23.2 Extent Ordering

Extents are compared using memory indices. There are two orderings for extents and both orders are kept current at all times. The normal or display order is as follows:

     Extent A is ``less than'' extent B,
     that is, earlier in the display order,
       if:    A-start < B-start,
       or if: A-start = B-start, and A-end > B-end

So if two extents begin at the same position, the larger of them is the earlier one in the display order (EXTENT_LESS is true).

For the e-order, the same thing holds:

     Extent A is ``less than'' extent B in e-order,
     that is, later in the buffer,
       if:    A-end < B-end,
       or if: A-end = B-end, and A-start > B-start

So if two extents end at the same position, the smaller of them is the earlier one in the e-order (EXTENT_E_LESS is true).

The display order and the e-order are complementary orders: any theorem about the display order also applies to the e-order if you swap all occurrences of “display order” and “e-order”, “less than” and “greater than”, and “extent start” and “extent end”.