[picojson] Let objects be ordered when serializing

This PR changes the serialization logic of objects to follow the insertion
order of elements to keep the output consistent across different platforms.

Author: Ubospica

3rdparty/picojson/picojson.h

@@ -727,6 +727,24 @@ void value::_serialize(Iter oi, int indent) const {
       if (indent != -1) {
         ++indent;
       }
+
+#if PICOJSON_USE_ORDERED_OBJECT
+      for (auto i = u_.object_->ordered_keys().begin();
+           i != u_.object_->ordered_keys().end(); ++i) {
+        if (i != u_.object_->ordered_keys().begin()) {
+          *oi++ = ',';
+        }
+        if (indent != -1) {
+          _indent(oi, indent);
+        }
+        serialize_str(*i, oi);
+        *oi++ = ':';
+        if (indent != -1) {
+          *oi++ = ' ';
+        }
+        u_.object_->at(*i)._serialize(oi, indent);
+      }
+#else
       for (object::const_iterator i = u_.object_->begin(); i != u_.object_->end(); ++i) {
         if (i != u_.object_->begin()) {
           *oi++ = ',';
@@ -741,6 +759,7 @@ void value::_serialize(Iter oi, int indent) const {
         }
         i->second._serialize(oi, indent);
       }
+#endif
       if (indent != -1) {
         --indent;
         if (!u_.object_->empty()) {

3rdparty/picojson/test_picojson.cpp

@@ -58,8 +58,21 @@ void test_modifier() {
   assert((obj.ordered_keys() == std::vector<std::string>{}));
 }
 
+void test_serializer() {
+  picojson::object obj;
+
+  obj["bar"] = picojson::value(static_cast<int64_t>(10));
+  obj["baz"] = picojson::value(10.5);
+  obj["foo"] = picojson::value(true);
+
+  picojson::value v(obj);
+
+  assert((v.serialize(false) == "{\"bar\":10,\"baz\":10.5,\"foo\":true}"));
+}
+
 int main() {
   test_constructor();
   test_modifier();
+  test_serializer();
   return 0;
 }