Day 9 Part 2

2024/src/main/kotlin/day9.kt

@@ -8,6 +8,7 @@ sealed class Span {
 
 fun main() {
     var currentFileId = 0
+
     val input = readInputString("day9.txt").mapIndexed { index, c ->
         if (index % 2 == 0) {
             Span.File(currentFileId++, c.digitToInt())
@@ -17,6 +18,7 @@ fun main() {
     }
 
     println("Part 1: ${part1(input)}")
+    println("Part 2: ${part2(input)}")
 }
 
 private fun part1(input: List<Span>): Long {
@@ -38,8 +40,42 @@ private fun part1(input: List<Span>): Long {
         }
     }
 
-    val checkSum = disk.calculateCheckSum()
-    return checkSum
+    return disk.calculateCheckSum()
+}
+
+private fun part2(input: List<Span>): Long {
+    val disk = input.toMutableList()
+
+    var index = disk.lastIndex
+    while (index > 0) {
+        val file = disk[index]
+        if (file !is Span.File) {
+            --index
+            continue
+        }
+
+        val target = disk.subList(0, index).indexOfFirst {
+            it is Span.Empty && it.length >= file.length
+        }
+
+        if (target != -1) {
+            disk[index] = Span.Empty(file.length)
+
+            val space = disk[target]
+
+            require(space is Span.Empty)
+
+            disk[target] = file
+            if (space.length > file.length) {
+                disk.add(target + 1, Span.Empty(space.length - file.length))
+                ++index
+            }
+        }
+
+        --index
+    }
+
+    return disk.calculateCheckSum()
 }
 
 private fun MutableList<Span>.takeFileBlocksFromEnd(blocks: Int): Span.File {
@@ -62,10 +98,11 @@ private fun List<Span>.calculateCheckSum(): Long {
     var currentBlock = 0L
 
     for (file in this) {
-        require(file is Span.File)
-
-        repeat(file.length) {
-            checkSum += currentBlock++ * file.id
+        when (file) {
+            is Span.Empty -> currentBlock += file.length
+            is Span.File -> repeat(file.length) {
+                checkSum += currentBlock++ * file.id
+            }
         }
     }