Add function for calculating serialised/RLP length without encoding d…

…ata (#775)

* Add function for calculating serialised/RLP length without encoding data

Ackn:
  Thanks to Chirag

* Add unit test

eth/rlp/writer.nim

@@ -276,3 +276,17 @@ macro encodeList*(args: varargs[untyped]): seq[byte] =
     var `writer` = initRlpList(`listLen`)
     `body`
     move(finish(`writer`))
+
+
+proc getEncodedLength*[T](v: T): int =
+  mixin append
+
+  const nestedListsDepth = countNestedListsDepth(T)
+  when nestedListsDepth > 0:
+    var tracker = StaticRlpLengthTracker[nestedListsDepth]()
+  elif nestedListsDepth == 0:
+    var tracker = DynamicRlpLengthTracker()
+
+  tracker.initLengthTracker()
+  tracker.append(v)
+  return tracker.finish()

tests/rlp/test_object_serialization.nim

@@ -83,4 +83,18 @@ proc suite() =
         Foo.rlpFieldsCount == 3
         Transaction.rlpFieldsCount == 3
 
+    test "RLP size w/o data encoding":
+      var
+        originalBar = Bar(b: "abracadabra",
+                            f: Foo(x: 5'u64, y: "hocus pocus", z: @[uint64 100, 200, 300]))
+        originalBarBytes = encode(originalBar)
+
+        origVal = CustomSerialized(customFoo: Foo(x: 10'u64, y: "y", z: @[]), ignored: 5)
+        origValBytes = encode(origVal)
+
+      check:
+        originalBarBytes.len == originalBar.getEncodedLength
+        origValBytes.len == origVal.getEncodedLength
+
+
 suite()