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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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);
@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
Business Accounts
Answer for Membership
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.