aoc_2024/day09.kts

#!/usr/bin/env kotlin
import java.lang.RuntimeException
import java.util.Scanner

val scanner = Scanner(System.`in`)
if (!scanner.hasNext()) {
    throw RuntimeException("No input given.")
}
val fileSystem = mutableListOf<Int>()
val fileSystemDesc = scanner.nextLine().split("").filter { it.isNotEmpty() }.map { it.toInt() }.toMutableList()
for (i in 0..fileSystemDesc.lastIndex) {
    for (j in 0 until fileSystemDesc[i]) {
        if ((i and 1) == 0) {
            fileSystem.add(i / 2)
        } else {
            fileSystem.add(-1)
        }
    }
}
val compressed = compressPartOne(fileSystem)
val checksumOne = calculateChecksum(compressed)

println("Part one checksum: $checksumOne")

val compressedTwo = compressPartTwo(fileSystem)
val checksumTwo = calculateChecksum(compressedTwo)

println("Part two checksum: $checksumTwo")

fun compressPartOne(fileSystem: MutableList<Int>): MutableList<Int> {
    val compressed = fileSystem.toMutableList()
    var freeSpacePointer = 0
    var lastSectorPointer = compressed.lastIndex
    while (freeSpacePointer <= lastSectorPointer) {
        if (compressed[freeSpacePointer] != -1) {
            freeSpacePointer++
            continue
        }
        if (compressed[lastSectorPointer] == -1) {
            lastSectorPointer--
            continue
        }
        val tmp = compressed[freeSpacePointer]
        compressed[freeSpacePointer] = compressed[lastSectorPointer]
        compressed[lastSectorPointer] = tmp
        lastSectorPointer--
        freeSpacePointer++
    }
    return compressed
}

fun calculateChecksum(compressed: MutableList<Int>): Long {
    var checksum = 0L
    for (i in 0..compressed.lastIndex) {
        if (compressed[i] != -1) {
            checksum += i.toLong() * compressed[i].toLong()
        }
    }
    return checksum
}

fun compressPartTwo(fileSystem: MutableList<Int>): MutableList<Int> {
    val compressed = fileSystem.toMutableList()
    var relevantFileId = compressed[compressed.lastIndex]

    for (i in compressed.lastIndex downTo 0) {
        if (compressed[i] == relevantFileId && relevantFileId >= 0) {
            val fileSize = findFileSize(compressed, i, relevantFileId)
            val startIndex = findStartIndexOfEmptySpaceOfLength(compressed, fileSize)
            if (startIndex != -1 && startIndex < i) {
                for (j in 0 until fileSize) {
                    compressed[startIndex + j] = compressed[i - j]
                    compressed[i - j] = -1
                }
            }
            relevantFileId--
        }
    }
    return compressed
}

fun findFileSize(compressed: MutableList<Int>, i: Int, relevantFileId: Int): Int {
    var fileSize = 0
    var j = i
    while (j >= 0 && compressed[j] == relevantFileId) {
        fileSize++
        j--
    }
    return fileSize
}

fun findStartIndexOfEmptySpaceOfLength(compressed: MutableList<Int>, fileSize: Int): Int {
    var startIndex = 0
    while (startIndex < compressed.size) {
        if (compressed[startIndex] != -1) {
            startIndex++
        } else {
            var emptySpaceLength = 0
            for (i in startIndex until (startIndex + fileSize)) {
                if (i >= compressed.size) {
                    return -1
                }
                if (compressed[i] != -1) {
                    break
                } else {
                    emptySpaceLength++
                }
                if (emptySpaceLength == fileSize) {
                    return startIndex
                }
            }
            startIndex += emptySpaceLength
        }
    }
    return -1
}