Question

Is there an easy way to bit-reflect a byte variable in Delphi so that the most significant bit (MSB) gets the least significant bit (LSB) and vice versa?

Answer 1

In code you can do it like this:

function ReverseBits(b: Byte): Byte;
var 
  i: Integer;
begin
  Result := 0;
  for i := 1 to 8 do
  begin
    Result := (Result shl 1) or (b and 1);
    b := b shr 1;
  end;
end;

But a lookup table would be much more efficient, and only consume 256 bytes of memory.

function ReverseBits(b: Byte): Byte; inline;
const
  Table: array [Byte] of Byte = (
    0,128,64,192,32,160,96,224,16,144,80,208,48,176,112,240,
    8,136,72,200,40,168,104,232,24,152,88,216,56,184,120,248,
    4,132,68,196,36,164,100,228,20,148,84,212,52,180,116,244,
    12,140,76,204,44,172,108,236,28,156,92,220,60,188,124,252,
    2,130,66,194,34,162,98,226,18,146,82,210,50,178,114,242,
    10,138,74,202,42,170,106,234,26,154,90,218,58,186,122,250,
    6,134,70,198,38,166,102,230,22,150,86,214,54,182,118,246,
    14,142,78,206,46,174,110,238,30,158,94,222,62,190,126,254,
    1,129,65,193,33,161,97,225,17,145,81,209,49,177,113,241,
    9,137,73,201,41,169,105,233,25,153,89,217,57,185,121,249,
    5,133,69,197,37,165,101,229,21,149,85,213,53,181,117,245,
    13,141,77,205,45,173,109,237,29,157,93,221,61,189,125,253,
    3,131,67,195,35,163,99,227,19,147,83,211,51,179,115,243,
    11,139,75,203,43,171,107,235,27,155,91,219,59,187,123,251,
    7,135,71,199,39,167,103,231,23,151,87,215,55,183,119,247,
    15,143,79,207,47,175,111,239,31,159,95,223,63,191,127,255
  );
begin
  Result := Table[b];
end;

This is more than 10 times faster than the version of the code that operates on individual bits.

---

Finally, I don’t normally like to comment too negatively on accepted answers when I have a competing answer. In this case there are very serious problems with the answer that you accepted that I would like to state clearly for you and also for any future readers.

You accepted @Arioch’s answer at the time when it contained the same Pascal code as can be seen in this answer, together with two assembler versions. It turns out that those assembler versions are much slower than the Pascal version. They are twice as slow as the Pascal code.

It is a common fallacy that converting high level code to assembler results in faster code. If you do it badly then you can easily produce code that runs more slowly than the code emitted by the compiler. There are times when it is worth writing code in assembler but you must not ever do so without proper benchmarking.

What is particularly egregious about the use of assembler here is that it is so obvious that the table based solution will be exceedingly fast. It’s hard to imagine how that could be significantly improved upon.