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It seems that, at cycle n, the context stores one selected snapshot (among 16) for each cycle between n-5 and n+5, and 16 snapshots for cycle n+6, and some number between 0 and 16 for cycle n+7 (depending where we are in cycle n). Is this right?

If yes, then couldn't one already select (at n) the snapshot for cycle n+6, using the seed for cycle n+5 (because the one for n+6 is not yet available)? In this way the context could store 15 snapshots less. Or does there need to be a certain correlation between the seed used for selecting the snapshot and the cycle for which the snapshot is selected? (Same question could be asked for baking/endorsement rights.) I imagine it is simply sufficient to not reuse a seed for the same purpose.

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  • If you use the seed of n+5 at both n+5 and n+6, what would you use for n+7 ? Looks like you are going to always use the same seed...
    – lefessan
    Feb 11, 2019 at 17:37
  • I think I'm suggesting to use the seed of n+5 to select the snapshot of n+6 (and the baking/endorsments rights of n+5); then the seed of n+6 to select the snaphsot of n+7 and so on. The same seed would not be used twice for the same purpose (like selecting a snapshot).
    – Eugen
    Feb 12, 2019 at 8:50

1 Answer 1

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This is because of when the seed is available. During cycle n, there are 128 commitment blocks where a hash of a nonce must be provided. These must then be revealed before the end of n+1. Once the nonces are revealed, we use them to construct the seed, which is used to determine the snapshot for n+7.

So, n+6 is calculated at the end of n when we are sure all of the nonces have been revealed. All snapshots for n+6 are held until the end as that's when we can use the revealed nonces to calculate the seed. I guess an optimization could be to calculate this seed as soon as all the nonces are revealed, or at the end of the cycle (which ever comes first).

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    Yes, but why not use the seed for n+5 (computed as you say at the end of cycle n-1 from the nonces revealed during that cycle and committed during cycle n-2) to select the snapshots at n+6 (which are already available)? In this way, those 16 snapshots need not to be stored until the end of a cycle n. I only see a problem in terminology, in the sense that one cannot talk about the "seed for a cycle" because the seed would now be used for more than one cycle (but again, for different purposes). But maybe I'm missing something.
    – Eugen
    Feb 12, 2019 at 9:14
  • It doesn't work for that - if you're reusing seed for n+5 for cycle n+6, then when the next cycle comes along n+6 becomes n+5, and then you're reusing the same seed for the n+6 (which was n+7).
    – Stephen Andrews
    Feb 12, 2019 at 15:03
  • One could use n+5 for n+6, then n+6 for n+7, and so on, thus not reusing the seed (for snapshot selection). Well, there is a bootstrap moment (the one when this change would be implemented), which I haven't thought about and one would have to deal with carefully...
    – Eugen
    Feb 12, 2019 at 15:29

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