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Comment by dalke | Hacker News Reader

I can't figure out the grammar. I wanted to do the Hilbert curve at https://en.wikipedia.org/wiki/Hilbert_curve:

    Alphabet : A, B
    Constants : F + −
    Axiom : A
    Production rules:
        A → +BF−AFA−FB+
        B → −AF+BFB+FA−

I thought it might be:

  A=|+[B]|−[A]|[A]-|[B]+
  B=|−[A]|+[B]|[B]+|[A]−

but that doesn't work, and I don't know what's wrong.

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I tried for a bit and couldn't get it to work either. The dragon curve example is a lot more complicated than the simplest definition, which is a hint. I don't even know exactly what | or [ or ] are supposed to do here.

Here's a version in Go instead: https://github.com/deosjr/GenGeo/blob/master/gen/lindenmayer... Which renders into: https://github.com/deosjr/GenGeo/blob/83c77f9433e569b30f71b3... (see Algorithmic Beauty of Plants for way more examples)

The rules are in https://eliben.github.io/lsystem/rules.js and the evaluation is in https://eliben.github.io/lsystem/lsys.js It looks like f and g are special rules for moving with the pen down or the pen up.

I was able to get it working, I think:

axiom: a

rules:

a = +b|-a|a-|b+

b = -a|+b|b+|a-

initial angle: 90

angle change: 90

scale: 1

simulation depth: 4

That did the trick - thanks! I couldn't figure out what the [] meant in this context and thought it was to make a recursive call.

(I've been working on basic space-filling curves, and came across that Wikipedia page last week, which is about all I know about L-systems.)

Wow never heard of space filling curves. Yet another rabit hole to go into.

Just posted my code as a "Show HN" at https://news.ycombinator.com/item?id=36762283 . The actual project is at https://hg.sr.ht/~dalke/rectfillcurve . I've implemented a few space-filling curves and wrapped them into an iterator API.

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