fix: AMM swap rounding

[seelabs]

High Level Overview of Change

fix amendment: AMM swap should honor invariants:

The AMM has an invariant for swaps where:

new_balance_1*new_balance_2 >= old_balance_1*old_balance_2

Due to rounding, this invariant could sometimes be violated (although by very small amounts).

This patch introduces an amendment fixAMMRounding that changes the rounding to always favor the AMM. Doing this should maintain the invariant.

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ x] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Refactor (non-breaking change that only restructures code)
• [ ] Performance (increase or change in throughput and/or latency)
• [ x] Tests (you added tests for code that already exists, or your new feature included in this PR)
• [ ] Documentation update
• [ ] Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• [ ] Release

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 60.52632% with 30 lines in your changes are missing coverage. Please review.

Project coverage is 70.9%. Comparing base (b422e71) to head (b90f58d).

Additional details and impacted files

[](https://app.codecov.io/gh/XRPLF/rippled/pull/5002?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF) ```diff @@ Coverage Diff @@ ## develop #5002 +/- ## ========================================= - Coverage 70.9% 70.9% -0.0% ========================================= Files 796 796 Lines 66727 66792 +65 Branches 10976 10996 +20 ========================================= + Hits 47339 47378 +39 - Misses 19388 19414 +26 ``` | [Files](https://app.codecov.io/gh/XRPLF/rippled/pull/5002?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF) | Coverage Δ | | |---|---|---| | [src/ripple/app/tx/impl/Transactor.cpp](https://app.codecov.io/gh/XRPLF/rippled/pull/5002?src=pr&el=tree&filepath=src%2Fripple%2Fapp%2Ftx%2Fimpl%2FTransactor.cpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF#diff-c3JjL3JpcHBsZS9hcHAvdHgvaW1wbC9UcmFuc2FjdG9yLmNwcA==) | `84.5% <100.0%> (+<0.1%)` | :arrow_up: | | [src/ripple/basics/Number.h](https://app.codecov.io/gh/XRPLF/rippled/pull/5002?src=pr&el=tree&filepath=src%2Fripple%2Fbasics%2FNumber.h&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF#diff-c3JjL3JpcHBsZS9iYXNpY3MvTnVtYmVyLmg=) | `100.0% <100.0%> (ø)` | | | [src/ripple/protocol/Feature.h](https://app.codecov.io/gh/XRPLF/rippled/pull/5002?src=pr&el=tree&filepath=src%2Fripple%2Fprotocol%2FFeature.h&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF#diff-c3JjL3JpcHBsZS9wcm90b2NvbC9GZWF0dXJlLmg=) | `100.0% <ø> (ø)` | | | [src/ripple/protocol/Rules.h](https://app.codecov.io/gh/XRPLF/rippled/pull/5002?src=pr&el=tree&filepath=src%2Fripple%2Fprotocol%2FRules.h&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF#diff-c3JjL3JpcHBsZS9wcm90b2NvbC9SdWxlcy5o) | `100.0% <100.0%> (ø)` | | | [src/ripple/protocol/impl/Feature.cpp](https://app.codecov.io/gh/XRPLF/rippled/pull/5002?src=pr&el=tree&filepath=src%2Fripple%2Fprotocol%2Fimpl%2FFeature.cpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF#diff-c3JjL3JpcHBsZS9wcm90b2NvbC9pbXBsL0ZlYXR1cmUuY3Bw) | `93.9% <ø> (ø)` | | | [src/ripple/protocol/impl/Rules.cpp](https://app.codecov.io/gh/XRPLF/rippled/pull/5002?src=pr&el=tree&filepath=src%2Fripple%2Fprotocol%2Fimpl%2FRules.cpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF#diff-c3JjL3JpcHBsZS9wcm90b2NvbC9pbXBsL1J1bGVzLmNwcA==) | `97.9% <100.0%> (+0.4%)` | :arrow_up: | | [src/ripple/ledger/impl/View.cpp](https://app.codecov.io/gh/XRPLF/rippled/pull/5002?src=pr&el=tree&filepath=src%2Fripple%2Fledger%2Fimpl%2FView.cpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF#diff-c3JjL3JpcHBsZS9sZWRnZXIvaW1wbC9WaWV3LmNwcA==) | `91.6% <75.0%> (-0.1%)` | :arrow_down: | | [src/ripple/protocol/AmountConversions.h](https://app.codecov.io/gh/XRPLF/rippled/pull/5002?src=pr&el=tree&filepath=src%2Fripple%2Fprotocol%2FAmountConversions.h&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF#diff-c3JjL3JpcHBsZS9wcm90b2NvbC9BbW91bnRDb252ZXJzaW9ucy5o) | `85.7% <0.0%> (-10.3%)` | :arrow_down: | | [src/ripple/app/misc/AMMHelpers.h](https://app.codecov.io/gh/XRPLF/rippled/pull/5002?src=pr&el=tree&filepath=src%2Fripple%2Fapp%2Fmisc%2FAMMHelpers.h&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF#diff-c3JjL3JpcHBsZS9hcHAvbWlzYy9BTU1IZWxwZXJzLmg=) | `69.0% <53.1%> (-16.1%)` | :arrow_down: | ... and [5 files with indirect coverage changes](https://app.codecov.io/gh/XRPLF/rippled/pull/5002/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF) [](https://app.codecov.io/gh/XRPLF/rippled/pull/5002?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=XRPLF)