ErrId.scala

import java.io.{ByteArrayOutputStream, PrintStream}
import java.security.MessageDigest

/** Identifies throwables by creating checksums (like MD5 or SHA1) of its stacktrace. */
object ErrId {
  /** Returns the absolute checksum of the specified throwable. */
  def absolute(t: Throwable): String = {
    val bytes = stackBytes(t)

    val md = sha1er
    md update bytes
    val digest = md.digest

    hex(digest)
  }

  /** Returns the relative checksum of the specified throwable. */
  def relative(t: Throwable): String = {
    val caller = Thread.currentThread().getStackTrace()(2)

    val relativeTrace = t.getStackTrace takeWhile { ste ⇒
      ste.getClassName != caller.getClassName && ste.getMethodName != caller.getMethodName
    }

    val bytes = relativeTrace flatMap (_.toString getBytes "UTF-8")
    val causeBytes = Option(t.getCause) map stackBytes

    val md = sha1er
    md update bytes
    causeBytes foreach md.update
    val digest = md.digest

    hex(digest)
  }

  private def sha1er = MessageDigest getInstance "SHA-1"

  private def stackBytes(t: Throwable): Array[Byte] = {
    val out = new ByteArrayOutputStream()
    t printStackTrace new PrintStream(out)
    out.toByteArray
  }

  /** Returns the hexadecimal string form of the specified byte array. */
  private def hex(bytes: Array[Byte]) = {
    bytes map { b ⇒
      val s = Integer toHexString 0xff & b
      if (s.length == 1) s"0$s" else s
    } mkString ""
  }
}