Fast way to update grids

[David Collier-Brown]

In article <690005 at hpfelg.HP.COM> jk at hpfelg.HP.COM (John Kessenich) writes:
>    Use an array (it is the fastest know data structure for linear 
>    traversal in an arbitrary direction), but take advantage of C's
>    pointer arithmetic.  Example:
[elided]
>    If you are going through the array in some order, the address step 
>    may not even be necessary.
>    Also, for addressing, subscript ranges equal to powers of two will
>    yield faster results.  Why?  They are compiled as << instead of *.
>
  This is true for compilers using real multiplicative indexing.  As
the white book is written to allow edge-vector addressing, and at
least one pcc-based compiler uses it, this turns out to be
compiler-dependent.  
  As it happens, the Waterloo (GCOS) C compiler uses edge-vectors, but
also (if memory serves) keeps the pointed-at data contiguous to
allow such assumptions to work nevertheless.  Something of a ``best
of both worlds'' effect, at no extra cost to the program/programmer.

 --dave (I was the bug-fix guy at the acceptor site) c-b
-- 
 David Collier-Brown.                 {mnetor yunexus utgpu}!geac!daveb
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