import java.security.MessageDigest;
import scala.annotation._
import sbt._
import sbt.Using._
 
object Checksums {
  private val bufferSize = 2048
 
  def generate(source:File, target:File, algorithm:String):Unit =
      IO write (target, compute(source, algorithm))
 
  def compute(file:File, algorithm:String):String =
      hex(raw(file, algorithm))
 
  def raw(file:File, algorithm:String):Array[Byte]    =
      (Using fileInputStream file) { is =>
        val md  = MessageDigest getInstance algorithm
        val buf = new Array[Byte](bufferSize)
        md.reset()
        @tailrec
        def loop() {
          val len = is read buf
          if (len != -1) {
            md update (buf, 0, len)
            loop()
          }
        }
        loop()
        md.digest()
      }
 
  def hex(bytes:Array[Byte]):String =
      bytes map { it => "%02x" format (it.toInt & 0xff) } mkString ""
}