Have I understood the basics of decompression theory, GF99 and SurfGF?

[David Carron]

If the DC is "throwing" away old predictions because the "idiot" diver is not following them then what's the point in making new predictions? The fact that a "good" DC has the power and memory to do parallel multiple predictions is beside the point. Again, what is gained by assuming an ascent rate or by doing a prediction for NDL or for deco stops?

Here's an example of the kind of mess you can get yourself into if you don't do predictive calculations

Note that at 7:30, subsurface is reporting that NDL has been exceeded because the instantaneous ceiling, calculated without taking into account offgassing during ascent, is below the surface (or alternatively, because the instantaneous GFsurf is greater than GFhi) but that almost one minute later at 8:20 a direct ascent to the surface is still possible.

Note also that the instaneous ceiling continues to get deeper due to ongassing during the first half of the ascent; I'm sure that it wouldn't be difficult to contrive a profile where a dive reported within NDL turns into a deco dive, or the first stop gets deeper, during the ascent.

This type of behaviour, possibly acceptable in a simulator, is clearly unacceptable in dive computer whos reported ascent profile must be smooth and continuous.