Add test for data_len in DF03 envelope

dataforge-io/src/jvmTest/kotlin/hep/dataforge/io/FileEnvelopeTest.kt

@@ -1,9 +1,10 @@
 package hep.dataforge.io
 
 import hep.dataforge.context.Global
+import java.nio.ByteBuffer
 import java.nio.file.Files
-import kotlin.test.Test
-import kotlin.test.assertTrue
+import java.util.*
+import kotlin.test.*
 
 
 class FileEnvelopeTest {
@@ -28,4 +29,28 @@ class FileEnvelopeTest {
         val restored: Envelope = Global.io.readEnvelopeFile(tmpPath)
         assertTrue { envelope.contentEquals(restored) }
     }
+
+
+    @Test
+    fun testDataSize() {
+        val tmpPath = Files.createTempFile("dataforge_test", ".df")
+        Global.io.writeEnvelopeFile(tmpPath,envelope)
+        println(tmpPath.toUri())
+        val scan = Scanner(tmpPath.toFile().inputStream()).useDelimiter("\n").nextLine()
+        println(scan)
+        val format = scan.slice(2..5)
+        if (format == "DF03"){
+            val buff = ByteBuffer.allocate(4)
+            buff.put(scan.slice(12..15).toByteArray())
+            buff.flip()
+            val size = buff.int
+            println(size)
+            assertEquals(8, size)
+        }
+        else{
+           fail("Format $format don't have test")
+        }
+
+    }
+
 }