What causes wall-slides?

[ultdoomer]

I know how to execute them in-game, but what in the code causes them?

[Fredrik]

See http://doom.wikicities.com/wiki/Wallrunning

[Grazza]

Fredrik: unless I'm missing something, that page doesn't explain why it happens - what the specific bug at the heart of it is. Actually, I don't recall ever hearing a full explanation of the phenomenon.

[Graf Zahl]

Grazza said:

Actually, I don't recall ever hearing a full explanation of the phenomenon.

From ZDoom's rh-log.txt:

- Fixed: When a multi-step move is blocked in P_XYMovement, the move needs
to actually stop right away. This seems to be what caused wallrunning: In
the first step, the player hit the wall, so his movement got clipped to it.
In the next step, the player would continue in his original direction and
travel further along the wall.

That's the closest to an explanation I ever found.