Invariant check in token swap program

[anoushk1234]

Why are we comparing invariant here with pool_a.amount * pool_a.amount

program-examples/tokens/token-swap/anchor/programs/token-swap/src/instructions/swap_exact_tokens_for_tokens.rs

Lines 137 to 140 in 4366669

	if invariant > ctx.accounts.pool_account_a.amount * ctx.accounts.pool_account_a.amount {
	return err!(TutorialError::InvariantViolated);
	}

[a-hariti]

the invariant is essentially the product of the pool reserves, the k in k = a * b formula of a CPMM.

Ideally it should remain constant (invariant = new_reserses_a * new_reservers_b),
but since we are accruing a small fee with each swap, we expect it to grow a bit, hence the comparison.

But the problem with this if check isn't the direction of the comparison, but the duplicated tx.accounts.pool_account_a.amount 💀.

You probably don't need this answer now, but I hope it helps someone in the future :) @anoushk1234