The library is irreplaceable for the tasks of sequential data reading or writing, especially if the requirements for the performance are increased and there are much data. The library is based on two main classes of the module CachedBuffers.pas: TCachedReader and TCachedWriter. The functionality of these two classes largely repeats the standard TStream class, but the difference is that you are using a convenient temporary memory buffer. Such architecture makes it possible to give the direct access to the memory and to increase productivity avoid calling virtual functions, universal implementations and API features of operating systems. An example of usage of the library and the illustration of a high performance can be seen in the demonstration projects.
The TCachedReader and TCachedWriter classes are supposed to indicate a callback in order to fulfill or commit a buffer. However, the library already contains a several standard classes which help to perform distributed tasks: TCachedFileReader, TCachedFileWriter, TCachedMemoryReader, TCachedMemoryWriter and TCachedResourceReader. In the future there also may be standard classes for the network interaction.
The library also contains TCachedReReader and TCachedReWriter classes which serve to be intermediate members. Those classes are necessary when the data is sequentially converted from one format to another. For example, if your parser receives data (TCachedReader) in the UTF-16LE encoding and an open file (TCachedFileReader) is written in another encoding, then you will need an intermediate member (TCachedReReader) processing the conversion into UTF-16LE. Another striking example of an intermediate member is the ZLib library data compression and decompression.
All the classes interacting among TCachedReader, TCachedWriter and TStream are located in CachedStreams.pas module. The TCachedStreamReader class (the heir of the TCachedReader) can read from TStream instance. The TCachedStreamWriter (the heir of the TCachedWriter) can write to TStream instance. The TCachedBufferStream (the heir of the TStream) uses TCachedReader or TCachedWriter. The TCachedBufferAdapter support the OLE interface IStream for the interaction with the TCachedReader or TCachedWriter.
As a bonus there is an NcMove function accessible in the CachedBuffers.pas module which on the average is two times faster than the standard Move but which is designed for the non-overlapping memory areas.
The TCachedBuffer is a mutual ancestor for the TCachedReader and the TCachedWriter. The main class property is the Memory. It is a allocated memory buffer aligned to 4KB which also has Previous и Additional areas (the description will be given below). The size of the buffer is set in a constructor where the default value is 64KB. If the size is not multiple of 4KB then the size automatically align (e.g. 5000 aligns to 8192). Please note that in some cases (for example, fixed or preallocated), the size of the memory buffer, as well as the previous/additional areas, may be aligned to 1KB or be completely absent.
To fill or read the buffer is used Current/Overflow pair. Current indicates the position in a buffer and allows the direct access to the memory. Overflow is an upper limit of the buffer. Margin defines the number of bytes accessible in a buffer (Overflow - Current). The Flush function updates the buffer and returns the number of bytes accessible in a buffer (Margin). The function may return 0 if the reading or writing is over.
The EOF is an indication of the end of the reading or writing, which also may be set to True. If needed it is possible to limit the reading or writing size by setting the Limit value.
TCachedBufferKind = (cbReader, cbWriter);
TCachedBufferCallback = function(const ASender: TCachedBuffer; AData: PByte; ASize: NativeUInt): NativeUInt of object;
TCachedBufferProgress = procedure(const ASender: TCachedBuffer; var ACancel: Boolean) of object;
TCachedBuffer = class(TCachedObject)
public
Current: PByte;
function Flush: NativeUInt;
property Kind: TCachedBufferKind read
property Overflow: PByte read
property Margin: NativeInt read
property EOF: Boolean read/write
property Limited: Boolean read
property Limit: Int64 read/write
property Memory: TCachedBufferMemory read
property Position: Int64 read
property OnProgress: TCachedBufferProgress read/write
end;The TCachedReader is used for the sequential data reading. The direct use of a buffer is allowed with the Current, Overflow properties and Flush function but for convenience sake you may use high-level TStream-like methods Read and ReadData. The Skip method allows to pass through a certain number of useless bytes. The Export method allows to write whole or a certain number of data bytes in the TCachedWriter.
The main constructor’s parameter is a Callback. It is a function which fills the buffer. During reading much data the Callback may be called passing the buffer. If the Callback returns less than Size parameter then the Finishing flag is set to True and at the next Flush the reading will be over (EOF = True).
