OK, this is what I want to do:

I want to define a spline (or set of points), then be able to say "draw 43%
of this spline" or whatever.  It would then draw the first 43% of this
spline, and not the rest.  I'm going to use this in an animation to show a
spline gradually getting longer at constant speed.

Now, this is a bit trickier than it sounds, because the spline will not be
equal lengths between each point, and because i'll be using cubic or
something, the spacing between the points will not be linear.

I tried to make a macro that stepped through the spline and measured the
length to work out where the "cut-off" point was, but to make it accurate
enough I had to use a really small step size, and it took forever to parse.

Anyone have any clever ideas? Or better yet, some code, or some weird
built-in function I'm missing?

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