Question

Bit Manipulation

Asked by: nacker2000

Hi all,

I am converting some C code to delphi and I am having some problems with working at the bit level with some routines.

I need to write 16 bits in total, the first 4 bits need to have a value and the last 12 bits need to have a value. It the C example it looks as follows (note that the 4 bits then 12 bits needs to be done twice):

init_put_bits(&pb, buf, buf_size*8);
put_bits(&pb, 4, (block_width/16)-1);
put_bits(&pb, 12, s->image_width);
put_bits(&pb, 4, (block_height/16)-1);
put_bits(&pb, 12, s->image_height);
flush_put_bits(&pb);

You see the first 4 bits is a blocksize and the 12 bits is a width, and then its repeated for the height. This is part of a simple codec I am working on.

Now I understand these are routines in C but suppose I have a memorystream or filestream how would I go about writing 4 bits and then 12 bits to the stream?

If I had a WORD variable to hold the 2 bytes (16-bits) could I build it up that way perhaps, if so how?

Also how would I go about converting the 4 bits and 12 bits to their values, reverse it if you like?

Thanks.

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Asked On
2008-04-22 at 13:03:23ID23344389
Tags

Delphi

Topic

Delphi Programming

Participating Experts
5
Points
500
Comments
7

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Answers

 

by: ziolkoPosted on 2008-04-22 at 13:20:47ID: 21415175

procedure SetBit(var AValue:Integer; ABitNo: Word);
begin
  AValue := AValue or (1 shl ABitNo);
end;

function IsBitSet(BitMask: Integer; ABitNo: Byte): Boolean;
var v: Integer;
begin
  v := 1 shl ABitNo;
  Result := BitMask and v = v;
end;


note that bit 0 is right most bit
there's no range checking

ziolko.

 

by: mokulePosted on 2008-04-22 at 13:25:15ID: 21415220

var
  i: word;
  j: word;
  k: word;
begin
  i := 7;
  j := 123;
  k := (i and $f) shl 12 or (j and $fff);   // merge 4 bits from i and12 from j
// and back split k
  i := (k shr 12) and $f;
  j := k and $fff;

  showMessage(IntToStr(k));
  showMessage(IntToStr(i));
  showMessage(IntToStr(j));

 

by: MerijnBPosted on 2008-04-22 at 13:28:24ID: 21415250

> If I had a WORD variable to hold the 2 bytes (16-bits) could I build it up that way perhaps, if so how?

yep, that's it, see example below for both ways:

var blocksize, Width, total: word;
begin
 blocksize := 4;
 Width := 12;
 
 // combine
 total := ((blocksize and $F) shl 12) or (Width and $FFF);
 
 // split
 blocksize := (total and $F000) shr 12;
 Width := total and $FFF;
end;
                                              
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by: MerijnBPosted on 2008-04-22 at 13:28:44ID: 21415254

lol

 

by: Geert_GruwezPosted on 2008-04-23 at 00:49:02ID: 21418441

bittotext('lol') = 'laughing out loud'

ughuh

 

by: gskoczylasPosted on 2008-04-23 at 01:45:20ID: 21418619

If number of bits is less or euqal 255 you can use *set*, e.g.

type
  TBits = set of 0..15;

var
  Bits:  TBits;

begin
  { initialize example }
  Bits := [1..3, 5];

  { setting bits example }
  Bits := Bits + [2..4, 9, 10];

  { clearing bits example }
  Bits := Bits - [3, 4];

  { checking if bit is set example }
  if  5 in Bits  then
    ActionWhenSet
  else
    ActionWhenClear;

 { clearing all bits example }
 Bits := [];

If you need to convert bits to number then you have to cast it to the number with the same number of bits. WORD size is 2 bytes, 16 bits. To convert 'Bits' to number use following assignment:

var
  BitsNum:  Word;

BitsNum := Word(Bits);

 

by: nacker2000Posted on 2008-04-23 at 03:33:04ID: 21419074

@mokule and @MerijnB

Thanks both of you. This seems to work fine. I had to do some endian swapping after the bit setting, this was throwing me off at first because it wasn't working hehe.

Since you both gave me the same answer within minutes of each other (although written slightly different) I have increased the points to 500 and will award you both 250.

Thanks

20120131-EE-VQP-002

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