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Date: Wed, 28 Mar 2012 00:01:49 +0200
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
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Subject: Re: xor
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Date: 2012-03-28T00:01:50+02:00
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On 27.03.12 23:44, Michael Moeller wrote:
> dealing with ciphers
> the size of file is well known in advance and must be verified in any event.
> In addition sometimes we'll only pick certain parts of a pretty huge file
> and we never hit EOF.


Just in case you'll have to read blocks, and while Ada.Streams
should be most efficient, another convenient and also venerable
method uses an instance Direct_IO, instantiated for blocks.
You can set the file position to the block you need.

with Ada.Direct_IO;
with Interfaces;

procedure Position is

    -- Read "xyz.bin", second block, where
    -- $ perl -e "print 'x' x 512, 'y' x 512, 'z' x 512" > xyz.bin

    subtype Byte is Interfaces.Unsigned_8;

    type Block_Index is range 0 .. 511;
    type Block is array (Block_Index) of Byte;

    package Block_IO is new Ada.Direct_IO (Block);

    -- second block should be all 'y's; in terms of Byte:
    Y : constant Byte := Byte'Val (Character'Pos ('y'));

    -- a virtual character not occuring in input:
    Rubbish: constant Byte := Byte'Val (Character'Pos ('*'));

    Input : Block_IO.File_Type;
    Data : Block := Block'(Block_Index => Rubbish);

    use Block_IO;
begin
    Open (Input, Name => "xyz.bin", Mode => In_File);
    Set_Index (Input, To => 2);
    Read (Input, Item => Data);
    Close (Input);

    if Data /= Block'(Block_Index => Y) then
       raise Program_Error;
    end if;
end Position;