chore: import upstream snapshot with attribution

cpp/json_ffi/json_ffi_engine.h

@@ -0,0 +1,74 @@
+/*!
+ *  Copyright (c) 2023-2025 by Contributors
+ * \file json_ffi/json_ffi_engine.h
+ * \brief The header of JSON FFI engine in MLC LLM.
+ */
+#ifndef MLC_LLM_JSON_FFI_JSON_FFI_ENGINE_H_
+#define MLC_LLM_JSON_FFI_JSON_FFI_ENGINE_H_
+
+#include <string>
+
+#include "../serve/threaded_engine.h"
+#include "../tokenizers/streamer.h"
+#include "conv_template.h"
+#include "openai_api_protocol.h"
+
+namespace mlc {
+namespace llm {
+namespace json_ffi {
+
+using namespace tvm::runtime;
+using namespace mlc::llm::serve;
+
+/*!
+ * \brief // Todo: document this class, fields and member functions
+ */
+class JSONFFIEngine {
+ public:
+  JSONFFIEngine();
+
+  ~JSONFFIEngine();
+
+  bool ChatCompletion(std::string request_json_str, std::string request_id);
+
+  bool AddRequest(std::string request_json_str, std::string request_id);
+
+  void StreamBackError(std::string request_id);
+
+  bool Abort(std::string request_id);
+
+  std::string GetLastError();
+
+  void ExitBackgroundLoop();
+
+ protected:
+  /*! \brief local request state entry, one per reply stream. */
+  struct RequestState {
+    /*! \brief model to fill in reply. */
+    std::string model;
+    /*! \brief text streamer for each stream */
+    std::vector<TextStreamer> streamer;
+  };
+
+  std::unique_ptr<ThreadedEngine> engine_;
+  std::string err_;
+  Function request_stream_callback_;
+  // tokenizer
+  Tokenizer tokenizer_;
+  // conversation template
+  Conversation conv_template_;
+  // generation config
+  GenerationConfig default_generation_config_;
+  // model config
+  ModelConfig model_config_;
+  // local device
+  DLDevice device_;
+  // request state map
+  std::unordered_map<String, RequestState> request_map_;
+};
+
+}  // namespace json_ffi
+}  // namespace llm
+}  // namespace mlc
+
+#endif  // MLC_LLM_JSON_FFI_JSON_FFI_ENGINE_H_