Add methods to remove unused symbols from a model #161
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| // This method will reset the topological sort if there is one. | ||
| // | ||
| // A node is considered unused if all of the following are true: | ||
| // * It is not a decision. |
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This is currently a constraint because of the difficulty of removing some types of decisions, correct?
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It's not actually difficult to implement but there are a few reasons:
- I can see how symbols might be accidentally added to the model, but for decisions I think there is a high chance they are there deliberately anyway. I.e. i doubt we'd be removing decisions often.
- This way if one was to use this method as part of a presolve step, the serialized states of the model after presolve would still be relevant to the original model.
It's a simple enough thing to add a toggle for at some later date if we really want.
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Good point, I didn't think about the presolve angle.
| // For performance we store a signalling value in the topological_index of the | ||
| // nodes. We need to take some care to not override the topological index of | ||
| // any decisions. | ||
| constexpr ssize_t keep = -2; |
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This is set to -2 because -1 already has a meaning of "unsorted", correct? But at this point you've topologically sorted so it should be guaranteed all nodes have an index >= 0.
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That's right. I just need a negative number. I chose -2 because -1 already has meaning as unsorted. Should never matter but 🤷
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What about the case of an unused |
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Ah! Good catch. Yes. Need to think about how to handle that. |
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Ok, added a mechanism for nodes to opt out of removal. Probably good for decisions as well (for safety). |
Closes #41