weak map gist

cache.ts

export class Cache<T extends object, K> {
	items = new WeakMap<T, K>()

	get<P extends T>(item: P, cb: (item: P) => K) {
		if (!this.items.has(item)) {
			this.items.set(item, cb(item))
		}
		return this.items.get(item)!
	}

	access(item: T) {
		return this.items.get(item)
	}

	set(item: T, value: K) {
		this.items.set(item, value)
	}

	has(item: T) {
		return this.items.has(item)
	}

	invalidate(item: T) {
		this.items.delete(item)
	}

	bust() {
		this.items = new WeakMap()
	}
}

gistfile1.ts

class Example {

  getOutline(shape) {
    const sides = 5 
        const ratio = 1
        const cx = w / 2
        const cy = h / 2
        const ix = (cx * ratio) / 2
        const iy = (cy * ratio) / 2
        const step = PI2 / sides / 2
        return Array.from(Array(sides * 2)).map((_, i) => {
          const theta = -TAU + i * step
          return new Vec2d(
            cx + (i % 2 ? ix : cx) * Math.cos(theta),
            cy + (i % 2 ? iy : cy) * Math.sin(theta)
          )
        })
  }

  outline(shape: T) {
    return outlines.get<T>(shape, (shape) => this.getOutline(shape))
  }

Fastest code:

function pointsOnEllipse(cx, cy, rx, ry, n) {
  const points = Array(n);

  const step = Math.PI / (n / 2);
  let sin = 0;
  let cos = 1;

  const a = Math.cos(step);
  const b = Math.sin(step);

  for (let i = 0; i < n; i++) {
    points[i] = { x: cx + rx * cos, y: cy + ry * sin };
    const ts = b * cos + a * sin;
    const tc = a * cos - b * sin;
    sin = ts;
    cos = tc;
  }

  return points;
}

https://jsbench.me/21l80nvjcf/1

So I see 5x boost on this hot path, numerical stability, same readability, and a better understanding of the geometry through linear algebra (debatable but yeah). We can get another 2-3x easily later through my above suggestions too. Can we remove the cache later? =)

Worth noting that 2 ternaries are gone from the loop. Those should have been cheap, but lower-level condition-less code is faster than conditional code. To be fast, remove ifs. Fewer ifs also means less debugging, win-win.

This is fun!

So here's the AoS->SoA transform. I don't have a JSBench account to share saved snippets, but you can copy the setup and the 2 tests from this gist:

https://gist.github.com/chenglou/ff603736ae48c09c9293651dc94a6adf

This isn't as big of a win than it should be (though 1/4 is pretty darn good for a hot path already given the very little readability difference). But the bigger win might be architectural: I'm wondering if you can just pass a single mutable array of ys and ys to all getOutlines, then fast iterate though xs to find the x bound, and fast iterate through ys to find the y bound. Or maybe that's not what you need.

Squeezing a little more out: https://jsbench.me/z5l819pk9w/1

The architectural change is interesting. I may be able to do that, yeah—the outline is primarily used for hit testing (ie point in polygon, or else as a series of line segment vs line segment or arc intersections), however it also used to render for certain shapes, where we pass the outline to perfect-freehand to create a path that way. The first case would just mean changing the way we do intersections (not so bad), but the second would mean creating a custom version of perfect-freehand that expects data in a certain shape.

Really though this isn't the hottest path in the app, though it is a good place to learn some of these strategies.

The points-to-svg-strings from that original tweet is much more impactful.

The matrix stuff is probably next on the block, and I've already been able to roll some of this into that!

(Since this gist's public now, here's the tweet referred to by points-to-svg-strings: https://twitter.com/_chenglou/status/1567269585585606659)

You've benchmarked creations; that SoA transform is for iteration. Here's the correct benchmark: https://jsbench.me/7gl81zd1xk/1

The memory layout rearrangement is for optimizing memory (memory is the bottleneck for modern computers, not compute). This will be especially relevant if you e.g. traverse only xs. An array of numbers is like 3x smaller than an array of objects of 2 numbers. The reason why you don't see 3x difference is because* in JavaScript everything's basically a pointer and instead of having a compact array of floats, we get an array of pointers to floats. In theory. In practice and outside of benchmarks, the JITs will do whatever.

