The teleport to safety routine would try to find a viable spot 400
times, the first 200 rejecting trap locations and the last 200
accepting such. While testing various escape from lava variations
recently, I noticed that I could fail to reach safety even when there
was an open spot immediately adjacent to me. I added a BandAid(tm)
to make another 400 tries if the first attempt failed, but that was
clumsy and still didn't guarantee picking a viable spot.
This adds a new routine, collect_coords(), which will gather a list
of coordinates for the entire map. safe_teleds() goes through them
one by one until either finding a spot or exhausting the possibilies,
without randomly trying and retrying the same spot multiple times and
without missing other potential spots, also without just scanning the
map from left to right and top to bottom or similar.
Various other things which retry over and over, and especially the
ones which make a bunch of random attempts and then fallback to trying
every spot on the map, could be switched over to this, at least for
the falling back phase. Right now collect_coords() is local to
teleport.c but that could be easily changed.
The try-at-random method is much quicker when there are lots of
available spots but the gather-shuffled-candidates method is
guaranteed to succeed if success is possible. The way safe_teleds()
is presently using it collects the list with all locations within
2 steps first, then those within 3 to 4, then 5 to 6, and so on out,
randomized within each block of ranges. So the destination will be
within one step of being as close to the starting spot as possible
but not always immediately adjacent when that happens to be available.
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