fix: handle non-BMP Unicode codepoints in foldl, foldr, and %c format (#606)

This PR fixes two more non-BMP Unicode bugs:

- foldl/foldr iterated strings by UTF-16 code unit (`for (char <-
s.value)`), splitting non-BMP characters like emoji into surrogate pair
halves. Use `codePointAt`/`codePointBefore` with `Character.charCount`
for correct codepoint iteration.
- The `%c` format conversion used `s.toChar.toString` which truncates
codepoints above U+FFFF to 16 bits. Use `Character.toString(s.toInt)`
instead.

---

All code written by Claude Opus 4.6.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: JoshRosen

sjsonnet/src/sjsonnet/Format.scala

@@ -196,7 +196,7 @@ object Format {
                 case 'f' | 'F'       => formatFloat(formatted, s)
                 case 'g'             => formatGeneric(formatted, s).toLowerCase
                 case 'G'             => formatGeneric(formatted, s)
-                case 'c'             => widenRaw(formatted, s.toChar.toString)
+                case 'c'             => widenRaw(formatted, Character.toString(s.toInt))
                 case 's'             =>
                   if (s.toLong == s) widenRaw(formatted, s.toLong.toString)
                   else widenRaw(formatted, s.toString)

sjsonnet/src/sjsonnet/stdlib/ArrayModule.scala

@@ -296,12 +296,16 @@ object ArrayModule extends AbstractFunctionModule {
 
         case s: Val.Str =>
           var current = init.force
-          for (char <- s.value) {
+          val str = s.value
+          var i = 0
+          while (i < str.length) {
             val c = current
-            current = func.apply2(c, Val.Str(pos, new String(Array(char))), pos.noOffset)(
+            val codePoint = str.codePointAt(i)
+            current = func.apply2(c, Val.Str(pos, Character.toString(codePoint)), pos.noOffset)(
               ev,
               TailstrictModeDisabled
             )
+            i += Character.charCount(codePoint)
           }
           current
 
@@ -324,9 +328,13 @@ object ArrayModule extends AbstractFunctionModule {
           current
         case s: Val.Str =>
           var current = init.force
-          for (char <- s.value.reverse) {
+          val str = s.value
+          var i = str.length
+          while (i > 0) {
+            val codePoint = str.codePointBefore(i)
+            i -= Character.charCount(codePoint)
             val c = current
-            current = func.apply2(Val.Str(pos, new String(Array(char))), c, pos.noOffset)(
+            current = func.apply2(Val.Str(pos, Character.toString(codePoint)), c, pos.noOffset)(
               ev,
               TailstrictModeDisabled
             )

sjsonnet/test/src/sjsonnet/UnicodeHandlingTests.scala

@@ -256,5 +256,34 @@ object UnicodeHandlingTests extends TestSuite {
       eval("""std.trim("🌍   ")""") ==> ujson.Str("🌍")
       eval("""std.trim("   🌍   ")""") ==> ujson.Str("🌍")
     }
+
+    test("foldl") {
+      // foldl must iterate by codepoint, not UTF-16 code unit
+      eval("""std.foldl(function(acc, c) acc + [c], "a😀b", [])""") ==>
+      ujson.Arr("a", "😀", "b")
+      eval("""std.foldl(function(acc, c) acc + 1, "a😀b", 0)""") ==> ujson.Num(3)
+      eval("""std.foldl(function(acc, c) acc + [c], "🎉🔥", [])""") ==>
+      ujson.Arr("🎉", "🔥")
+      // Round-trip concatenation
+      eval("""std.foldl(function(acc, c) acc + c, "a😀b", "")""") ==> ujson.Str("a😀b")
+    }
+
+    test("foldr") {
+      // foldr must iterate by codepoint, not UTF-16 code unit
+      eval("""std.foldr(function(c, acc) acc + [c], "a😀b", [])""") ==>
+      ujson.Arr("b", "😀", "a")
+      eval("""std.foldr(function(c, acc) acc + [c], "🎉🔥", [])""") ==>
+      ujson.Arr("🔥", "🎉")
+      // Round-trip concatenation (right-to-left: 'b' then '😀' then 'a')
+      eval("""std.foldr(function(c, acc) acc + c, "a😀b", "")""") ==> ujson.Str("b😀a")
+    }
+
+    test("formatPercentC") {
+      // %c must handle non-BMP codepoints
+      eval("""std.format("%c", [128512])""") ==> ujson.Str("😀") // U+1F600
+      eval("""std.format("%c", [128293])""") ==> ujson.Str("🔥") // U+1F525
+      eval("""std.format("%c", [127757])""") ==> ujson.Str("🌍") // U+1F30D
+      eval("""std.format("%c", [65])""") ==> ujson.Str("A") // BMP char
+    }
   }
 }