try find the fastest way to convert a group string into index?

[Vladimir Panteleev]

On Sunday, 16 September 2018 at 10:04:09 UTC, learnfirst1 wrote:
> how to make this more fast like with one loop and get the 
> results.

This is a more general problem than any specific programming 
language; you may want to look into perfect hashing:

https://en.wikipedia.org/wiki/Perfect_hash_function

There exists software to generate perfect hash functions, e.g. 
GNU gperf:

https://www.gnu.org/software/gperf/

It looks like Basile wrote one for D:

https://github.com/BBasile/IsItThere

If you don't want to go that far, you can generate a string 
switch using string mixins:

void main()
{
	static immutable string[] keywords = ["foo", "bar", "baz"];
	enum Keyword { foo, bar, baz }

	Keyword parseKeyword(string s)
	{
		switch (s)
		{
			mixin({
				string code;
				foreach (keyword; keywords)
					code ~= `case "` ~ keyword ~ `": return Keyword.` ~ keyword 
~ `;`;
				return code;
			}());
			default:
				throw new Exception("No such keyword!");
		}
	}
	assert(parseKeyword("bar") == Keyword.bar);
}