• BitGuy (10/10) Jul 06 2016 I'm trying to implement a feistel cipher that'll give the same
• Lodovico Giaretta (8/18) Jul 06 2016 You can just use bitwise operations, without casts (with Value
• Alex Parrill (5/15) Jul 06 2016 Bitshifts and binary operators work the same regardless of
• Gary Willoughby (25/35) Jul 07 2016 What about something like:
• Lodovico Giaretta (4/28) Jul 07 2016 Hi,
• Gary Willoughby (4/6) Jul 07 2016 It shouldn't matter. You're just interested in the high and low 4
• Lodovico Giaretta (4/11) Jul 07 2016 But are the high 4 bytes the first 4 or the second 4? It depends
• Gary Willoughby (25/37) Jul 07 2016 Ah, I see. You could modify it like this:

BitGuy <fsdfsd sdfs.com> writes:

I'm trying to implement a feistel cipher that'll give the same 
results regardless of the endianness of the machine it runs on. 
To make the cipher I need to split a 64bit value into two 32bit 
values, mess with them, and then put them back together. I can 
think of a few ways to split a 64bit value with versions or the 
endianness functions in bitmanip but it all seems pretty messy 
for just wanting to split a value... I'm thinking maybe I can 
just cast and bitshift so I can forget about the endianness but 
I'm not really sure about the casting down rules and if that'd 
work?

Lodovico Giaretta <lodovico giaretart.net> writes:

On Wednesday, 6 July 2016 at 21:44:37 UTC, BitGuy wrote:
 I'm trying to implement a feistel cipher that'll give the same 
 results regardless of the endianness of the machine it runs on. 
 To make the cipher I need to split a 64bit value into two 32bit 
 values, mess with them, and then put them back together. I can 
 think of a few ways to split a 64bit value with versions or the 
 endianness functions in bitmanip but it all seems pretty messy 
 for just wanting to split a value... I'm thinking maybe I can 
 just cast and bitshift so I can forget about the endianness but 
 I'm not really sure about the casting down rules and if that'd 
 work?

You can just use bitwise operations, without casts (with Value 
Range Propagation, the compiler knows that the two halves fit a 
uint each). The recomposition will require a cast, however.

ulong value = ...
uint low = value & uint.max;
uint high = value >>> 32;
assert(value == ((cast(ulong)high << 32) | low));

Alex Parrill <initrd.gz gmail.com> writes:

On Wednesday, 6 July 2016 at 21:44:37 UTC, BitGuy wrote:
 I'm trying to implement a feistel cipher that'll give the same 
 results regardless of the endianness of the machine it runs on. 
 To make the cipher I need to split a 64bit value into two 32bit 
 values, mess with them, and then put them back together. I can 
 think of a few ways to split a 64bit value with versions or the 
 endianness functions in bitmanip but it all seems pretty messy 
 for just wanting to split a value... I'm thinking maybe I can 
 just cast and bitshift so I can forget about the endianness but 
 I'm not really sure about the casting down rules and if that'd 
 work?

Bitshifts and binary operators work the same regardless of 
endianness. Just shift by 32/mask  with 0xFFFFFFFF and cast.

It's only if you are accessing a value as a byte array that you 
have to worry about endianness.

Gary Willoughby <dev nomad.so> writes:

On Wednesday, 6 July 2016 at 21:44:37 UTC, BitGuy wrote:
 I'm trying to implement a feistel cipher that'll give the same 
 results regardless of the endianness of the machine it runs on. 
 To make the cipher I need to split a 64bit value into two 32bit 
 values, mess with them, and then put them back together. I can 
 think of a few ways to split a 64bit value with versions or the 
 endianness functions in bitmanip but it all seems pretty messy 
 for just wanting to split a value... I'm thinking maybe I can 
 just cast and bitshift so I can forget about the endianness but 
 I'm not really sure about the casting down rules and if that'd 
 work?

What about something like:

import std.stdio;

union Value
{
     ulong full;

     static struct Bits
     {
         uint high;
         uint low;
     }

     Bits bits;
     alias bits this;

     this(ulong value)
     {
         this.full = value;
     }
}

void main(string[] args)
{
     auto value = Value(77309411348);

     writefln("%s, (%b)", value.high, value.high);
     writefln("%s, (%b)", value.low, value.low);
     writefln("%s, (%b)", value.full, value.full);
}

Lodovico Giaretta <lodovico giaretart.net> writes:

On Thursday, 7 July 2016 at 08:14:40 UTC, Gary Willoughby wrote:
 What about something like:

 import std.stdio;

 union Value
 {
     ulong full;

     static struct Bits
     {
         uint high;
         uint low;
     }

     Bits bits;
     alias bits this;

     this(ulong value)
     {
         this.full = value;
     }
 }

 void main(string[] args)
 {
     auto value = Value(77309411348);

     writefln("%s, (%b)", value.high, value.high);
     writefln("%s, (%b)", value.low, value.low);
     writefln("%s, (%b)", value.full, value.full);
 }

Hi,
Are you sure that this works in both big-endian and little-endian 
systems?

Gary Willoughby <dev nomad.so> writes:

On Thursday, 7 July 2016 at 08:21:53 UTC, Lodovico Giaretta wrote:
 Are you sure that this works in both big-endian and 
 little-endian systems?

It shouldn't matter. You're just interested in the high and low 4 
byte chunks (which are to be interpreted as an int) which will 
return in the relevant endianess of your machine.

Lodovico Giaretta <lodovico giaretart.net> writes:

On Thursday, 7 July 2016 at 10:45:12 UTC, Gary Willoughby wrote:
 On Thursday, 7 July 2016 at 08:21:53 UTC, Lodovico Giaretta 
 wrote:
 Are you sure that this works in both big-endian and 
 little-endian systems?

 It shouldn't matter. You're just interested in the high and low 
 4 byte chunks (which are to be interpreted as an int) which 
 will return in the relevant endianess of your machine.

But are the high 4 bytes the first 4 or the second 4? It depends 
on endianness. So your high and low variables may be switched, if 
I understand correctly.

Gary Willoughby <dev nomad.so> writes:

On Thursday, 7 July 2016 at 10:48:56 UTC, Lodovico Giaretta wrote:
 On Thursday, 7 July 2016 at 10:45:12 UTC, Gary Willoughby wrote:
 On Thursday, 7 July 2016 at 08:21:53 UTC, Lodovico Giaretta 
 wrote:
 Are you sure that this works in both big-endian and 
 little-endian systems?

 It shouldn't matter. You're just interested in the high and 
 low 4 byte chunks (which are to be interpreted as an int) 
 which will return in the relevant endianess of your machine.

 But are the high 4 bytes the first 4 or the second 4? It 
 depends on endianness. So your high and low variables may be 
 switched, if I understand correctly.

Ah, I see. You could modify it like this:

union Value
{
     ulong full;

     static struct Bits
     {
         version (BigEndian)
         {
             uint high;
             uint low;
         }
         else
         {
             uint low;
             uint high;
         }

     }

     Bits bits;
     alias bits this;

     this(ulong value)
     {
         this.full = value;
     }
}