how to access a large datastructure efficiently?

[Christian Wittern]

Hi there,

Here is the problem I am trying to solve:

I have a large list of items which I want to access.  The items are in 
sequential order, but many are missing in between, like:

(1 8 17 23 25 34 45 47 50)  [in reality, there is a value associated 
with this, but I took it out for simplicity]

Now when I am trying to access with a key that is not in the list, I 
want to have the one with the closest smaller key returned, so for 6 
and 7 this would be 1, but for 8 and 9 this would be 8.

Since the list will have thousands of elements, I do not want to simply 
loop through it but am looking for better ways to do this in Emacs lisp.  
Any ideas how to achieve this?

Christian