fix issue reading off the end of the ByteBuffer if ptr > 0

CHANGES.md

@@ -2,6 +2,8 @@
 
 ### Unreleased
 
+* Fix `IndexOutOfBoundsException` in the JRuby extension when encoding shared strings.
+
 ### 2025-09-18 (2.14.0)
 
 * Add new `allow_duplicate_key` generator options. By default a warning is now emitted when a duplicated key is encountered.

java/src/json/ext/SWARBasicStringEncoder.java

@@ -24,14 +24,14 @@ void encode(ByteList src) throws IOException {
         int pos = 0;
 
         ByteBuffer bb = ByteBuffer.wrap(ptrBytes, 0, len);
-        while (pos + 8 <= len) {
+        while (ptr + pos + 8 <= len) {
             long x = bb.getLong(ptr + pos);
             if (skipChunk(x)) {
                 pos += 8;
                 continue;
             }
-            int chunkEnd = pos + 8;
-            while (pos < chunkEnd) {
+            int chunkEnd = ptr + pos + 8;
+            while (ptr + pos < chunkEnd) {
                 int ch = Byte.toUnsignedInt(ptrBytes[ptr + pos]);
                 int ch_len = ESCAPE_TABLE[ch];
                 if (ch_len > 0) {
@@ -43,7 +43,7 @@ void encode(ByteList src) throws IOException {
             }
         }
 
-        if (pos + 4 <= len) {
+        if (ptr + pos + 4 <= len) {
             int x = bb.getInt(ptr + pos);
             if (skipChunk(x)) {
                 pos += 4;

test/json/json_encoding_test.rb

@@ -31,6 +31,12 @@ def test_generate
     assert_equal @generated, JSON.generate(@utf_16_data, ascii_only: true)
   end
 
+  def test_generate_shared_string
+    # Ref: https://github.com/ruby/json/issues/859
+    s = "01234567890"
+    assert_equal '"234567890"', JSON.dump(s[2..-1])
+  end
+
   def test_unicode
     assert_equal '""', ''.to_json
     assert_equal '"\\b"', "\b".to_json