Monsters should not spawn on unwalkable tiles

[Playtester]

rAthena Hash

30928a0

Client Date

2021-11-03

Server Mode

Pre-Renewal

Result

Monsters spawn on unwalkable tiles, for example the Hydras in iz_dun03.

Relevant Log Output

No response

Expected Result

Monsters never spawn on walkable tiles, if no walkable tile could be found after a while (I assume it does a number of attempts), the monster is just spawned on a random location on the map instead.

For example there are ~30 hydras that spawn randomly on iz_dun03 instead of just 10.

How to Reproduce

Check spawns in iz_dun03.

Official Information

If you check iz_dun03, you will see that those fixed position Hydras that rAthena has are not there on any official server.

Modifications that may affect results

No response

[Playtester]

This is how it works on official servers:

• The spawn area is divided into 4 quadrants (-x,-y), (x,-y), (-x,y) and (x,y), 4 lines (-x,0), (x,0), (0,-y) and (0, y) and 1 dot (exact tile)
• Starting with a random quadrant, a random tile is picked
• A picked tile is valid if it's not a wall and it's not within 4 (north, east) or 5 cells (south, west) from the edge of the map
• Once a valid tile was found this tile is used and the search is done
• If no valid tile was found, it continues to search in: (Line)->(Square)->(Line)->(Dot)->(Line)->(Square)->(Line)->(Square)
• If there are no valid tiles, it will search 50 times map-wide for a valid cell, ignoring 15 cells from the edge of the map (the same code is used for teleporation except warp portals are invalid cells for teleportation)

[Playtester]

Further tests on spawns area:

• Both (0,0) and (1,1) result in a single fixed spawn cell, there's no 3x3 area variance on (1,1)
• (2,2) on the other hand spawns in a 5x5 region, but the center 3x3 area has a much higher chance to be picked
• (3,3) results in a 9x9 region already
• (2,1) results in a 5x1 region
• (5,5) results in a 17x17 region

Conclusion: The area for finding a spawn cell is one radius lower than we expected (1->1x1, 2->3x3, 3->5x5), but the "find spawn cell" code is actually called twice, which doubles the potential area but results in a much higher chance for center cells to be picked (1x1 stays 1x1, 3x3 becomes 5x5 with 3x3 focus, 5x5 becomes 9x9 with 5x5 focus).

[Playtester]

From further testing we found out that first randomization of the area already happens at server start.

So for example on (3,3), at server start, each monster's center cell will be placed in a 5x5 area around the spawn center. And then each time the monster spawn, it will then be placed in a 5x5 around its own center cell. That's why the total possible area is 9x9, but until the next server restart, the same monster will still always respawn in a 5x5 area.

The center cell can even be outside of the map.