fix(currency-math): correct within-step fractional bonding-curve pricing - #1172
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DiscreteBondingCurve.tokensToValue returned a negative value for a within-step
purchase with a fractional token amount — e.g. tokensToValue(0, 12.5) yielded
-0.750 instead of 0.125.
Root cause: the `startStep == endStep` short-circuit used BigDecimal `==`
(scale-sensitive equals). For a fractional within-step purchase, startStep ("0",
scale 0) and endStep ("0.0", scale 1) are value-equal but `==`-unequal, so the
code fell through to the multi-step branch and computed
middleCost = cumulative[0] - cumulative[1] = -1.0. Whole-token inputs share
scale 0, so `==` happened to work — which is why the bug stayed hidden.
Fix: compare with compareTo; also harden the sibling `tokens == ZERO` check to
signum() (same scale-sensitivity class).
Adds DiscreteBondingCurveVectorTest, asserting the curve against canonical
vectors derived from the on-chain Rust curve (both apps load identical tables),
including the fractional cases that regressed. Verified on device.
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Problem
DiscreteBondingCurve.tokensToValuereturns a negative value for a within-step purchase with a fractional token amount:(currentSupply, tokens)(0, 12.5)0.125-0.750(0, 0.0000000001)0.000000000001-0.999999999998Root cause
The
startStep == endStepshort-circuit uses Kotlin==→BigDecimal.equals, which is scale-sensitive. For a fractional within-step purchase,startStep = "0"(scale 0) andendStep = "0.0"(scale 1) are equal in value but==-unequal, so the code falls through to the multi-step branch and computesmiddleCost = cumulative[0] - cumulative[1] = -1.0.Whole-token inputs share scale 0, so
==happened to work — which is why the bug stayed hidden.Fix
compareToinstead of==in the within-step check.tokens == BigDecimal.ZEROcheck totokens.signum() == 0(same scale-sensitivity class).Testing
Adds
DiscreteBondingCurveVectorTest(instrumented) asserting the curve against canonical vectors derived from the on-chain Rust curve (flipcash-program/api) — both apps load bit-identical pricing/cumulative tables — including the fractional cases that regressed.Verified on device (SM-S938U1): before the fix the within-step fractional cases returned negative; after, they match the reference (
0.125,1e-12). iOS already computes these correctly, so this brings Android into agreement.Reachability: the production sell path uses large (multi-step)
new_supply, so this is unlikely to be hit today — but it's a latent money-math divergence in a shared primitive.