Kotlinの末尾再帰(tailrec)のサンプルコード

Scalaの末尾再帰(tailrec)のサンプルコード - QiitaをKotlinに翻訳したもの。

interface Node
data class Directory(val name: String, val children: List<Node> = listOf()) : Node
data class File(val name: String) : Node

val fileTree =
        Directory("Users", listOf(
                Directory("alice", listOf(
                        File(".bashrc"),
                        Directory(".Trash"),
                        Directory("Desktop", listOf(
                                File("photo.jpg"),
                                File("document.docx"),
                                File("spreadsheet.xlsx")
                        ))
                ))
        ))

tailrec fun flattenFilesytem(tree: List<Node>, filenames: List<String> = listOf()): List<String> {
    val first = tree.firstOrNull()
    val rest = tree.drop(1)
    return when (first) {
        is File -> flattenFilesytem(rest, filenames.plus(first.name))
        is Directory -> flattenFilesytem(rest + first.children, filenames.plus(first.name))
        else -> filenames
    }
}

flattenFilesytem(listOf<Node>(fileTree))
//=> res27: kotlin.collections.List<kotlin.String> = [Users, alice, .bashrc, .Trash, Desktop, photo.jpg, document.docx, spreadsheet.xlsx]