Initializing static array with contents of (static and dynamic) arrays
Johannes Loher via Digitalmars-d-learn
digitalmars-d-learn at puremagic.com
Tue Jul 5 08:28:52 PDT 2016
Am 05.07.2016 um 17:22 schrieb Marc Schütz:
> auto concat(T : E[n], E, size_t n)(const E[][] args...) @nogc
> {
> size_t offset = 0;
> T result = void;
> foreach(arr; args) {
> result[offset .. offset+arr.length] = arr;
> offset += arr.length;
> }
> assert(offset == result.length);
> return result;
> }
>
> 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 ref ubyte[32] key, in ref ubyte[16] n) @nogc
> {
> 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..$];
> buf[60..64] = sigma3;
>
> auto buf2 = concat!(ubyte[64])(sigma0, key[0..16], sigma1, n,
> sigma2, key[16..$], sigma3);
>
> assert(buf == buf2);
> }
>
> void main() {
> ubyte[32] key =
> [0,1,2,3,4,5,6,7,8,9,0,1,2,3,4,5,6,7,8,9,0,1,2,3,4,5,6,7,8,9,0,1];
> ubyte[16] n = key[0..16];
> func(key, n);
> }
>
>
> Some remarks:
>
> * I added `ref` to `func`'s parameters, because the arrays are
> relatively large, so passing them by value might be costly (you should
> measure it if you care).
>
> * The `void` initialization in `concat` is an optimization that is valid
> only for POD types.
>
> * Returning the array is cheap because of NRVO.
This seems to be exactly what I was looking for. Thanks a lot!
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