Add Size symbol

[arcondello]

Closes #48

[arcondello]

I am toying with renaming these to Length and LengthNode. Need to think about it but wanted to get the logic up to be tested with.

[arcondello]

Still ruminating on this, but a few thoughts.

One issue with calling it Len or Length is that the len() of a NumPy array is the size of the first dimension. I am worried that would be confusing.

I am currently inclined to do something like

s: Size = a.size(symbol=True)

It's ugly, but nicely explicit.

Alternatives

• We could make ArraySymbol.size() always return a symbol, but there are many legitimate uses that want an integer.
• If we had Consider adding as_symbol() method or similar #38 we could consider having ArraySymbol.size() return a symbol when it's dynamic and an int otherwise. But that's pretty ugly switching behavior.

[arcondello]

Ah, if we implement __index__() for Constant then returning a constant always from size() will work in most places. Interesting.

Something like

def __index__(self) -> int:
    return int(np.asarray(self))

would let us fall back on NumPy's error message albeit at the cost of some extra object creations. Or we could just copy their error message.

[arcondello]

With the addition of #72, we could now always return a symbol from .size(). Though another downside of that approach is that without some sort of constant caching or de-duplication scheme we might end up with many constants in the model. So returning a symbol only when dynamic might still be preferable.

[arcondello]

Ended up going with the approach where we return a symbol only for dynamic nodes. I hate the switching behavior of the return type, but I think it's the least confusion option for now.

[elham-azd]

typo

[elham-azd]

LenNode => SizeNode ?

[elham-azd]

I'm trying to get the length for disjoint_lists:

model = Model()
model.states.resize(1)

x, ys = model.disjoint_lists(5, 3)
length0 = ys[0].size()
length1 = ys[1].size()
length2 = ys[2].size()

x.set_state(0, [[0, 1, 2, 3, 4], [], []])
with model.lock():
    print(length0.state(0))
    print(length1.state(0))
    print(length2.state(0))

Is there a better way for doing this? For example, can we make an array of all lengths?

[arcondello]

Currently no way to join them into another array unfortunately, we'd need #46 for that I think