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Refine AMM offer generation for integral MPT pairs

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This commit is contained in:
Gregory Tsipenyuk
2026-05-16 19:33:00 -04:00
parent c6918f5915
commit a27596f988
2 changed files with 101 additions and 11 deletions
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82
src/test/app/AMMMPT_test.cpp
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@@ -5645,6 +5645,87 @@ private:
});
}
void
testAMMOfferGenerationPolicy(FeatureBitset features)
{
testcase("AMM payment offer generation picks economically coarser integral side");
using namespace jtx;
enum class GeneratedFirst { TakerPays, TakerGets };
auto const check = [&](std::uint64_t mptUnitsPerXRP, GeneratedFirst generatedFirst) {
TAmounts<XRPAmount, MPTAmount> const pool{
XRPAmount{1'000'000}, MPTAmount{1'000'000'125}};
TAmounts<XRPAmount, MPTAmount> const clobOffer{
kDROPS_PER_XRP, MPTAmount{static_cast<std::int64_t>(mptUnitsPerXRP)}};
Quality const clobQuality{clobOffer};
auto const expectedAmounts = generatedFirst == GeneratedFirst::TakerGets
? getAMMOfferStartWithTakerGets(pool, clobQuality, 0)
: getAMMOfferStartWithTakerPays(pool, clobQuality, 0);
auto const otherAmounts = generatedFirst == GeneratedFirst::TakerGets
? getAMMOfferStartWithTakerPays(pool, clobQuality, 0)
: getAMMOfferStartWithTakerGets(pool, clobQuality, 0);
BEAST_EXPECT(expectedAmounts);
BEAST_EXPECT(otherAmounts);
if (!expectedAmounts || !otherAmounts)
return;
// Make the tested branch observable: these cases are chosen so the
// payment consumes different AMM amounts depending on which side
// is generated first.
BEAST_EXPECT(*expectedAmounts != *otherAmounts);
Env env(*this, features);
auto const gw = Account("gw");
auto const lp = Account("lp");
auto const maker = Account("maker");
auto const taker = Account("taker");
auto const dst = Account("dst");
env.fund(XRP(10'000), gw, lp, maker, taker, dst);
env.close();
MPTTester const token(
{.env = env, .issuer = gw, .holders = {lp, maker, dst}, .flags = kMPT_DEX_FLAGS});
env(pay(gw, lp, token(pool.out.value())));
env(pay(gw, maker, token(10'000'000)));
env.close();
AMM const amm(env, lp, drops(pool.in), token(pool.out.value()));
auto const makerOfferSeq = env.seq(maker);
env(offer(maker, XRP(1), token(mptUnitsPerXRP)), Txflags(tfPassive));
env.close();
env(pay(taker, dst, token(expectedAmounts->out.value())),
Sendmax(drops(expectedAmounts->in)));
env.close();
BEAST_EXPECT(amm.expectBalances(
drops(pool.in + expectedAmounts->in),
token((pool.out - expectedAmounts->out).value()),
amm.tokens()));
env.require(Balance(dst, token(expectedAmounts->out.value())));
BEAST_EXPECT(env.le(keylet::offer(maker.id(), makerOfferSeq)));
};
// CLOB price: 10'000'000 MPT per 1 XRP, so one raw MPT unit is worth
// 0.1 drops. One drop is the economically coarser unit and the AMM
// offer is generated from takerPays.
check(10 * kDROPS_PER_XRP.drops(), GeneratedFirst::TakerPays);
// CLOB price: 1'000'000 MPT per 1 XRP, so one raw MPT unit is worth
// one drop. Ties use takerGets to preserve the historical XRP-output
// behavior.
check(kDROPS_PER_XRP.drops(), GeneratedFirst::TakerGets);
// CLOB price: 100'000 MPT per 1 XRP, so one raw MPT unit is worth
// 10 drops. MPT is the economically coarser unit and the AMM offer is
// generated from takerGets.
check(kDROPS_PER_XRP.drops() / 10, GeneratedFirst::TakerGets);
}
void
testTradingFee(FeatureBitset features)
{
@@ -7101,6 +7182,7 @@ private:
testAMMTokens();
testAmendment();
testAMMAndCLOB(all);
testAMMOfferGenerationPolicy(all);
testTradingFee(all);
testTradingFee(all - fixAMMv1_3);
testAdjustedTokens(all);