DelayNoMore/ConcerningEdgeCases.md

Potential avalanche from local lag

Under the current "input delay" algorithm, the lag of a single player would cause all the other players to receive outdated commands, e.g. when at a certain moment

• player#1: renderFrameId = 100, significantly lagged due to local CPU overheated
• player#2: renderFrameId = 240
• player#3: renderFrameId = 239
• player#4: renderFrameId = 242

players #2, #3 #4 would receive "outdated(in their subjective feelings) but all-confirmed commands" from then on, thus forced to rollback and chase many frames - the lag due to "large range of frame-chasing" would then further deteriorate the situation - like an avalanche.

In a "no-server & p2p" setup, I couldn't think of a proper way to cope with such edge case. Solely on the frontend we could only mitigate the impact to players #2, #3, #4, e.g. a potential lag due to "large range of frame-chasing" is proactively avoided in <proj-root>/frontend/assets/scripts/Map.js, function update(dt).

However in a "server as authority" setup, the server could force confirming an inputFrame without player#1's upsync, and notify player#1 to apply a "roomDownsyncFrame" as well as drop all its outdated local inputFrames.

Start up frames

renderFrameId	generatedInputFrameId	toApplyInputFrameId
0, 1, 2, 3	0, EMP, EMP, EMP	0
4, 5, 6, 7	1, EMP, EMP, EMP	0
8, 9, 10, 11	2, EMP, EMP, EMP	1
12, 13, 14, 15	3, EMP, EMP, EMP	2

It should be reasonable to assume that inputFrameId=0 is always of all-empty content, because human has no chance of clicking at the very first render frame.

Alignment of the current setup

The following setup is chosen deliberately for some "%4" number coincidence.

• NstDelayFrames = 2
• InputDelayFrames = 4
• InputScaleFrames = 2

If "InputDelayFrames" is changed, the impact would be as follows, kindly note that "372%4 == 0".

pR.InputDelayFrames = 4

renderFrameId	toApplyInputFrameId
368, 369, 370, 371	91
372, 373, 374, 375	92

pR.InputDelayFrames = 5

renderFrameId	toApplyInputFrameId
..., ..., ..., 368	90
369, 370, 371, 372	91
373, 374, 375, ...	92