Make empty arrays and dictionaries render compactly even when indent …

…> 0 (#501)

Fixes #499

We take a similar strategy as we do for adding commas: rather than
calling `renderIndent` immediately after opening a new array/dict and at
`visitEnd`, we instead set an `indentBuffered` flag and defer the
`renderIndent` until later. Then each `flushBuffer` call can render the
indent where necessary, and `visitEnd` can perform some additional
checks that the collection is not empty before rendering the final
indent

Added some basic tests to `ExampleTests.scala`, since some of this
behavior is probably worth showing to users in the documentation

ujson/templates/BaseElemRenderer.scala

@@ -28,12 +28,16 @@ class BaseElemRenderer[T <: upickle.core.ElemOps.Output]
   private[this] var visitingKey = false
 
   private[this] var commaBuffered = false
+  private[this] var indentBuffered = false
   private[this] var quoteBuffered = false
 
   def flushBuffer() = {
     if (commaBuffered) {
       commaBuffered = false
       elemBuilder.append(',')
+    }
+    if (indentBuffered){
+      indentBuffered = false
       renderIndent()
     }
     if (quoteBuffered) {
@@ -47,16 +51,21 @@ class BaseElemRenderer[T <: upickle.core.ElemOps.Output]
     elemBuilder.append('[')
 
     depth += 1
-    renderIndent()
+    indentBuffered = true
+
     def subVisitor = BaseElemRenderer.this
+
     def visitValue(v: T, index: Int): Unit = {
       flushBuffer()
       commaBuffered = true
+      indentBuffered = true
     }
+
     def visitEnd(index: Int) = {
-      commaBuffered = false
       depth -= 1
-      renderIndent()
+      if (indentBuffered && commaBuffered) renderIndent()
+      commaBuffered = false
+      indentBuffered = false
       elemBuilder.append(']')
       flushElemBuilder()
       out
@@ -67,26 +76,32 @@ class BaseElemRenderer[T <: upickle.core.ElemOps.Output]
     flushBuffer()
     elemBuilder.append('{')
     depth += 1
-    renderIndent()
+    indentBuffered = true
+
     def subVisitor = BaseElemRenderer.this
     def visitKey(index: Int) = {
       quoteBuffered = true
       visitingKey = true
       BaseElemRenderer.this
     }
+
     def visitKeyValue(s: Any): Unit = {
       elemBuilder.append('"')
       visitingKey = false
       elemBuilder.append(':')
       if (indent != -1) elemBuilder.append(' ')
     }
+
     def visitValue(v: T, index: Int): Unit = {
       commaBuffered = true
+      indentBuffered = true
     }
+
     def visitEnd(index: Int) = {
-      commaBuffered = false
       depth -= 1
-      renderIndent()
+      if (indentBuffered && commaBuffered) renderIndent()
+      commaBuffered = false
+      indentBuffered = false
       elemBuilder.append('}')
       flushElemBuilder()
       out

upickle/test/src/upickle/example/ExampleTests.scala

@@ -156,9 +156,11 @@ object ExampleTests extends TestSuite {
         write("omg")                      ==> "\"omg\""
       }
       test("seqs"){
+        write(Array.empty[Int])           ==> "[]"
         write(Array(1, 2, 3))             ==> "[1,2,3]"
 
         // You can pass in an `indent` parameter to format it nicely
+        write(Array.empty[Int], indent = 4)  ==> "[]"
         write(Array(1, 2, 3), indent = 4)  ==>
           """[
             |    1,
@@ -176,6 +178,18 @@ object ExampleTests extends TestSuite {
         write(Map(1 -> 2, 3 -> 4))         ==> """{"1":2,"3":4}"""
         write(Map("hello" -> "world"))     ==> """{"hello":"world"}"""
         write(Map(Seq(1, 2) -> Seq(3, 4))) ==> """[[[1,2],[3,4]]]"""
+        write(Map.empty[Int, Int])         ==> """{}"""
+        write(Map(Seq.empty[Int] -> Seq.empty[Int])) ==> """[[[],[]]]"""
+
+        write(Map(Seq.empty[Int] -> Seq.empty[Int]), indent = 4) ==>
+        """[
+          |    [
+          |        [],
+          |        []
+          |    ]
+          |]""".stripMargin
+
+        write(Map.empty[Int, Int], indent = 4) ==> """{}"""
       }
       test("options"){
         write(Some(1))                    ==> "[1]"
@@ -227,7 +241,6 @@ object ExampleTests extends TestSuite {
 
         write(Bar("bearrr", Seq(Foo(1), Foo(2), Foo(3)))) ==>
           """{"name":"bearrr","foos":[{"i":1},{"i":2},{"i":3}]}"""
-
       }
       test("null"){
         write(Bar(null, Seq(Foo(1), null, Foo(3)))) ==>