Fast way to update grids

[John Kessenich]

    Use an array (it is the fastest know data structure for linear 
    traversal in an aribitrary direction), but take advantage of C's
    pointer arithmetic.  Example:

    int a[16][16];

    #define NO   1
    #define SO  -1
    #define EA  16
    #define WE -16

    Then if you want to know about a[x][y] neighbors, look at

    b = &a[x][y];
    b + NO;
    b + SO;
    b + EA;
    b + WE;

    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 *.

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John Kessenich