perf: cache sorted visible key names on Val.Obj

Motivation:
Non-inline objects (>8 fields, super chains, excludedKeys) call
.sorted(CodepointStringOrdering) on visibleKeyNames at every
materialization. This re-sorts and allocates a new array each time.

Modification:
Add _sortedVisibleKeyNames cache on Val.Obj with lazy initialization.
Replace all 5 call sites (Materializer recursive/stackless, Renderer,
ByteRenderer, Val.Obj.foreachElement) with the cached accessor.

Result:
Objects with stable schemas sort their key names once and reuse the
result on subsequent materializations.

Author: He-Pin

sjsonnet/src/sjsonnet/ByteRenderer.scala

@@ -391,7 +391,7 @@ class ByteRenderer(out: OutputStream = new java.io.ByteArrayOutputStream(), inde
       matDepth: Int,
       ctx: Materializer.MaterializeContext)(implicit evaluator: EvalScope): Unit = {
     val keys =
-      if (ctx.sort) obj.visibleKeyNames.sorted(Util.CodepointStringOrdering)
+      if (ctx.sort) obj.sortedVisibleKeyNames
       else obj.visibleKeyNames
 
     openObjBrace()

sjsonnet/src/sjsonnet/Materializer.scala

@@ -100,7 +100,7 @@ abstract class Materializer {
         else materializeInlineObj(obj, visitor, depth, ctx)
       } else {
         val keys =
-          if (ctx.sort) obj.visibleKeyNames.sorted(Util.CodepointStringOrdering)
+          if (ctx.sort) obj.sortedVisibleKeyNames
           else obj.visibleKeyNames
         val ov = visitor.visitObject(keys.length, jsonableKeys = true, -1)
         var i = 0
@@ -432,7 +432,7 @@ abstract class Materializer {
     storePos(obj.pos)
     obj.triggerAllAsserts(ctx.brokenAssertionLogic)
     val keyNames =
-      if (ctx.sort) obj.visibleKeyNames.sorted(Util.CodepointStringOrdering)
+      if (ctx.sort) obj.sortedVisibleKeyNames
       else obj.visibleKeyNames
     val objVisitor = visitor.visitObject(keyNames.length, jsonableKeys = true, -1)
     stack.push(

sjsonnet/src/sjsonnet/Val.scala

@@ -886,6 +886,17 @@ object Val {
      */
     @volatile private[sjsonnet] var _sortedInlineOrder: Array[Int] = null
 
+    @volatile private[sjsonnet] var _sortedVisibleKeyNames: Array[String] = null
+
+    private[sjsonnet] def sortedVisibleKeyNames: Array[String] = {
+      var r = _sortedVisibleKeyNames
+      if (r == null) {
+        r = visibleKeyNames.sorted(Util.CodepointStringOrdering)
+        _sortedVisibleKeyNames = r
+      }
+      r
+    }
+
     /**
      * When true, field caching can be skipped during materialization because no field body
      * references `self` or `super`. This eliminates HashMap allocation overhead for objects with >2
@@ -1402,7 +1413,7 @@ object Val {
 
     def foreachElement(sort: Boolean, pos: Position)(f: (String, Val) => Unit)(implicit
         ev: EvalScope): Unit = {
-      val keys = if (sort) visibleKeyNames.sorted(Util.CodepointStringOrdering) else visibleKeyNames
+      val keys = if (sort) sortedVisibleKeyNames else visibleKeyNames
       for (k <- keys) {
         val v = value(k, pos)
         f(k, v)