Remove loop from elapsed slots calculation

[LucasSte]

Problem

A loop that calculates the number of elapsed slots between two epochs depend on the difference between the start and the end epoch:

agave/sdk/src/rent_collector.rs

Line 93 in bb70670

let slots_elapsed: u64 = (account_rent_epoch..=self.epoch)

This loop can get quite slow (>20 minutes) if the difference between them is too large.

Summary of Changes

Remove the loop and perform a constant calculation.

I wrote a big comment explaining where I got the expressions from. The math behind it is quite involved, so I included a sanity and a fuzzy test.

[brooksprumo]

This loop can get quite slow (>20 minutes) if the difference between them is too large.

What constitutes as "too large"?

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This loop can get quite slow (>20 minutes)

Do you know what specifically causes the slowness?

[LucasSte]

This loop can get quite slow (>20 minutes) if the difference between them is too large.

What constitutes as "too large"?

The input my fuzzer generated has zero for the account_rent_epoch and 1334399889604 (this is around 2^40) for the EpochSchedule's epoch. A loop with these conditions causes a timeout.

This loop can get quite slow (>20 minutes)

Do you know what specifically causes the slowness?

Running 2^40 iterations takes a long time.

[LucasSte]

Math on a paper is different from math on a computer, because on a paper nothing overflows. I found out I wasn't handling u64::MAX cases correctly due to saturating_ operations. I have to revise the PR, so I'm marking it as a draft again.

[LucasSte]

Closing in favor of #2411.