Hi,

following the advice of howardpc, I repost here my question from the forum (https://forum.lazarus.freepascal.org/index.php/topic,60287.0.html).

Recently I wanted to rewind a file to read again a previous line. After some search I came to the conclusion to modify the TStreamReader class. Do you think that would be a useful addition? Should it be an a modification to TStreamReader or a new class?

Ciao,

Gilles Marcou

Below is a sketch of the modifications.

1. Add a list of line start positions in the stream:

Code: Pascal [Select][+]

1.

    TListInt64 = specialize TFPGList<Int64>;

2.
(...)
3.
 TStreamReader = class(TTextReader)
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    private

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          flPos: TListInt64;

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    function ReadPos(Idx: integer): Int64;

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    procedure WritePos(Idx: integer; AValue: Int64);

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(...)
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    public

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    property lPos[Idx: integer]: Int64 read ReadPos write WritePos;

11.

2. Manage this list
Code: Pascal [Select][+]

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    function TStreamReader.ReadPos(Idx: integer): Int64;

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    begin

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       Result:=flPos[Idx];

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    end;

5.
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    procedure TStreamReader.WritePos(Idx: integer; AValue: Int64);

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    begin

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       flPos[idx]:=AValue;

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    end;

10.
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    constructor TStreamReader.Create(AStream: TStream; ABufferSize: Integer;

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    AOwnsStream: Boolean);

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    begin

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(...)
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       flPos:=TListInt64.Create;

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    end;

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    procedure TStreamReader.Close;

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    begin

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(...)
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    FreeAndNil(flPos);

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    end;

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3. Store line start positions
Code: Pascal [Select][+]

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    procedure TStreamReader.ReadLine(out AString: string);

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    var

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       VPByte: PByte;

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       VPosition, VStrLength, VLength: Integer;

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    begin

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       flPos.Add(FBufferPosition);

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4. Add a procedure to jump to the desired line
Code: Pascal [Select][+]

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    procedure GoToLn(n: integer);

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(...)
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    procedure TStreamReader.GoToLn(n: integer);

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    var

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       apos: Int64;

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    begin

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       apos:=flPos[n-1];//flPos is 0-based and lines are 1-based

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       Reset;

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       FStream.Seek(apos,soBeginning);

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    end;

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I think this should be in a new class, with a descendent that one could attach to a TStream and a descendent to attach to a TStreamReader.

Michael.