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

[learnfirst1]

On Sunday, 16 September 2018 at 11:30:11 UTC, learnfirst1 wrote:
> On Sunday, 16 September 2018 at 10:14:24 UTC, Vladimir 
> Panteleev wrote:

It has to be case Case Insensitive,  so before I run mpfh for 
each new request headers,  I need to convert the keys into low or 
up case.  this value can be storage on a stack local tmp cache.   
  and I has to run it on the get(string key) method again.