* Well, also because in this benchmark you're accessing both xs and ys. Theoretically if you're only accessing one of the two arrays then your perf increase goes from ~25% to >30%

Tldr modern performance is mostly about shuffling your memory into the right representation*. Which is a nice endeavor because starting from data structures is important anyway.

But yes, if it's not in the hot path, then don't bother! Though there might be an argument that this is the better data layout even just for ergonomics. I wouldn't know that without trying perfect-freehand though

* again, my rough approximation is that if you're doing the sort of optimizations that increases compute at the expense of memory, by e.g. caching or whatever, then you might have already lost, especially in latency

Quick update—this is the fastest that I've gotten it so far. Surprisingly the destructuring trick is fast!

function pointsOnEllipse4(cx, cy, rx, ry, n) {
  const xs = new Float32Array(n);
  const ys = new Float32Array(n);

  const step = Math.PI / (n / 2);
  let sin = 0;
  let cos = 1;

  const a = Math.cos(step);
  const b = Math.sin(step);

  for (let i = 0; i < n; i++) {
    xs[i] = cx + rx * cos;
    ys[i] = cy + ry * sin;
    ;[sin, cos] = [b * cos + a * sin, a * cos - b * sin];
  }

  return { xs, ys }
}

I wouldn't pay too much attention to that destructuring, which is more microbench-y really. It invokes array iterator, but here the engine clearly sees there's nothing to be invoked. Won't necessarily be true if the code shifts around. Plus, readability. Aim for simple and fast code that looks like C (this has been a solid advice for the past 20 years no matter what magic JIT has done); C doesn't have a JIT, so coding while pretending there's no JIT helps too.

Regarding Float32Array: they might be faster, but the reason is likely that they're 32-bits instead of 64-bits. So yeah here again it's about having a smaller memory footprint. But on the other hand, you've now traded off your number precision (that we just fixed) that you might need in the future. Imo this is especially relevant for SVGs (as opposed to raster graphics and 3D), since e.g. if you have 2 same shapes with different colors perfectly on top of each other, you should see a single outline, not some colors from the background shape bleeding out (aka numerical imprecision). I don't know though. Just a general observation.

The structure-of-array transform we did is fast for 2 reasons:

• Way fewer tiny allocations. Tiny objects are still relatively fast to create because they're are usually allocated around the same places ("arenas"), so, good memory locality, thus perf, and so their drawback of being more allocate-y doesn't show up as much. But when readability is the same, I try not to rely on that assumption too much and just do the predictable less allocate-y thing. Plus there's also deallocation: GC pauses etc.
• The transform makes iteration and branch predictor very happy. Iterating over a plain array of numbers is the best-case scenario for modern CPUs (and GPUs, and eh, TPUs or whatever comes next I guess. This will always be relevant).

So that's why you see that 25% perf boost at basically no cost.

One last note on your initial problem: I think if we have to cache the outlines, we should at least lift the cache to a higher level, to a single big one. Having so many little disparate caches is a bit scary.

Update: final follow-up tweet at https://twitter.com/_chenglou/status/1571430079829528577

Verify Github on Galxe. gid:YZP8XDSaNU8GruWpvhJpFZ

It's mentioned that the cache is slow. This is to be expected when using a WeakMap.

Was inverting the cache considered? Meaning storing the x,y cached properties inside each associated original shape (either as public or private fields). This ensures there is no increase in the count of objects and makes cache access super fast. I am wondering if fast caching changes the balance here.

Good point. For Steve's use-case it seems storing the entire outline array on the instance every time the shape transforms, might be viable. It does nothing for latency and uses more memory, but still, seems better than a WeakMap.