• Christian Manning (20/20) Jul 04 2011 Hi, when I populate a BitArray using .init, the bits are in reverse
• Steven Schveighoffer (11/30) Jul 04 2011 What exactly are you expecting? In other words, when using the void[]
• Christian Manning (6/41) Jul 04 2011 Sorry, I assumed it was BitArray causing this, but that's just what made...

Christian Manning <cmanning999 gmail.com> writes:

Hi, when I populate a BitArray using .init, the bits are in reverse 
order, although the bytes are in the correct order.
eg:

import std.stdio,std.bitmanip;

void main() {
     ubyte[] arr = [130,56,43,2];

     BitArray ba;
     ba.init(cast(void[])arr,32);
     foreach(b; ba)
        write(cast(ubyte) b); //bits in each byte reversed
                              //01000001000111001101010001000000
     writeln();

     ba.init(cast(void[])arr.reverse,32);
     foreach(b; ba.reverse)
        write(cast(ubyte) b); //bits printed in intended order
                              //10000010001110000010101100000010
     writeln();
}

Why does this happen?

Thanks

"Steven Schveighoffer" <schveiguy yahoo.com> writes:

On Mon, 04 Jul 2011 14:03:35 -0400, Christian Manning  
<cmanning999 gmail.com> wrote:

 Hi, when I populate a BitArray using .init, the bits are in reverse  
 order, although the bytes are in the correct order.
 eg:

 import std.stdio,std.bitmanip;

 void main() {
      ubyte[] arr = [130,56,43,2];

      BitArray ba;
      ba.init(cast(void[])arr,32);
      foreach(b; ba)
         write(cast(ubyte) b); //bits in each byte reversed
                               //01000001000111001101010001000000
      writeln();

      ba.init(cast(void[])arr.reverse,32);
      foreach(b; ba.reverse)
         write(cast(ubyte) b); //bits printed in intended order
                               //10000010001110000010101100000010
      writeln();
 }

 Why does this happen?

What exactly are you expecting?  In other words, when using the void[]  
version of init, you are specifying the exact memory to use, which implies  
you know how BitArray's internals work.  If you create a BitArray by  
adding bits to the BitArray, then it's internal storage doesn't matter to  
you, it can store however it wants.

I'd guess the most logical reason to store the bits in reverse order would  
be for performance.  Not sure what the merits are, but typically, it's  
faster to access and manipulate the 0 bit than some arbitrary bit.

-Steve

Christian Manning <cmanning999 gmail.com> writes:

On 04/07/2011 19:34, Steven Schveighoffer wrote:
 On Mon, 04 Jul 2011 14:03:35 -0400, Christian Manning
 <cmanning999 gmail.com> wrote:

 Hi, when I populate a BitArray using .init, the bits are in reverse
 order, although the bytes are in the correct order.
 eg:

 import std.stdio,std.bitmanip;

 void main() {
 ubyte[] arr = [130,56,43,2];

 BitArray ba;
 ba.init(cast(void[])arr,32);
 foreach(b; ba)
 write(cast(ubyte) b); //bits in each byte reversed
 //01000001000111001101010001000000
 writeln();

 ba.init(cast(void[])arr.reverse,32);
 foreach(b; ba.reverse)
 write(cast(ubyte) b); //bits printed in intended order
 //10000010001110000010101100000010
 writeln();
 }

 Why does this happen?

 What exactly are you expecting? In other words, when using the void[]
 version of init, you are specifying the exact memory to use, which
 implies you know how BitArray's internals work. If you create a BitArray
 by adding bits to the BitArray, then it's internal storage doesn't
 matter to you, it can store however it wants.

 I'd guess the most logical reason to store the bits in reverse order
 would be for performance. Not sure what the merits are, but typically,
 it's faster to access and manipulate the 0 bit than some arbitrary bit.

 -Steve

Sorry, I assumed it was BitArray causing this, but that's just what made 
me aware of it.

It seems dmd does this when accessing pointers directly, so your 
reasoning is probably right!

Thanks!