[Polyworld-develop] single x-sorted list checked in

[Larry Y. <lsy...@be...>]

Matt Whitehead's changes to put all objects into a single x-sorted 
list are now checked in.  (Before I committed the changes, I tagged 
the existing source with "pre-object-x-sort".)

Detailed behavior is identical up until one encounters a (mostly 
inconsequential) bug in old code.*  As expected, coarser scale 
behavior, measured by performance on one of the Ideal Free 
Distribution runs, is also consistent with the older code.

The new code appears to be marginally faster than the old code, but I 
need to run a longer simulation using the old code on a particular 
CPU to make a precise statement.

Virgil, you can update your source now and have at.  You and MattW 
are both going to be pounding on the gene code, so you'll want to be 
a little careful and maybe keep in touch.

Cullen, I don't think this hurts or helps you, aside from the modest 
speed-up.  And, Matt Ira, I now you're not quite ready to re-sync yet.

- larryy

*Previously, the food was not re-sorted each time step, and while 
food centers remain sorted automatically, because food doesn't move, 
the list is actually sorted on leading edge, which changes as food is 
eaten and shrinks.  The effect is minimal, but detectable.