intermediate changes

ref:cde9a383711a11544ce7e107a78147fb96cc4029

1 files changed, 85 insertions, 0 deletions

diff --git a/library/cpp/yson_pull/event.h b/library/cpp/yson_pull/event.h
new file mode 100644
index 0000000000..b41d5ea3b5
--- /dev/null
+++ b/library/cpp/yson_pull/event.h
@@ -0,0 +1,85 @@
+#pragma once
+
+#include "cyson_enums.h"
+#include "scalar.h"
+
+#include <util/generic/strbuf.h>
+#include <util/system/types.h>
+#include <util/system/yassert.h>
+
+namespace NYsonPull {
+    //! A well-formed decoded YSON stream can be described by the following grammar:
+    //!
+    //! STREAM[node]          ::= begin_stream VALUE end_stream
+    //! STREAM[list_fragment] ::= begin_stream LIST_FRAGMENT end_stream
+    //! STREAM[map_fragment]  ::= begin_stream MAP_FRAGMENT end_stream
+    //! LIST_FRAGMENT         ::= { VALUE; }
+    //! MAP_FRAGMENT          ::= { KEY VALUE; }
+    //! KEY                   ::= key(String)
+    //! VALUE                 ::= VALUE_NOATTR | ATTRIBUTES VALUE_NOATTR
+    //! ATTRIBUTES            ::= begin_attributes MAP_FRAGMENT end_attributes
+    //! VALUE_NOATTR          ::= scalar(Scalar) | LIST | MAP
+    //! LIST                  ::= begin_list LIST_FRAGMENT end_list
+    //! MAP                   ::= begin_map MAP_FRAGMENT end_map
+
+    //! \brief YSON event type tag. Corresponds to YSON grammar.
+    enum class EEventType {
+        BeginStream = YSON_EVENT_BEGIN_STREAM,
+        EndStream = YSON_EVENT_END_STREAM,
+        BeginList = YSON_EVENT_BEGIN_LIST,
+        EndList = YSON_EVENT_END_LIST,
+        BeginMap = YSON_EVENT_BEGIN_MAP,
+        EndMap = YSON_EVENT_END_MAP,
+        BeginAttributes = YSON_EVENT_BEGIN_ATTRIBUTES,
+        EndAttributes = YSON_EVENT_END_ATTRIBUTES,
+        Key = YSON_EVENT_KEY,
+        Scalar = YSON_EVENT_SCALAR,
+    };
+
+    //! \brief YSON event variant type.
+    class TEvent {
+        EEventType Type_;
+        TScalar Value_;
+
+    public:
+        //! \brief Construct a tag-only event.
+        explicit constexpr TEvent(EEventType type = EEventType::BeginStream)
+            : Type_{type} {
+        }
+
+        //! \brief Construct a tag+value event.
+        //!
+        //! Only \p EEventType::key is meaningful.
+        constexpr TEvent(EEventType type, const TScalar& value)
+            : Type_{type}
+            , Value_{value} {
+        }
+
+        //! \brief Construct a \p EEventType::scalar event.
+        explicit constexpr TEvent(const TScalar& value)
+            : Type_{EEventType::Scalar}
+            , Value_{value} {
+        }
+
+        EEventType Type() const {
+            return Type_;
+        }
+
+        //! \brief Get TScalar value.
+        //!
+        //! Undefined behaviour when event type is not \p EEventType::scalar.
+        const TScalar& AsScalar() const {
+            Y_ASSERT(Type_ == EEventType::Scalar || Type_ == EEventType::Key);
+            return Value_;
+        }
+
+        //! \brief Get string value.
+        //!
+        //! Undefined behaviour when event type is not \p EEventType::key.
+        TStringBuf AsString() const {
+            Y_ASSERT(Type_ == EEventType::Key || (Type_ == EEventType::Scalar && Value_.Type() == EScalarType::String));
+            return Value_.AsString();
+        }
+    };
+
+}