Generator.kt

val manifestWriter = System.out.writer()

manifestWriter.use {
  sourceDirectory
    .walk()
    .filter {
      it.isFile && it.extension.equals("mp3", ignoreCase = true)
    }
    .mapNotNull { file ->
      readAudioFileOrNull(file)
    }
    .mapNotNull { audioFile ->
      extractMetadataOrNull(audioFile)
    }
    .joinTo(manifestWriter, prefix = """{ "music": [""", postfix = "]}") {
      it.toJsonString(true)
    }