Compiletime Table

[D_Learner via Digitalmars-d-learn]

On Thursday, 13 August 2015 at 19:54:18 UTC, anonymous wrote:
> On Thursday, 13 August 2015 at 19:13:55 UTC, D_Learner wrote:
>>     [...]
>
> I think you may have some fundamental misunderstandings 
> regarding CTFE, templates, etc. Your code seems to be half-way 
> between a template-based and a CTFE-based solution.
>
> The simple way to do compile-time computation in D is CTFE 
> (Compile Time Function Evaluation). That is, you write a pure 
> function that can be evaluated both at run-time and at 
> compile-time. CTFE doesn't need template parameters.
>
> Here's some code that should do the same as yours and is 
> compatible with CTFE:
>
> ----
> import std.conv: to;
> import std.stdio;
>
> int[char] calculatebmBc(string pattern) pure
> {
>     const int size = to!int(pattern.length);
>     int[char] result;
>
>     foreach(i; 0 .. size - 1)
>     {
>         result[pattern[i]] = to!int(size - i - 1);
>     }
>
>     return result;
> }
>
> void main()
> {
>     auto bmBc = calculatebmBc("GCAGAGAG");
>     enum bmBcTable = calculatebmBc("GCAGAGAG");
> }
> ----
>
> The key is that calculatebmBc is pure, i.e. it doesn't read or 
> write any module-level mutables.
>
> I touched some things here and here I didn't like that are not 
> related to purity/CTFE.
>
> Adam D. Ruppe already mentioned an issue with associative 
> arrays: This doesn't work:
> ----
> static foo = calculatebmBc("GCAGAGAG");
> ----
> It's annoying, but if you need that you have to use some other 
> data structure. See Adam's post.

Thanks so much for the advice, it works fine. Just experimented 
with the code . I had no idea about it. My mind had the  some 
pre-occupation that compile-time execution is only achievable 
through templates. Had no idea about pure functions...awesone , 
code. Thanks again for the clear explanation.