Function to print a diamond shape

[monarch_dodra via Digitalmars-d-learn]

On Monday, 21 April 2014 at 00:11:14 UTC, Jay Norwood wrote:
> So this printDiamonde2b example had the fastest time of the 
> solutions, and had similar times on all three builds. The ldc2 
> compiler build is performing best in most examples on ubuntu.
>
> void printDiamonde2b(in uint N)
> {
>     uint N2 = N/2;
>     char pSpace[] = uninitializedArray!(char[])(N2);
>     pSpace[] = ' ';
>
>     char pStars[] = uninitializedArray!(char[])(N+1);
>     pStars[] = '*';
>
>     pStars[$-1] = '\n';
>
>     auto w = appender!(char[])();
>     w.reserve(N*3);
>
>     foreach (n ; 0 .. N2 + 1){
>         w.put(pSpace[0 .. N2 - n]);
> 	w.put(pStars[$-2*n-2 .. $]);
> 	}
>
>     foreach_reverse (n ; 0 .. N2){
>         w.put(pSpace[0 .. N2 - n]);
> 	w.put(pStars[$-2*n-2 .. $]);
> 	}
>
>     write(w.data);
> }

With this slightly tweaked solution, I can get times of roughly 
50% to 100% faster, on my dmd-linux box:

//----
void printDiamonde2monarch(in uint N)
{
     uint N2 = N/2;

     char[] pBuf = uninitializedArray!(char[])(N + N2);
     pBuf[ 0 .. N2] = ' ';
     pBuf[N2 ..  $] = '*';

     auto slice = uninitializedArray!(char[])(3*N2*N2 + 4*N);

     size_t i;
     foreach (n ; 0 .. N2 + 1){
         auto w = 1 + N2 + n;
         slice[i .. i + w] = pBuf[n .. w + n];
         slice[(i+=w)++]='\n';
     }

     foreach_reverse (n ; 0 .. N2){
         auto w = 1 + N2 + n;
         slice[i .. i + w] = pBuf[n .. w + n];
         slice[(i+=w)++]='\n';
     }

     write(slice[0 .. i]);
}
//----

The two "key" points here, first, is to avoid using appender. 
Second, instead of having two buffer: "    " and "******\n", and 
two do two "slice copies", to only have 1 buffer "    *****", and 
to do 1 slice copy, and a single '\n' write. At this point, I'm 
not sure how we could be going any faster, short of using 
alloca...

How does this hold up on your environment?