fix: std.round preserves identity for |x| >= 2^52

sjsonnet/src/sjsonnet/stdlib/MathModule.scala

@@ -314,7 +314,12 @@ object MathModule extends AbstractFunctionModule {
      * The official docs list std.round(x) as a mathematical function.
      */
     builtin("round", "x") { (pos, ev, x: Double) =>
-      if (x >= 0) math.floor(x + 0.5) else math.ceil(x - 0.5)
+      // For |x| >= 2^52 the double is already an exact integer (ULP >= 1.0);
+      // adding 0.5 would invoke IEEE 754 round-to-even and produce the wrong
+      // result for odd inputs (e.g. 2^53 - 1). Short-circuit to avoid it.
+      if (x != x || math.abs(x) >= 4503599627370496.0) x
+      else if (x >= 0) math.floor(x + 0.5)
+      else math.ceil(x - 0.5)
     },
     /**
      * [[https://jsonnet.org/ref/stdlib.html#math std.ceil(x)]].

sjsonnet/test/resources/new_test_suite/math_round_isEven_isOdd_isInteger_isDecimal.jsonnet

@@ -12,6 +12,14 @@ assert std.round(-1.8) == -2 : 'round(-1.8)';
 assert std.round(0) == 0 : 'round(0)';
 assert std.round(1) == 1 : 'round(1)';
 assert std.round(-1) == -1 : 'round(-1)';
+// Large-integer regression: for |x| >= 2^52 the double is already an exact
+// integer (ULP >= 1.0), so std.round must be the identity. go-jsonnet and
+// jrsonnet agree on all four values below.
+assert std.round(9007199254740991) == 9007199254740991 : 'round(2^53 - 1)';
+assert std.round(9007199254740990) == 9007199254740990 : 'round(2^53 - 2)';
+assert std.round(4503599627370497) == 4503599627370497 : 'round(2^52 + 1)';
+assert std.round(-9007199254740991) == -9007199254740991 : 'round(-(2^53 - 1))';
+assert std.round(1e20) == 1e20 : 'round(1e20)';
 
 // === std.isEven: truncation of integral part (go-jsonnet + jrsonnet agree) ===
 // Integer inputs