == null }
+		var lastValue = defragmented.indexOfLast { it != null }
 
 		while(currentGap < lastValue) {
-			fileDb[currentGap] = fileDb[lastValue]
-			fileDb[lastValue] = null
-			currentGap = fileDb.indexOfFirst { it == null }
-			lastValue = fileDb.indexOfLast { it != null }
+			defragmented[currentGap] = defragmented[lastValue]
+			defragmented[lastValue] = null
+			currentGap = defragmented.indexOfFirst { it == null }
+			lastValue = defragmented.indexOfLast { it != null }
 		}
-		return fileDb.filter { it != null }
+		return defragmented.filter { it != null }
 			.mapIndexed { index, entry ->
 				index.toLong() * entry!!
 			}.sum()
 	}
 
 	override fun part2(): Long {
-		TODO("Not yet implemented")
+		val defragmented = FileSystem(fileDb.mapIndexed { index, (fileSize, gapSize) ->
+			FileWithGap(index, fileSize, gapSize)
+		}.toMutableList())
+
+		for (currentFile in (fileDb.size - 1 downTo 0)) {
+			var currentGap = 0
+			val currentFilePos = defragmented.files.indexOfFirst { it.id == currentFile }
+			val (_, currentFileSize, currentGapSize) = defragmented.files[currentFilePos]
+			while (defragmented.files[currentGap].id != currentFile) {
+				if (defragmented.files[currentGap].gap >= currentFileSize) {
+					val (file, fileSize, gap) = defragmented.files[currentGap]
+					defragmented.files[currentGap] = FileWithGap(file, fileSize, 0)
+					defragmented.files.add(currentGap + 1, FileWithGap(currentFile, currentFileSize, gap-currentFileSize))
+					defragmented.files.removeAt(currentFilePos+1)
+					val (previousFile, previousFileSize, previousGap) = defragmented.files[currentFilePos]
+					defragmented.files[currentFilePos] = FileWithGap(previousFile, previousFileSize, previousGap+currentFileSize+currentGapSize)
+					break
+				}
+				currentGap++
+			}
+		}
+
+		val reduced = defragmented.files.fold(Pair(0L,0L)) { (index, acc), nextFile ->
+			val value = (index until index + nextFile.size).sumOf { it * nextFile.id }
+			Pair(index + nextFile.size + nextFile.gap, value + acc)
+		}
+		return reduced.second
 	}
 }
 

src/main/kotlin/de/jball/aoc2024/day09/FileSystem.kt

@@ -0,0 +1,7 @@
+package de.jball.aoc2024.day09
+
+data class FileSystem(val files: MutableList<FileWithGap>) {
+	override fun toString(): String {
+		return files.joinToString("")
+	}
+}

src/main/kotlin/de/jball/aoc2024/day09/FileWithGap.kt

@@ -0,0 +1,7 @@
+package de.jball.aoc2024.day09
+
+data class FileWithGap(val id: Int, val size: Int, val gap: Int) {
+	override fun toString(): String {
+		return List(size) { id.toString() }.joinToString("") + List(gap) { "." }.joinToString("")
+	}
+}

src/test/kotlin/de/jball/aoc2024/Tests2024.kt

@@ -54,7 +54,6 @@ class Tests2024 {
 	}
 
 	@Test
-	@Ignore
 	fun day09() {
 		Day09(true).run()
 	}