Help optimize D solution to phone encoding problem: extremely slow performace.

[Renato]

On Tuesday, 16 January 2024 at 22:15:04 UTC, Siarhei Siamashka 
wrote:
> On Tuesday, 16 January 2024 at 21:15:19 UTC, Renato wrote:
>> For the record (I already posted this on GitHub)... here's [my 
>> current fastest 
>> solution](https://github.com/renatoathaydes/prechelt-phone-number-encoding/blob/dlang-key-hash-incremental/src/d/src/dencoder.d) time using the same algorithm as Rust ...
>
> [...]
>
>> ... what I am really curious about is what the code I wrote is 
>> doing wrong that causes it to run 4x slower than Rust despite 
>> doing "the same thing"...
>
> It's a GC allocations fest. Things like this make it slow:
>
> ```diff
>      {
> -        string digit = [digits[0]];
> +        string digit = digits[0 .. 1];
>          words.insertBack(digit);
> ```

I was under the impression that `[digits[0]]` would just use a 
stack allocation??

The profiler does not show any GC anymore, are you sure it's a 
"GC allocations fest"???


> And at the top is the associative array lookup (when profiling 
> the handling of the "phones_1_000_000_with_empty.txt" input 
> file):
>
> ```
>     36.85%  dencoder  dencoder              [.] _aaInX
>     12.38%  dencoder  dencoder              [.] void


Well, I know, but I did everything to make the hash "cheap" to 
compute so I still don't see a way to improve it.