[feat] Utreexo: removing item from the accumulator

[TAdev0]

• resolves [feat] Utreexo: removing item from the accumulator #209
• follows contribution guide
• code change includes tests

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Name	Status	Preview	Comments	Updated (UTC)
raito	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 27, 2024 11:21am

[TAdev0]

@m-kus first draft.

Need to add more tests to it, not 100% sure of my implementation.
Works well for a simple test where i remove one leave to a 2 leaves - 1 root forest merkle tree

[TAdev0]

can you confirm that desired behaviour is that once an UtreexoState has contained n roots, then even if we remove all leaves, we should still have n roots, all set to Option::None ?

I think there is no choice actually. Because we need to ''copy paste'' all the remaining roots that are not concerned by the removal, we need to append residual roots to the utreexostate. Hence, in case where we remove a lot of leaves, we'll have multiple Option::None appended. I dont see any way (that is not costly and doesnt require to loop over all the remaining roots) to check whether we should append or not remaining roots to stick to only one Option::None at the end of our array of roots.
not sure if i'm totally clear

[TAdev0]

Also, can you confirm that the deletion algorithm doesnt require to use leaf_index binary representation in any step? just to make sure

[m-kus]

can you confirm that desired behaviour is that once an UtreexoState has contained n roots, then even if we remove all leaves, we should still have n roots, all set to Option::None ?

I think there is no choice actually. Because we need to ''copy paste'' all the remaining roots that are not concerned by the removal, we need to append residual roots to the utreexostate. Hence, in case where we remove a lot of leaves, we'll have multiple Option::None appended. I dont see any way (that is not costly and doesnt require to loop over all the remaining roots) to check whether we should append or not remaining roots to stick to only one Option::None at the end of our array of roots. not sure if i'm totally clear

There always has to be at least one root (Option::None if the forest is empty).
It's ok to have multiple None roots, the total number will remain pretty small (e.g. in ZeroSync they have a fixed array)

[m-kus]

Also, can you confirm that the deletion algorithm doesnt require to use leaf_index binary representation in any step? just to make sure

It requires leaf index (which is part of the proof) to determine the position of the sibling node from the proof when you compute parent hash.

[TAdev0]

@m-kus ready for final review