fix: preserve exact binary64 digits in decimal formats

Motivation:
High-precision %f and fixed-form %g formatting started from the shortest
round-tripping decimal string, hiding meaningful trailing digits of the
binary64 value.

Modification:
Use the exact binary64 decimal value when fixed formatting needs more than 15
digits, propagate the original %g significant precision, and use half-even
rounding for the exact path. Add non-tie and exact-halfway regressions while
preserving the public five-argument JVM descriptor.

Result:
High-precision fixed decimal output matches Python from the same IEEE-754
value, including ties-to-even, without changing the legacy low-precision
rounding path or JVM binary compatibility.

References:
- #1097
- https://jsonnet.org/ref/stdlib.html#std-format

Author: He-Pin

sjsonnet/src/sjsonnet/DecimalFormat.scala

@@ -19,7 +19,16 @@ object DecimalFormat {
       hashes: Int,
       alternate: Boolean,
       expLengthOpt: Option[Int],
-      number: Double): String = {
+      number: Double): String =
+    format(zeroes, hashes, alternate, expLengthOpt, number, useExactDecimal = false)
+
+  private[sjsonnet] def format(
+      zeroes: Int,
+      hashes: Int,
+      alternate: Boolean,
+      expLengthOpt: Option[Int],
+      number: Double,
+      useExactDecimal: Boolean): String = {
     expLengthOpt match {
       case Some(expLength) =>
         var expNum =
@@ -75,9 +84,15 @@ object DecimalFormat {
           if (alternate) prefix + "." else prefix
         } else {
           val denominator = BigDecimal(10).pow(precision)
-          val bd = BigDecimal(number).abs
+          val exactDecimal = useExactDecimal || precision > 15
+          val bd =
+            if (exactDecimal) BigDecimal.exact(number).abs
+            else BigDecimal(number).abs
           val scaled =
-            (bd * denominator + BigDecimal("0.5")).setScale(0, BigDecimal.RoundingMode.FLOOR)
+            if (exactDecimal)
+              (bd * denominator).setScale(0, BigDecimal.RoundingMode.HALF_EVEN)
+            else
+              (bd * denominator + BigDecimal("0.5")).setScale(0, BigDecimal.RoundingMode.FLOOR)
           val wholeBD = (scaled / denominator).setScale(0, BigDecimal.RoundingMode.FLOOR)
           val fracBD = (scaled - wholeBD * denominator).abs
 

sjsonnet/src/sjsonnet/Format.scala

@@ -1222,7 +1222,8 @@ object Format {
             if (formatted.alternate) 0 else fractionalPrecision,
             formatted.alternate,
             None,
-            math.abs(s)
+            math.abs(s),
+            useExactDecimal = precision > 15
           ),
         numeric = true,
         signedConversion = !isNegative(s)

sjsonnet/test/resources/new_test_suite/format_exact_binary64_precision.jsonnet

@@ -0,0 +1,11 @@
+std.assertEqual(std.format("%.16f", 3.14159265358979323), "3.1415926535897931") &&
+std.assertEqual(std.format("%.17f", 1.0 / 3.0), "0.33333333333333331") &&
+std.assertEqual(std.format("%.17f", 3.14159265358979323), "3.14159265358979312") &&
+std.assertEqual(std.format("%.17f", 0.1), "0.10000000000000001") &&
+std.assertEqual(std.format("%.17g", 3.14159265358979323), "3.1415926535897931") &&
+std.assertEqual(std.format("%.17g", 31.4159265358979323), "31.415926535897931") &&
+std.assertEqual(std.format("%.16f", 1.00000762939453125), "1.0000076293945312") &&
+std.assertEqual(std.format("%.16f", 1.00002288818359375), "1.0000228881835938") &&
+std.assertEqual(std.format("%.16f", -1.00000762939453125), "-1.0000076293945312") &&
+std.assertEqual(std.format("%.17g", 1.00000762939453125), "1.0000076293945312") &&
+true

sjsonnet/test/resources/new_test_suite/format_exact_binary64_precision.jsonnet.golden

@@ -0,0 +1 @@
+true