Individual vote count per option

[0xmad]

This discussion is related to issue #2315.

The idea is to add some additional operations in VoteTally circuit where we construct a temporary Merkle tree and to save an amount of voters voted per each option. Each leaf will be hash3(voteOptionIndex, totalVotersCount, salt) (salt needed to allow publish this tree without worrying it can be reconstructed using only voteOptionIndex and totalVotersCount).

Per each batch we will calculate new temporary tree (similar as ballotTree) and add root of this tree to tallyCommitment, so we can ensure that this data is verifiable and we can continue calculating individual vote count per option. For next batch, we check that tallyCommitment is valid and add/remove/merge new values into the temporary tree and then continue the process until there are no batches.

Also, coordinator keeps previous version of temporary tree and based on it build new tree with updated values, so it's important to check that previous tree root is included in tallyCommitment.

Finally, we have tree that contains all the individual vote count values per option and its root.