allow tasty version to be inspected

Author: bishabosha

tasty/src-bootstrapped/dotty/tools/tasty/TastyHeader.scala

@@ -0,0 +1,5 @@
+package dotty.tools.tasty
+
+import java.util.UUID
+
+case class TastyHeader(uuid: UUID, majorVersion: Int, minorVersion: Int)

tasty/src-bootstrapped/dotty/tools/tasty/TastyHeaderUnpickler.scala

@@ -0,0 +1,46 @@
+package dotty.tools.tasty
+
+import java.util.UUID
+
+import TastyFormat.{MajorVersion, MinorVersion, header}
+
+class TastyHeaderUnpickler(reader: TastyReader) {
+  import reader._
+
+  def this(bytes: Array[Byte]) = this(new TastyReader(bytes))
+
+  /**delegates to `readFullHeader`, expecting the following
+   * - `majorVersion == TastyFormat.MajorVersion`
+   * - `0 <= minorVersion <= TastyFormat.MinorVersion`
+   */
+  def readHeader(): UUID = {
+    val h = readFullHeader()
+    val validVersion = (
+      h.majorVersion == MajorVersion
+      && h.minorVersion >= 0 && h.minorVersion <= MinorVersion
+    )
+    check(validVersion,
+      s"""TASTy signature has wrong version.
+         | expected: $MajorVersion.$MinorVersion
+         | found   : ${h.majorVersion}.${h.minorVersion}""".stripMargin
+    )
+    h.uuid
+  }
+
+  /**Reads the header of a Tasty File
+   */
+  def readFullHeader(): TastyHeader = {
+    for (i <- 0 until header.length)
+      check(readByte() == header(i), "not a TASTy file")
+    val majorVersion = readNat()
+    val minorVersion = readNat()
+    val uuid = new UUID(readUncompressedLong(), readUncompressedLong())
+    TastyHeader(uuid, majorVersion, minorVersion)
+  }
+
+  def isAtEnd: Boolean = reader.isAtEnd
+
+  private def check(cond: Boolean, msg: => String): Unit = {
+    if (!cond) throw new UnpickleException(msg)
+  }
+}