// I, Danny Milosavljevic, hereby release this code into the public domain.

unit ring_buffers;

{$MODE OBJFPC}
{$M+}

interface

type
  TRingBuffer = class
  private
    fBuffer : array[0..8191] of Byte; // GIF LZW has 4K decompression size per code.
    fBufferReadPosition : Cardinal;
    fBufferWritePosition : Cardinal;
    fBufferFillCount : Cardinal;
  protected
    function GetBufferSize() : Cardinal; inline;
  published
    property FillCount : Cardinal read fBufferFillCount;
    property Size : Cardinal read GetBufferSize;
    procedure Write(const aBuffer; aSize : Cardinal); inline;
    function Read(var aBuffer; aSize : Cardinal) : Cardinal; inline;
  end;

implementation

{ TRingBuffer }

function TRingBuffer.GetBufferSize() : Cardinal; inline;
begin
  Result := Length(fBuffer);
end;

procedure TRingBuffer.Write(const aBuffer; aSize : Cardinal); inline;
var
  vBuffer : PByte;
begin
  vBuffer := @aBuffer;
  // TODO optimize.

  while aSize > 0 do begin  
    assert(fBufferFillCount < Length(fBuffer));
    fBuffer[fBufferWritePosition] := vBuffer^;
    Inc(vBuffer);
    Inc(fBufferFillCount);
    Inc(fBufferWritePosition);
    if fBufferWritePosition >= Length(fBuffer) then
      fBufferWritePosition := 0;
    
    assert(fBufferWritePosition <> fBufferReadPosition);
    Dec(aSize);
  end;
end;

function TRingBuffer.Read(var aBuffer; aSize : Cardinal) : Cardinal; inline;
var
  vBuffer : PByte;
  vBufferCount : Cardinal;
  vBufferCount1 : Cardinal;
  vBufferCount2 : Cardinal;
begin
  vBuffer := @aBuffer;
  Result := 0;
    
  if fBufferFillCount > 0 then begin
      vBufferCount := aSize; // try to take as much as requested by the client...
      if vBufferCount > fBufferFillCount then // ... if possible.
        vBufferCount := fBufferFillCount;
      
      if fBufferReadPosition < fBufferWritePosition then begin {  ------RXXXXXXW-------- }
        vBufferCount1 := fBufferWritePosition - fBufferReadPosition; // max count for the first Move.
        assert(vBufferCount <= vBufferCount1);
        Move(fBuffer[fBufferReadPosition], vBuffer^, vBufferCount);
        Inc(vBuffer, vBufferCount);
      end else begin                                           {  XXXW-----RXXXXXXXXXXXX }
        vBufferCount1 := Length(fBuffer) - fBufferReadPosition; // count for the first Move.
        if vBufferCount < vBufferCount1 then
          vBufferCount1 := vBufferCount;

        if vBufferCount1 > 0 then begin
          Move(fBuffer[fBufferReadPosition], vBuffer^, vBufferCount1);
          Inc(vBuffer, vBufferCount1);
        end;

        vBufferCount2 := vBufferCount - vBufferCount1; // remaining count for the second Move.
        
        if vBufferCount2 > 0 then begin
          Move(fBuffer[0], vBuffer^, vBufferCount2);
          Inc(vBuffer, vBufferCount2);
        end;
      end;
    
      Inc(fBufferReadPosition, vBufferCount);
      while fBufferReadPosition >= Length(fBuffer) do
        Dec(fBufferReadPosition, Length(fBuffer));

      assert(fBufferFillCount >= vBufferCount);      
      Dec(fBufferFillCount, vBufferCount);
      Inc(Result, vBufferCount);
  end;
end;

end.