Day 9 part 1

2024/src/main/kotlin/day9.kt

@@ -0,0 +1,65 @@
+sealed class Span {
+    abstract val length: Int
+
+    data class Empty(override val length: Int): Span()
+    data class File(val id: Int, override val  length: Int): Span()
+}
+
+fun main() {
+    var currentFileId = 0
+    val disk = readInputString("day9.txt").mapIndexed { index, c ->
+        if (index % 2 == 0) {
+            Span.File(currentFileId++, c.digitToInt())
+        } else {
+            Span.Empty(c.digitToInt())
+        }
+    }.toMutableList()
+
+    fun take(blocks: Int): Span.File {
+        var block: Span
+        do {
+            block = disk.removeLast()
+        } while (block !is Span.File)
+
+        if (block.length > blocks) {
+            disk.add(Span.File(block.id, block.length - blocks))
+            return Span.File(block.id, blocks)
+        }
+
+        return block
+    }
+
+    var index = 1
+    while (index < disk.count()) {
+        require(disk[index] is Span.Empty)
+
+        val freeSpace = disk[index].length
+        val fillData = take(freeSpace)
+        disk[index] = fillData
+
+        if (fillData.length < freeSpace) {
+            disk.add(index + 1, Span.Empty(freeSpace - fillData.length))
+            index += 1
+        } else {
+            index += 2
+        }
+    }
+
+    val checkSum = calculateCheckSum(disk)
+    println("Part 1: $checkSum")
+}
+
+private fun calculateCheckSum(disk: List<Span>): Long {
+    var checkSum = 0L
+    var currentBlock = 0L
+
+    for (file in disk) {
+        require(file is Span.File)
+
+        repeat(file.length) {
+            checkSum += currentBlock++ * file.id
+        }
+    }
+
+    return checkSum
+}