String Metaprogramming

[Nicholas Wilson via Digitalmars-d-learn]

On Saturday, 18 July 2015 at 13:48:20 UTC, Clayton wrote:
> Am new to D programming, am considering it since it supports 
> compile-time function execution . My challenge is how can I 
> re-implement the function below so that it is fully executed in 
> compile-time. The function should result to tabel1 being 
> computed at compile-time. There seems to be a lot of mutation 
> happening here yet I have heard no mutation should take place 
> in meta-programming as it subscribes to functional programming 
> paradigm.


> void computeAtCompileTime( ref string pattern ,ref int[char] 
> tabel1){

change function signature to
int[char] function(string) or as the char type is the index 
probably better of as
int[256] function(string). also probably no need to take pattern 
by ref as it is effectively struct{ size_t length; char* ptr;}. 
also we aren't going to modify it.

int[256] computeAtCompileTime(string pattern)
{
> 	int size = to!int(pattern.length) ;
pattern.length is a size_t no need to change its type in another 
variable.  you are unlikely to be dealing with string longer than 
2^32 (also signedness) but w/e
         int[256] ret; // implicitly initialised to int.init (i.e. 
0)

> 	
> 	foreach( c; ALPHABET){
> 		tabel1[c] = size;
> 	}
> 	
> 	for( int i=0;i<size -1 ; ++i){   //Initialise array
> 		tabel1[pattern[i]] = size -i-1;
can just foreach over pattern
         foreach(i, c; pattern)
                 ret[c] = pattern.length - i -1;

>         pragma(msg, format("reached pattern  
> table1[pattern[i]]=(%s) here",
>         table1[pattern[i]].stringof  ~"     v="~ (size 
> -i-1).stringof));
> 	}
	
> }

if you want this to be not callable at runtime then wrap the main 
body (sans variable declaration) with
if (__ctfe)
{
      ...

}