Pre-expanding alloc cell(s) / reserving space for an associative array

[IchorDev]

On Sunday, 9 July 2023 at 20:24:24 UTC, Cecil Ward wrote:
> Before I posted a question about avoiding unnecessary 
> allocs/reallocs when adding entries to an array like so
>     uint[ dstring ]  arr;
>
> when I build it up from nothing with successive insertions.
>
> The array is accessed by a key that is a dstring. I was told 
> that I can’t use .reserve or the like on it? Is that correct? 
> My memory fails me, powerful pain drugs.
>
> Is there an alternate method I could use ? To be honest, the 
> number of entries is likely to be extremely modest, so this is 
> not a huge performance issue, six entries would be considered 
> quite a lot. The string keys are probably not very very long. 
> But I’m learning good habits for the day when I might have a 
> bigger such database.

 From the spec it sounds as though (but good luck testing for 
sure) that if you have (for example) 6 big dummy key-value pairs 
in the AA to begin with, then if you use `.clear` it "Removes all 
remaining keys and values from [the] associative array. The array 
is not rehashed after removal, __to allow for the existing 
storage to be reused.__" 
([source](https://dlang.org/spec/hash-map.html#properties)) I'm 
not 100% sure this would work how you want, and it's such a hack, 
but it's something.
D has a few user-made hash-map libraries, so it might be worth 
investigating those to see if they offer a pre-allocation method, 
too.