Rotating arbitrary sets of elements within a list

[wes weston]

phil_nospam_schmidt at yahoo.com wrote:
> I'm trying to come up with a good algorithm to do the following:
> 
> Given a list 'A' to be operated on, and a list 'I' of indices into 'A',
> rotate the i'th elements of 'A' left or right by one position.
> 
> Here's are some examples:
> 
> A = [a, b, c, d, e, f]
> I = [0, 3, 4]
> 
> rotate(A, I, 'left') --> [b, c, d, e, f, a]
> rotate(A, I, 'right') --> [b, a, c, f, d, e]
> 
> I = [1, 3, 4]
> 
> rotate(A, I, 'left') --> [b, a, d, e, c, f]
> rotate(A, I, 'right') --> [a, c, b, f, d, e]
> 
> Any ideas?
> 

phil,
    Could you do a circlular buffer where the front
starts at 'a' (0) and rotate left increments front.
The i'th element is gotten as mod 6 (front+i) where
6 would be the length of the list.
wes