mail_header.scala

import java.nio.file._

object mail_header {
    def read_file(path: String): Option[String] = {
        val p = Paths.get(path)
        if (Files.exists(p)) Some(new String(Files.readAllBytes(p))) else None
    }
    
    def split_header_body(text: String): Option[(String, String)] = {
        text.split("\r\n\r\n", 2) match {
            case Array(header, body) => Some(header, body)
            case _ => None
        }
    }
    
    def unfold_header(header: String): String =
        header.replaceAll("\r\n[ \t]+", " ")
    
    def split_field(line: String): Option[(String, String)] = {
        val pat = """(\S+): *(.+)?""".r
        pat.findFirstMatchIn(line) match {
            case Some(m) => Some(m.group(1), Option(m.group(2)).getOrElse(""))
            case None => None
        }
    }
    
    def parse_header(header: String): List[(String, String)] =
        header.lines.flatMap(split_field).toList
        
    def tuple_to_hash(header: List[(String, String)]): Map[String, List[String]] =
        header.groupBy(_._1).map { case (k, v) => (k.toLowerCase(), v.map(_._2)) }
    
    def main(args: Array[String]): Unit = {
        val text = read_file("data.txt").getOrElse(sys.error("File not found"))
        val (header, _) = split_header_body(text).getOrElse(sys.error("Invalid mail-header"))
        val unfolded_header = unfold_header(header)
        val tupled_header = parse_header(unfolded_header)
        val mail_header = tuple_to_hash(tupled_header)
        
        println(mail_header)
    }
}