[x/gamm][Testing] generalize TestCalculateAmountOutAndIn_InverseRelationship

[p0mvn]

Background

The following PRs contributed to TestCalculateAmountOutAndIn_InverseRelationship in balancer package:

• chore: inverse unit tests for CalcInAmtGivenOut and CalcOutAmtGivenIn with zero swap fee #1254 - tests with zero swap fee
• test: improve TestCalculateAmountOutAndIn with non-zero swap fee tests #1344 - tests with non-zero swap fee

From "Gamm Refactor", CalcInAmtGivenOut and CalcOutAmtGivenIn are defined on the PoolI interface.

There are 2 pool implementations for PoolI: balancer and stableswap. The previously added tests only test balancer at the moment. However, since the inverse relationship applies to both balancer and stableswap, these tests can be generalized to apply to all possible implementations.

Suggested Design

• create a test file in x/gamm/pool-models
• extract TestCalculateAmountOutAndIn_InverseRelationship test from x/gamm/pool-models/balancer to x/gamm/pool-models
• generalize the test to apply to both balancer and stableswap pool implementations

Acceptance Criteria

• TestCalculateAmountOutAndIn_InverseRelationship tests both pool implementations
• the test is easily extensible to add more implementations if we have any in the future
• all new and existing unit tests pass

[p0mvn]

@xBalbinus would you be interested in working on this? Thanks for offering to help with the tests

[xBalbinus]

Yo @p0mvn ! Of course! Taking a look.

[p0mvn]

From @ValarDragon :

Recently, great tests have been added that test certain invariants (in the tests called "inverse relations") that are expected to hold trrue for any pool: https://github.com/osmosis-labs/osmosis/blob/main/x/gamm/pool-models/balancer/amm_test.go#L89-L216

We should make code reuse possible for using the same core logic in stableswap tests. Theres a couple ways this can be done:

I suggest we do the following:

• Make a general function, perhaps in pool-models/internal/test_helpers that defines a testcase struct, which takes in a PoolI and some input to try the invariant relation on
• Define testcases in amm_test.go (in the case of balancer, adapt the existing tests, to use a single function call to generate the pool)
  • This should not increase the length of testcase definition notably. Ideally its a one or two liner.
• Add the invariant relation tests to stableswap

There are alternative approaches if for some reason this produces extra overheads though.

[xBalbinus]

^ Huge shoutout to @mattverse for helping with this PR, I also fixed a small thing to keep the linter running. All tests are passing & ready for merge upon approval.