Initializing static array with contents of (static and dynamic) arrays

[Johannes Loher via Digitalmars-d-learn]

Am 04.07.2016 um 20:33 schrieb Ali Çehreli:
> On 07/04/2016 07:31 AM, Johannes Loher wrote:
>> In a project I am currently working on, I have lot's of code of the
>> following form:
>>
>> static immutable ubyte[4] sigma0 = [101, 120, 112,  97];
>> static immutable ubyte[4] sigma1 = [110, 100,  32,  51];
>> static immutable ubyte[4] sigma2 = [ 50,  45,  98, 121];
>> static immutable ubyte[4] sigma3 = [116, 101,  32, 107];
>>
>> void func(in ubyte[32] key, in ubyte[16] n)
>> {
>>      ubyte[64] buf;
>>      buf[0..4] = sigma0;
>>      buf[4..20] = key[0..16];
>>      buf[20..24] = sigma1;
>>      buf[24..40] = n;
>>      buf[40..44] = sigma2;
>>      buf[44..60] = key[16..$];
>>      buf[60..64] = sigma3;
>>
>>     /* do something with buf */
>> }
> 
> Here's an option that overlays a struct on top of the bytes:
> 
> struct Buf {
>     union {
>         struct {
>             ubyte[4] sigma0 = [101, 120, 112,  97];
>             ubyte[16] keyBeg;
>             ubyte[4] sigma1 = [110, 100,  32,  51];
>             ubyte[16] n;
>             ubyte[4] sigma2 = [ 50,  45,  98, 121];
>             ubyte[16] keyEnd;
>             ubyte[4] sigma3 = [116, 101,  32, 107];
>         }
> 
>         ubyte[64] bytes;
>     }
> 
>     this(const ubyte[] key, const ubyte[] n) {
>         this.keyBeg = key[0..16];
>         this.keyEnd = key[16..$];
>         this.n = n;
>     }
> }
> 
> static assert(Buf.sizeof == 64);
> 
> void func(in ubyte[] key, in ubyte[] n) {
>     auto buf = Buf(key, n);
>     writeln(buf.bytes);
> }
> 
> import std.stdio;
> import std.range;
> import std.array;
> 
> void main() {
>     func(ubyte(32).iota.array,
>          ubyte(16).iota.array);
> }
> 
> Prints:
> 
> [101, 120, 112, 97, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
> 15, 110, 100, 32, 51, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
> 15, 50, 45, 98, 121, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28,
> 29, 30, 31, 116, 101, 32, 107]
> 
> Ali
> 
This looks really nice, but I have several occurences of this, with
different arrays (and lengths), so i would need to create several of
those structs. But it looks really clean :)