On what block levels within a cycle can roll snapshots happen?

Question

According to this documentation:

Roll snapshots represent the state of rolls for a given block. Roll snapshots are taken every BLOCKS_PER_ROLL_SNAPSHOT = 256 blocks, which is 16 times per cycle. There is a tradeoff between memory consumption and economic efficiency. If roll snapshots are too frequent, they will consume a lot of memory. If they are too rare, strategic participants could purchase many tokens in anticipation of a snapshot and resell them right after.

But this does not say on which levels the snapshots are taken. It's likely that the answer is either 0, 256, ..., 3840 or 256, 512, ..., 4096 but it's not specified which one it is.

Answer 1

It is done at the levels corresponding to the cycle positions 255, 511, ..., 4095. Given that cycle_position == (level - 1) % 4096 (the first cycle has started at level 1, genesis being special), the levels are 256, 512, ...

The relevant code is here:

let may_snapshot_roll ctxt =
  let level = Alpha_context.Level.current ctxt in
  let blocks_per_roll_snapshot = Constants.blocks_per_roll_snapshot ctxt in
  if
    Compare.Int32.equal
      (Int32.rem level.cycle_position blocks_per_roll_snapshot)
      (Int32.pred blocks_per_roll_snapshot)
  then Alpha_context.Roll.snapshot_rolls ctxt
  else return ctxt

So the relevant cycle_position are those cp such that cp % 256 == 255.