How to fix byte ordering issue in this piece of code? - delphi

Question

To read a index file in a specific format, I cooked the following piece of code without considering byte ordering:
unit uCBI;
interface
uses
SysUtils,
Classes,
Generics.Collections;
type
TIndexList = class
private
FIndexList:TList<Cardinal>;
FOwnedStream:Boolean;
FMemoryStream: TMemoryStream;
function GetCount: Integer;
protected
public
constructor Create(AStream:TMemoryStream; OwnedStream:Boolean=True);
destructor Destroy; override;
function Add(const Value: Cardinal): Integer;
procedure Clear;
procedure SaveToFile(AFileName:TFileName);
procedure LoadFromFile(AFileName:TFileName);
property Count: Integer read GetCount;
end;
implementation
{ TIndexList }
function TIndexList.Add(const Value: Cardinal): Integer;
begin
Result := FIndexList.Add(Value)
end;
procedure TIndexList.Clear;
begin
FIndexList.Clear;
end;
constructor TIndexList.Create(AStream: TMemoryStream; OwnedStream: Boolean);
begin
FMemoryStream := AStream;
FOwnedStream := OwnedStream;
FIndexList := TList<Cardinal>.Create;
end;
destructor TIndexList.Destroy;
begin
if (FOwnedStream and Assigned(FMemoryStream)) then
FMemoryStream.Free;
FIndexList.Free;
//
inherited;
end;
function TIndexList.GetCount: Integer;
begin
Result := FIndexList.Count;
end;
procedure TIndexList.LoadFromFile(AFileName: TFileName);
var
lMemoryStream:TMemoryStream;
lCount:Cardinal;
begin
lMemoryStream := TMemoryStream.Create;
try
lMemoryStream.LoadFromFile(AFileName);
lMemoryStream.ReadBuffer(lCount,SizeOf(Cardinal));
if (lCount = Cardinal((lMemoryStream.Size-1) div SizeOf(Cardinal))) then
begin
FMemoryStream.Clear;
lMemoryStream.Position :=0;
FMemoryStream.CopyFrom(lMemoryStream,lMemoryStream.Size)
end else
raise Exception.CreateFmt('Corrupted CBI file: %s',[ExtractFileName(AFileName)]);
finally
lMemoryStream.Free;
end;
end;
procedure TIndexList.SaveToFile(AFileName: TFileName);
var
lCount:Cardinal;
lItem:Cardinal;
begin
FMemoryStream.Clear;
lCount := FIndexList.Count;
FMemoryStream.WriteBuffer(lCount,SizeOf(Cardinal));
for lItem in FIndexList do
begin
FMemoryStream.WriteBuffer(lItem,SizeOf(Cardinal));
end;
//
FMemoryStream.SaveToFile(AFileName);
end;
end.
It tested it and seems to work well as needed. Great was my suprise when I pursue extensive tests with real sample file. In fact the legacy format was devised with Amiga computer with a different byte ordering.
My Question:
How can I fix it ?
I want to keep the code unchanged and wonder wether a decorated TMemorySream will do so that I can transparently switch between big endian and little endian.

Answer 1

To change 'endianness' of Cardinals you can use the following:
function EndianChange(Value: Cardinal): Cardinal;
var
A1: array [0..3] of Byte absolute Value;
A2: array [0..3] of Byte absolute Result;
I: Integer;
begin
for I:= 0 to 3 do begin
A2[I]:= A1[3 - I];
end;
end;
If you want to keep your code unchanged, you can write your own TMemoryStream descendant and override its Read and Write methods using the above function, like that:
function TMyMemoryStream.Read(var Buffer; Count: Integer): Longint;
var
P: PCardinal;
I, N: Integer;
begin
inherited;
P:= #Buffer;
Assert(Count and 3 = 0);
N:= Count shr 2;
while N > 0 do begin
P^:= EndianChange(P^);
Inc(P);
Dec(N);
end;
end;