The Memory.Additional (Overflow) area is always bigger than 4KB and may also serve for any reading needs the main of which is an AccessViolation avoidance with specific algorithms of data parsing. At the Flush moment the Current pointer may be less than Overflow. In this case the margin data is transferred to the Memory.Previous area. For example it is useful when the algorithm offers the struct processing and it is not fully stored in the buffer. The same is with lexeme parsing. The size of Memory.Previous area is minimum 4KB and when necessary it automatically grows during the Flush moment. But if the Current pointer is bigger than Overflow an exception will be thrown.
The DirectRead procedure allows data reading from a random place even without the buffer memory. Some instances of the TCachedReader allow addressing to random places (e.g. files or memory). If the addressing to random places is not supported, better be careful. While reading "before" the buffer an exception will throw. While reading "after" the buffer, it will be enlarged to the necessary size what can lead to memory shortage.
TCachedReader = class(TCachedBuffer)
public
constructor Create(const ACallback: TCachedBufferCallback; const ABufferSize: NativeUInt = 0);
procedure DirectRead(const APosition: Int64; var ABuffer; const ACount: NativeUInt);
property Finishing: Boolean read
procedure Skip(const ACount: NativeUInt);
procedure Export(const AWriter: TCachedWriter; const ACount: NativeUInt = 0);
procedure Read(var ABuffer; const ACount: NativeUInt);
procedure ReadData(var AValue: Boolean);
procedure ReadData(var AValue: AnsiChar);
procedure ReadData(var AValue: WideChar);
procedure ReadData(var AValue: ShortInt);
procedure ReadData(var AValue: Byte);
procedure ReadData(var AValue: SmallInt);
procedure ReadData(var AValue: Word);
procedure ReadData(var AValue: Integer);
procedure ReadData(var AValue: Cardinal);
procedure ReadData(var AValue: Int64);
procedure ReadData(var AValue: UInt64);
procedure ReadData(var AValue: Single);
procedure ReadData(var AValue: Double);
procedure ReadData(var AValue: TExtended80Rec);
procedure ReadData(var AValue: Currency);
procedure ReadData(var AValue: TPoint);
procedure ReadData(var AValue: TRect);
procedure ReadData(var AValue: ShortString);
procedure ReadData(var AValue: AnsiString; ACodePage: Word = 0);
procedure ReadData(var AValue: WideString);
procedure ReadData(var AValue: UnicodeString);
procedure ReadData(var AValue: TBytes);
procedure ReadData(var AValue: Variant);
end;The TCachedWriter is used for the sequential data writing. The class has only several differences from the TCachedReader.
The size of the Memory.Previous area may be equal 0 that is why do not write anything “before” the buffer. The size of the Memory.Additional (Overflow) is minimum 4KB. A writing in this area is permitted (Current is bigger than Overflow) and then at the next Flush an overflowed data will be transferred to the beginning of the buffer. If while Flush the Current pointer is less than Overflow then the writing is considered to be over and the EOF is set to True.
TCachedWriter = class(TCachedBuffer)
public
constructor Create(const ACallback: TCachedBufferCallback; const ABufferSize: NativeUInt = 0);
procedure DirectWrite(const APosition: Int64; const ABuffer; const ACount: NativeUInt);
procedure Import(const AReader: TCachedReader; const ACount: NativeUInt = 0);
procedure Write(const ABuffer; const ACount: NativeUInt);
procedure WriteData(const AValue: Boolean);
procedure WriteData(const AValue: AnsiChar);
procedure WriteData(const AValue: WideChar);
procedure WriteData(const AValue: ShortInt);
procedure WriteData(const AValue: Byte);
procedure WriteData(const AValue: SmallInt);
procedure WriteData(const AValue: Word);
procedure WriteData(const AValue: Integer);
procedure WriteData(const AValue: Cardinal);
procedure WriteData(const AValue: Int64);
procedure WriteData(const AValue: UInt64);
procedure WriteData(const AValue: Single);
procedure WriteData(const AValue: Double);
procedure WriteData(const AValue: TExtended80Rec);
procedure WriteData(const AValue: Currency);
procedure WriteData(const AValue: TPoint);
procedure WriteData(const AValue: TRect);
procedure WriteData(const AValue: ShortString);
procedure WriteData(const AValue: AnsiString);
procedure WriteData(const AValue: WideString);
procedure WriteData(const AValue: UnicodeString);
procedure WriteData(const AValue: TBytes);
procedure WriteData(const AValue: Variant);
end;The TCachedReReader is an intermediate member in reading tasks where a sequential data converting from one format to another is required. If during "destructor" the Owner is True then the Source is destroyed too. Pay attention to the fact that while calling the Callback the TCachedReReader itself is a Sender parameter but not the Source.
