Add IoUtils.writeFrameAsDelimited (#246)

The utility will be useful in nanopub-registry to quickly return the nanopub from the DB, just by prepending a delimiter.

core/src/main/scala/eu/ostrzyciel/jelly/core/IoUtils.scala

@@ -1,6 +1,9 @@
 package eu.ostrzyciel.jelly.core
 
-import java.io.{ByteArrayInputStream, InputStream, SequenceInputStream}
+import com.google.protobuf.CodedOutputStream
+import scalapb.LiteParser
+
+import java.io.{ByteArrayInputStream, InputStream, OutputStream, SequenceInputStream}
 
 object IoUtils:
   /**
@@ -28,3 +31,19 @@ object IoUtils:
       scout(0) != 0x0A || scout(1) == 0x0A && scout(2) != 0x0A
     )
     (isDelimited, newInput)
+
+  /**
+   * Utility method to transform a non-delimited Jelly frame (as a byte array) into a delimited one,
+   * writing it to a byte stream.
+   *
+   * This is useful if you for example store non-delimited frames in a database, but want to write them to a stream.
+   *
+   * @param nonDelimitedFrame EXACTLY one non-delimited Jelly frame
+   * @param output the output stream to write the frame to
+   */
+  def writeFrameAsDelimited(nonDelimitedFrame: Array[Byte], output: OutputStream): Unit =
+    // Don't worry, the buffer won't really have 0-size. It will be of minimal size able to fit the varint.
+    val codedOutput: CodedOutputStream = CodedOutputStream.newInstance(output, bufferSize = 0)
+    codedOutput.writeUInt32NoTag(nonDelimitedFrame.length)
+    codedOutput.flush()
+    output.write(nonDelimitedFrame)

core/src/test/scala/eu/ostrzyciel/jelly/core/IoUtilsSpec.scala

@@ -119,4 +119,15 @@ class IoUtilsSpec extends AnyWordSpec, Matchers:
         newIn.readAllBytes() shouldBe Array[Byte](0x12, 0x34)
       }
     }
+
+    "writeFrameAsDelimited" in {
+      val os = ByteArrayOutputStream()
+      IoUtils.writeFrameAsDelimited(frameLarge.toByteArray, os)
+      val bytes = os.toByteArray
+
+      val in = new ByteArrayInputStream(bytes)
+      val (isDelimited, newIn) = IoUtils.autodetectDelimiting(in)
+      isDelimited shouldBe true
+      RdfStreamFrame.parseDelimitedFrom(newIn).get shouldBe frameLarge
+    }
   }