From a742ce1d321de204f0f70c61f83bc08af56cf4f1 Mon Sep 17 00:00:00 2001
From: Wojtek Figat <wojtek@figat.pl>
Date: Mon, 13 May 2024 22:40:27 +0200
Subject: [PATCH] Optimize `FileReadStream` seeking if new position is within
 the cached buffer

---
 .../Engine/Serialization/FileReadStream.cpp   | 41 ++++++++++++++++++-
 Source/Engine/Serialization/FileReadStream.h  |  1 +
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/Source/Engine/Serialization/FileReadStream.cpp b/Source/Engine/Serialization/FileReadStream.cpp
index c542c9490..4aaee55cc 100644
--- a/Source/Engine/Serialization/FileReadStream.cpp
+++ b/Source/Engine/Serialization/FileReadStream.cpp
@@ -4,6 +4,8 @@
 #include "Engine/Core/Log.h"
 #include "Engine/Platform/File.h"
 
+#define USE_FILE_POS (1)
+
 FileReadStream* FileReadStream::Open(const StringView& path)
 {
     const auto file = File::Open(path, FileMode::OpenExisting, FileAccess::Read, FileShare::Read);
@@ -24,8 +26,12 @@ FileReadStream::FileReadStream(File* file)
     : _file(file)
     , _virtualPosInBuffer(0)
     , _bufferSize(0)
+#if USE_FILE_POS
+    , _filePosition(file->GetPosition())
+#else
+    , _filePosition(0)
+#endif
 {
-    ASSERT_LOW_LAYER(_file);
 }
 
 FileReadStream::~FileReadStream()
@@ -62,16 +68,40 @@ uint32 FileReadStream::GetLength()
 
 uint32 FileReadStream::GetPosition()
 {
+#if USE_FILE_POS
+    return _filePosition - _bufferSize + _virtualPosInBuffer;
+#else
     return _file->GetPosition() - _bufferSize + _virtualPosInBuffer;
+#endif
 }
 
 void FileReadStream::SetPosition(uint32 seek)
 {
+#if USE_FILE_POS
+    // Skip if position won't change
+    if (GetPosition() == seek)
+    {
+        return;
+    }
+
+    // Try to seek with virtual position
+    uint32 bufferStartPos = _filePosition - _bufferSize;
+    if (seek >= GetPosition() && seek < _filePosition)
+    {
+        _virtualPosInBuffer = seek - bufferStartPos;
+        return;
+    }
+#endif
+
     // Seek
     _file->SetPosition(seek);
+    _filePosition = _file->GetPosition();
 
     // Update buffer
     _hasError |= _file->Read(_buffer, FILESTREAM_BUFFER_SIZE, &_bufferSize) != 0;
+#if USE_FILE_POS
+    _filePosition += _bufferSize;
+#endif
     _virtualPosInBuffer = 0;
 }
 
@@ -88,6 +118,9 @@ void FileReadStream::ReadBytes(void* data, uint32 bytes)
     {
         CHECK(_virtualPosInBuffer == 0);
         _hasError |= _file->Read(_buffer, FILESTREAM_BUFFER_SIZE, &_bufferSize) != 0;
+#if USE_FILE_POS
+        _filePosition += _bufferSize;
+#endif
     }
 
     // Check if buffer has enough data for this read
@@ -107,6 +140,9 @@ void FileReadStream::ReadBytes(void* data, uint32 bytes)
             bytes -= bufferBytesLeft;
             _virtualPosInBuffer = 0;
             _hasError |= _file->Read(_buffer, FILESTREAM_BUFFER_SIZE, &_bufferSize) != 0;
+#if USE_FILE_POS
+            _filePosition += _bufferSize;
+#endif
         }
 
         // Read as much as can using whole buffer
@@ -116,6 +152,9 @@ void FileReadStream::ReadBytes(void* data, uint32 bytes)
             data = (byte*)data + FILESTREAM_BUFFER_SIZE;
             bytes -= FILESTREAM_BUFFER_SIZE;
             _hasError |= _file->Read(_buffer, FILESTREAM_BUFFER_SIZE, &_bufferSize) != 0;
+#if USE_FILE_POS
+            _filePosition += _bufferSize;
+#endif
         }
 
         // Read the rest of the buffer but without flushing its data
diff --git a/Source/Engine/Serialization/FileReadStream.h b/Source/Engine/Serialization/FileReadStream.h
index c5a9c22c7..ff88efcc1 100644
--- a/Source/Engine/Serialization/FileReadStream.h
+++ b/Source/Engine/Serialization/FileReadStream.h
@@ -15,6 +15,7 @@ private:
     File* _file;
     uint32 _virtualPosInBuffer; // Current position in the buffer (index)
     uint32 _bufferSize; // Amount of loaded bytes from the file to the buffer
+    uint32 _filePosition; // Cached position in the file (native)
     byte _buffer[FILESTREAM_BUFFER_SIZE];
 
 public: