Script to update the next/prev/num header fields in the Scala Book markdown files

UpdateAllMdFiles.scala

//> using scala "2.12"
//> using jar "sed_2.12-0.4.jar"

import scala.util.matching.Regex
import com.alvinalexander.sed.tostring._
import scala.io.Source
import java.io._

// RUN:  scala-cli UpdateAllMdFiles.scala --main-class UpdateAllMdFiles

/**
 * Requirements:
 *    - this script expects to be run in the same directory as the MD files
 *    - this script depends on the OrderedListOfFiles file
 *
 *  1 - get the list of filenames
 *  2 - figure out the NextPage & PrevPage values
 *  3 - go through the files and update the desired fields
 */
object UpdateAllMdFiles extends App {

    val filenames = Source.fromFile("OrderedListOfMdFiles")
        .getLines
        .filter(_.trim != "")
        .toList

    // loop through those filenames and replace/update everything
    var fileCounter = 1
    for (inputFile <- filenames) {

        // figure out prevPage and nextPage
        var prevPage = ""
        var nextPage = ""
        if (fileCounter == 1) {
            prevPage = ""
            nextPage = filenames(1)
        } 
        else if (fileCounter == filenames.size) {
            prevPage = filenames(fileCounter-2)
            nextPage = ""
        } 
        else {
            prevPage = filenames(fileCounter-2)
            nextPage = filenames(fileCounter)
        }
        
        // rm the trailing ".scala" from the filenames
        prevPage = prevPage.split("\\.")(0)
        nextPage = nextPage.split("\\.")(0)
        
        val mapNeededBySedFunction = Map(
            "num"       -> s"$fileCounter",
            "next-page" -> nextPage,
            "prev-page" -> prevPage
        )

        println(s"working on $inputFile ...")

        val source = Source.fromFile(inputFile)
        val sed: SedTrait = SedFactory.getSed(
            source, 
            updateHeaderFields _,
            mapNeededBySedFunction
        )
        val sedOutput: String = sed.run

        // now have `newLines`. write the content back to the file.
        writeFile(inputFile, sedOutput)

        fileCounter += 1
    }

    def updateHeaderFields(
        currentLine: String, 
        currentLineNum: Int, 
        kvMap: Map[String, String]
    ): SedAction = currentLine match {
        case _ if (new Regex("^num:.*")).findAllIn(currentLine).hasNext =>
            val fileNumber = kvMap("num")
            UpdateLine(s"num: $fileNumber")
        case _ if (new Regex("^next-page:.*")).findAllIn(currentLine).hasNext =>
            val nextPage = kvMap("next-page")
            UpdateLine(s"next-page: $nextPage")
        case _ if (new Regex("^previous-page:.*")).findAllIn(currentLine).hasNext =>
            val prevPage = kvMap("prev-page")
            UpdateLine(s"previous-page: $prevPage")
        case _ => UpdateLine(currentLine)
    }

    def writeFile(filename: String, s: String): Unit = {
        val file = new File(filename)
        val bw = new BufferedWriter(new FileWriter(file))
        bw.write(s)
        bw.close()
    }

}