Bit rotation question/challenge

[vitamin]

On Saturday, 30 January 2021 at 14:56:14 UTC, burt wrote:
> On Saturday, 30 January 2021 at 14:41:59 UTC, Afgdr wrote:
>> On Saturday, 30 January 2021 at 14:40:49 UTC, Afgdr wrote:
>>> On Saturday, 30 January 2021 at 13:30:49 UTC, burt wrote:
>>>> [...]
>>>
>>> cast as uint and shift. cast the result as ubyte[4].
>>
>> obiously, that works for n=4 with uint and n=8 for ulong, only.
>
> Yes I used to do this, but then I needed it for n > 8.

You can try somethink like this:

https://run.dlang.io/is/POQgnb

import std.range : cycle, take, drop;
import std.algorithm : copy;
import std.stdio;

version (LittleEndian)
ubyte[n] rotateRight(size_t n)(ref const ubyte[n] array, uint 
rotation){
     typeof(return) result;

     array[]
         .cycle()
         .drop(n - (rotation / 8) % n)
         .take(n)
         .copy(result[]);

     const ubyte bit_rotation = rotation % 8;

     enum ubyte full = 0b1111_1111;

     if(bit_rotation == 0)
         return result;

     ubyte next_prefix(const ubyte elm){
         const ubyte suffix = (elm & ~(full << bit_rotation));
         const ubyte prefix = cast(ubyte)(suffix << (8 - 
bit_rotation));
         return prefix;
     }

     ubyte prefix = next_prefix(result[$-1]);

     foreach(ref ubyte elm; result[]){
         const new_prefix = next_prefix(elm);
         elm = (elm >> bit_rotation) | prefix;
         prefix = new_prefix;
     }

     return result;
}

void main(){
     ubyte[4] x = [
         0b00011000,
         0b00100001,
         0b00010101,
         0b11110010,
     ];

	writefln!"%(%8b,\n%)"(x.rotateRight(4));
}