perf: O(1) sort and equality for range arrays

sjsonnet/src/sjsonnet/Evaluator.scala

@@ -2012,8 +2012,10 @@ class Evaluator(
     case x: Val.Arr =>
       y match {
         case y: Val.Arr =>
+          if (x eq y) return true
           val xlen = x.length
           if (xlen != y.length) return false
+          if (x.rangeEquals(y)) return true
           // Skip shared ConcatView prefix — elements are reference-identical
           var i = x.sharedConcatPrefixLength(y)
           while (i < xlen) {

sjsonnet/src/sjsonnet/Val.scala

@@ -647,6 +647,27 @@ object Val {
       true
     }
 
+    /**
+     * If this array is known to be sorted in ascending numeric order (e.g. a forward range), return
+     * it directly. If it is a reversed range, return the equivalent forward range in O(1). Returns
+     * null if sort order is unknown and a full sort is needed.
+     */
+    private[sjsonnet] def asSortedIfKnown(newPos: Position): Arr = this match {
+      case _: RangeArr => if (_reversed) reversed(newPos) else this
+      case _           => null
+    }
+
+    /**
+     * O(1) equality check for range arrays. Two ranges are equal if they describe the same integer
+     * sequence (same start, length, and direction). Returns false if either array is not a range.
+     */
+    private[sjsonnet] def rangeEquals(other: Arr): Boolean =
+      (this, other) match {
+        case (r1: RangeArr, r2: RangeArr) =>
+          r1._length == r2._length && r1._reversed == r2._reversed && r1.rangeFrom == r2.rangeFrom
+        case _ => false
+      }
+
     /**
      * Create a reversed view of this array without copying. The returned Arr shares the same
      * backing array but flips the reversed flag, so element access is O(1) with zero allocation.
@@ -671,7 +692,7 @@ object Val {
    *
    * Inspired by jrsonnet's RangeArray (arr/spec.rs).
    */
-  final class RangeArr(pos0: Position, private val rangeFrom: Int, size: Int)
+  final class RangeArr(pos0: Position, private[sjsonnet] val rangeFrom: Int, size: Int)
       extends Arr(pos0, null) {
     _length = size
 

sjsonnet/src/sjsonnet/stdlib/SetModule.scala

@@ -82,7 +82,21 @@ object SetModule extends AbstractFunctionModule {
     Val.Arr(pos, out.result())
   }
 
-  private def sortArr(pos: Position, ev: EvalScope, arr: Val, keyF: Val) = {
+  private def sortArr(pos: Position, ev: EvalScope, arr: Val, keyF: Val): Val = {
+    // Fast path: range arrays are already sorted ascending by construction.
+    // Avoids O(n) materialization + O(n log n) sort for already-sorted data.
+    if (keyF == null || keyF.isInstanceOf[Val.False]) {
+      arr match {
+        case a: Val.Arr =>
+          val sorted = a.asSortedIfKnown(pos)
+          if (sorted != null) return sorted
+        case _ =>
+      }
+    }
+    sortArrSlow(pos, ev, arr, keyF)
+  }
+
+  private def sortArrSlow(pos: Position, ev: EvalScope, arr: Val, keyF: Val) = {
     val vs = toArrOrString(arr, pos, ev)
     if (vs.length <= 1) {
       arr