Map generation limit: Fix checks for block/sector over-limit

[paramat]

Fix the maths that check if any part of a mapblock or sector is over the
set map_generation_limit.
Therefore avoid the loading of any over-limit blocks that were previously
generated when map_generation_limit was larger. The set limit can vary
for a world because it is not yet a per-world mapgen parameter, even when
it is sometimes it will be changed deliberately.
Therefore avoid a player being returned to world centre if they re-enter
a world while being over-limit.

Fix the createSector() crash caused by a mob spawning over-limit in an
over-limit mapblock
/////////////////////////////////////////////////////////
Map generation limit: Cache as 'const' not 'const static'
//////////////////////////////////////

Addresses #4923
See #4923 (comment) and onwards for an explanation of the bugs. 2 of the bugs have now been fixed in Lua.

This commit fixes the loading of over-limit mapblocks that were generated before a reduction in map_generation_limit. The maths of blockpos_over_limit() were wrong in the positive direction.

This commit also fixes the *ServerMap::createSector() crash caused by mobs spawning over-limit, this has the same maths error and is fixed in the same way.

//////////////////////////////

Mapblock minp is at n * 16, maxp is at n * 16 + 15 since mapblock zero is from 0 to 15.

In the positive direction we are checking for
mapblockmaxp > limit
n * 16 + 15 > limit
which is
p.X * blocksize + blocksize - 1 > limit
(p.X + 1) * blocksize - 1 > limit

In the negative direction we are checking for
mapblockminp < -limit
n * 16 < -limit
which is
p.X * blocksize < -limit

All variables are integers so If we work in units of nodes we can avoid division float rounding errors.

Sectors are vertical columns of mapblocks, so we can use the same maths for sector over limit.

//////////////////////////////

Tested.
The mapgen limit should probably not be 'static' but i will fix these in following PR.

[paramat]

With map_gen_limit is set to 300 before starting a new world this code seems to make a world too small by a mapchunk in the positive directions, with mapgen limit 300 world stops at 207 when you would expect 287.
In negative directions it stops at -272 which is expected.
I think i know why, mapgen works not only with the 5^3 block mapchunk but with a block shell around it. generating caves and dungeons up to 16 nodes beyond the mapchunk border, 287 + 16 goes over-limit while -272 - 16 does not.
So this seems correct behaviour.

When world is generated beyond 300 first then the limit set later, the world loads out to 287 as expected, blocks within the limit are loaded.

[paramat]

I tested the crash in *ServerMap::createSector() caused by mobs spawning at the world edge, now fixed by this commit.
So currently i'm happy with this, ready for review.

[nerzhul]

why remove 1 from (p.X + 1) * MAP_BLOCKSIZE ? are you sure it's not (p.X + 1) * (MAP_BLOCKSIZE -1) ?

[paramat]

I'm sure, see my working in first post, it does look weird though i know.

[paramat]

Should i add my maths working as a comment?

[nerzhul]

I think due to the complexity of the calcul and explanation in the code is a good idea

[paramat]

Will do.

[paramat]

The min nodes of blocks are at (n * mapblocksize)
So the max nodes are at (n * mapblocksize) - 1
This is where the -1 comes from.

[paramat]

@nerzhul added comments, will fix 'static' in a following PR.

[nerzhul]

thanks for the comment. Please just fix the static in this bugfix too as you modify the related code part, no interest to have 2 PR here

[paramat]

Ok.

[paramat]

@nerzhul done.

[nerzhul]

Seems okay to me, i like the static const settings fix also

[paramat]

ddcf842
8c1b4f2