TCachedReReader = class(TCachedReader)
public
constructor Create(const ACallback: TCachedBufferCallback; const ASource: TCachedReader; const AOwner: Boolean = False; const ABufferSize: NativeUInt = 0);
property Source: TCachedReader read
property Owner: Boolean read/write
end;The TCachedReWriter is an intermediate member in writing tasks where a sequential data converting from one format to another is required. If during "destructor" the Owner is True then the Target is destroyed too. Pay attention to the fact that while calling the Callback the TCachedReWriter itself is a Sender parameter but not the Target.
TCachedReWriter = class(TCachedWriter)
public
constructor Create(const ACallback: TCachedBufferCallback; const ATarget: TCachedWriter; const AOwner: Boolean = False; const ABufferSize: NativeUInt = 0);
property Target: TCachedWriter read
property Owner: Boolean read/write
end;The TCachedFileReader is a standard class designed for files reading of their parts. The Limit property is automatically set and equals the size of the file or the Size parameter.
TCachedFileReader = class(TCachedReader)
public
constructor Create(const AFileName: string; const AOffset: Int64 = 0; const ASize: Int64 = 0);
constructor CreateHandled(const AHandle: THandle; const ASize: Int64 = 0; const AHandleOwner: Boolean = False);
property FileName: string read
property Handle: THandle read
property HandleOwner: Boolean read/write
property Offset: Int64 read
end;The TCachedFileWriter is a standard class designed for files writing or their parts. The Limit parameter is set if the Size parameter is defined.
TCachedFileWriter = class(TCachedWriter)
public
constructor Create(const AFileName: string; const ASize: Int64 = 0);
constructor CreateHandled(const AHandle: THandle; const ASize: Int64 = 0; const AHandleOwner: Boolean = False);
property FileName: string read
property Handle: THandle read
property HandleOwner: Boolean read/write
property Offset: Int64 read
end;TCachedMemoryReader is a standard class designed for compatibility of the TCachedReader with the reading from a specified memory area. The Limit property is automatically set and it equals Size. The Fixed parameter allows you to read data from memory directly, but this can lead to errors, because the standard logic of memory alignment and vailability of previous/additional areas is broken.
TCachedMemoryReader = class(TCachedReader)
public
constructor Create(const APtr: Pointer; const ASize: NativeUInt; const AFixed: Boolean = False);
property Ptr: Pointer read
property Size: NativeUInt read
end;TCachedMemoryWriter is a standard class designed for compatibility of the TCachedWriter with the writing to the temporary or specified memory area. If it is a CreateTemporary then the Ptr memory resizes with every Flush calling. The Fixed parameter allows you to write data into memory directly, but this can lead to errors, because the standard logic of memory alignment and vailability of previous/additional areas is broken.
TCachedMemoryWriter = class(TCachedWriter)
public
constructor Create(const APtr: Pointer; const ASize: NativeUInt; const AFixed: Boolean = False);
constructor CreateTemporary;
property Temporary: Boolean read
property Ptr: Pointer read
property Size: NativeUInt read
end;TCachedResourceReader is a standard class designed for compatibility of the TCachedReader with the reading of the resources. It repeats interface of the Classes.TResourceStream.
TCachedResourceReader = class(TCachedMemoryReader)
public
constructor Create(const AInstance: THandle; const AResName: string; const AResType: PChar; const AFixed: Boolean = False);
constructor CreateFromID(const AInstance: THandle; const AResID: Word; const AResType: PChar; const AFixed: Boolean = False);
end;