Add stableswap

[JanKuczma]

This PR adds the StablePool contract and all necessary changes from the common-amm-stable-swap repository.

[deuszx]

Wouldn't it be OK to return self.init_amp_coef as well? We modify it in the { } above, if we're still ramping up/down. It just seems weird to return "future" as the target.

[JanKuczma]

If there was ramping, there's a difference between the init_amp_coef and the future_amp_coef. Over the ramping period the init_amp_coef +/- amp_delta gets closer to the future_amp_coef as current_time increases. When the ramping is over it should return the future_amp_coef because future_time <= current_time (the else {...} block).

Maybe we can reconsider the naming: rename future_* to target_*?

[deuszx]

Ah, I didn't notice we don't return from the first block.

When the ramping is over, isn't it the case that init_amp_coef and future_amp_coef will be equal?

Also, while the ramping lasts, this method will return future value of Amp coef – is that correct? Imagine ramping is spread across 1000 blocks, so every block the A increases by 1/1000 (a a_delta) . We'd return the A_init + 1000 * a_delta here though.

[JanKuczma]

When the ramping is over, isn't it the case that init_amp_coef and future_amp_coef will be equal?

Only if the ramping was stopped with stop_ramp_amp_coef(...) which sets both init_amp_coef and future_amp_coef to the value computed for the current time (block).

Also, while the ramping lasts, this method will return future value of Amp coef – is that correct?

When it lasts (current_time < future_time) it will return init_amp_coef +/- amp_delta.
The value of init_amp_coef does not change after the ramping ends because is not necessary since we know that after the ramping ends the value of the A should be equal to the future_amp_coef

Imagine ramping is spread across 1000 blocks, so every block the A increases by 1/1000 (a a_delta) . We'd return the A_init + 1000 * a_delta here though.

Something like that. Let's assume that A = 11, the current_block = 50 and we want to ramp it to 100 when we reach 1000 block. The values are set to:

• init_amp_coef = 11
• init_time = 50
• future_amp_coef = 100
• future_time = 1000

At a given block N where N < future_time, we can compute A like this:
A = init_amp_coef + (future_amp_coef - init_amp_coef) * (N - init_time) / (future_time - init_time) rounded down

E.g. N = 500: 11 + (100 - 11) * (500 - 50) / (1000 - 50) = ~53,16 = 53

If N is greater than future_time we should return the future_amp_coef.

Because we can't handle floats we can't tell by how much A will change every block (using integers) but we are able to tell what the value of A should be at a given block and that's how the value of A is computed in the ramping period.