From d519d4cb030bda67b4023c9677116801c858330c Mon Sep 17 00:00:00 2001
From: Nicholas Van Sickle <nvsickle@amazon.com>
Date: Wed, 1 Dec 2021 16:22:48 -0800
Subject: [PATCH] Improve performance for both in-place and copy parsing
 (faster than `AZ::JsonSerializationUtils::ReadJsonString` now!)

Signed-off-by: Nicholas Van Sickle <nvsickle@amazon.com>
---
 .../Backends/JSON/JsonSerializationUtils.cpp  | 51 ------------
 .../Backends/JSON/JsonSerializationUtils.h    | 77 ++++++++++++++++---
 2 files changed, 66 insertions(+), 62 deletions(-)

diff --git a/Code/Framework/AzCore/AzCore/DOM/Backends/JSON/JsonSerializationUtils.cpp b/Code/Framework/AzCore/AzCore/DOM/Backends/JSON/JsonSerializationUtils.cpp
index 84f663f97c..104082ce54 100644
--- a/Code/Framework/AzCore/AzCore/DOM/Backends/JSON/JsonSerializationUtils.cpp
+++ b/Code/Framework/AzCore/AzCore/DOM/Backends/JSON/JsonSerializationUtils.cpp
@@ -413,57 +413,6 @@ namespace AZ::Dom::Json
         return true;
     }
 
-    //
-    // struct AzStringStream
-    //
-    // rapidjson stream wrapper for AZStd::string suitable for in-situ parsing
-    AzStringStream::AzStringStream(AZStd::string& buffer)
-    {
-        m_cursor = buffer.data();
-        m_begin = m_cursor;
-    }
-
-    char AzStringStream::Peek() const
-    {
-        return *m_cursor;
-    }
-
-    char AzStringStream::Take()
-    {
-        return *m_cursor++;
-    }
-
-    size_t AzStringStream::Tell() const
-    {
-        return static_cast<size_t>(m_cursor - m_begin);
-    }
-
-    char* AzStringStream::PutBegin()
-    {
-        m_write = m_cursor;
-        return m_cursor;
-    }
-
-    void AzStringStream::Put(char c)
-    {
-        (*m_write++) = c;
-    }
-
-    void AzStringStream::Flush()
-    {
-    }
-
-    size_t AzStringStream::PutEnd(char* begin)
-    {
-        return m_write - begin;
-    }
-
-    const char* AzStringStream::Peek4() const
-    {
-        AZ_Assert(false, "Not implemented, encoding is hard-coded to UTF-8");
-        return m_cursor;
-    }
-
     //
     // Serialized JSON util functions
     //
diff --git a/Code/Framework/AzCore/AzCore/DOM/Backends/JSON/JsonSerializationUtils.h b/Code/Framework/AzCore/AzCore/DOM/Backends/JSON/JsonSerializationUtils.h
index 4197209b8e..5e7976568b 100644
--- a/Code/Framework/AzCore/AzCore/DOM/Backends/JSON/JsonSerializationUtils.h
+++ b/Code/Framework/AzCore/AzCore/DOM/Backends/JSON/JsonSerializationUtils.h
@@ -113,19 +113,65 @@ namespace AZ::Dom::Json
     };
 
     //! rapidjson stream wrapper for AZStd::string suitable for in-situ parsing
+    //! Faster than rapidjson::MemoryStream for reading from AZStd::string / AZStd::string_view (because it requires a null terminator)
+    //! \note This needs to be inlined for performance reasons.
     struct AzStringStream
     {
         using Ch = char; //<! Denotes the string character storage type for rapidjson
 
-        AzStringStream(AZStd::string& buffer);
-        char Peek() const;
-        char Take();
-        size_t Tell() const;
-        char* PutBegin();
-        void Put(char c);
-        void Flush();
-        size_t PutEnd(char* begin);
-        const char* Peek4() const;
+        AZ_FORCE_INLINE AzStringStream(AZStd::string& buffer)
+        {
+            m_cursor = buffer.data();
+            m_begin = m_cursor;
+        }
+
+        AZ_FORCE_INLINE AzStringStream(AZStd::string_view buffer)
+        {
+            // rapidjson won't actually call PutBegin or Put unless kParseInSituFlag is set, so this is safe
+            m_cursor = const_cast<char*>(buffer.data());
+            m_begin = m_cursor;
+        }
+
+        AZ_FORCE_INLINE char Peek() const
+        {
+            return *m_cursor;
+        }
+
+        AZ_FORCE_INLINE char Take()
+        {
+            return *m_cursor++;
+        }
+
+        AZ_FORCE_INLINE size_t Tell() const
+        {
+            return static_cast<size_t>(m_cursor - m_begin);
+        }
+
+        AZ_FORCE_INLINE char* PutBegin()
+        {
+            m_write = m_cursor;
+            return m_cursor;
+        }
+
+        AZ_FORCE_INLINE void Put(char c)
+        {
+            (*m_write++) = c;
+        }
+
+        AZ_FORCE_INLINE void Flush()
+        {
+        }
+
+        AZ_FORCE_INLINE size_t PutEnd(char* begin)
+        {
+            return m_write - begin;
+        }
+
+        AZ_FORCE_INLINE const char* Peek4() const
+        {
+            AZ_Assert(false, "Not implemented, encoding is hard-coded to UTF-8");
+            return m_cursor;
+        }
 
         char* m_cursor; //!< Current read position.
         char* m_write; //!< Current write position.
@@ -180,10 +226,19 @@ namespace AZ::Dom::Json
     Visitor::Result VisitSerializedJson(AZStd::string_view buffer, Lifetime lifetime, Visitor& visitor)
     {
         rapidjson::Reader reader;
-        rapidjson::MemoryStream stream(buffer.data(), buffer.size());
         RapidJsonReadHandler handler(&visitor, lifetime);
 
-        reader.Parse<static_cast<int>(parseFlags)>(stream, handler);
+        // If the string is null terminated, we can use the faster AzStringStream path - otherwise we fall back on rapidjson::MemoryStream
+        if (buffer.data()[buffer.size()] == '\0')
+        {
+            AzStringStream stream(buffer);
+            reader.Parse<static_cast<int>(parseFlags)>(stream, handler);
+        }
+        else
+        {
+            rapidjson::MemoryStream stream(buffer.data(), buffer.size());
+            reader.Parse<static_cast<int>(parseFlags)>(stream, handler);
+        }
         return handler.TakeOutcome();
     }