Mostly working Surf

[Zrocket]

I think I finally figured it out. Your choice to subtract toi from the Y velocity only at ground tick 1 was messing with later gravity calculations. From my testing, subtracting it every time doesn't seem to hamper bunny hopping acceleration, but let me know if there's a problem.

There are still some very minor details that make surfing here technically different to source engine surfing (e.g. holding forward doesn't pull you down), but I'd say it's pretty close.

[Zrocket]

Having given this some thought, this is probably still a naive solution, but it does help me narrow things down. From what I can tell, having debugged and stepped though the execution, the problem is still related to the gravity calculation:

$g = gravity \cdot g_e$

Considering we only care about the player entity, the entity gravity variable $g_e$ doesn't concern us. And you do calculate gravity in relation to deltatime. This leads me to believe that our problem might rest with how player movement physics in Half-Life is calculated via the virtual frame rate:

$f_p = \tau_p^{-1}$

Where $\tau_p$ is player frame rate, and $f_p$ is the resulting virtual frame rate used for calculations.