provider: schedule

[guillaumemichel]

Part of #1095

Depends on #1116

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Schedule

The crux of the Reprovide Sweep is to hold a schedule to know when to reprovide parts of the keyspace to avoid the burst of reproviding everything at once.

The schedule is a datastructure storing the prefixes associated with the explored keyspace regions, and the time for which they should be reprovided next. The schedule contains no overlapping prefix (e.g no prefix included in the schedule is a superstring of another prefix in the schedule).

The schedule only contains the keyspace regions for which the provider is (re)providing keys. E.g if the provider is only reproviding a single key, the schedule will contain a single region, whose identifier is a prefix of this key.

Provide Cycle

• The ReprovideInterval determine the duration of the provide cycle.
• Each region is reprovided exactly once every cycle.
• Regions reprovides are spread all over the cycle.
• If there are 4 prefixes, the schedule would look like this:
  • 00 at time offset 0
  • 01 at time offset ReprovideInterval / 4
  • 10 at time offset ReprovideInterval / 2
  • 11 at time offset 3 * ReprovideInterval / 4

The time at which a keys matching a region should be reprovided is not a timestamp, but a time offset compared to the provide cycle. The provide cycle starts when the SweepingProvider is initialized.

• Merged provider: schedule prefix length #1116
• Change merge target to be provider after provider: schedule prefix length #1116 is merged