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fix(amm): normalize price scales before computing price impact #1017

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fix(amm): normalize price scales before computing price impact #1017

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15 changes: 11 additions & 4 deletions src/ammCalculator.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -435,16 +435,23 @@ function computePriceImpact(spotPrice: string, effectivePrice: string): string {

const spot = parseDecimal(spotPrice);
const effective = parseDecimal(effectivePrice);

const spotScaled = spot.int * spot.scale + spot.frac;
const effectiveScaled = effective.int * effective.scale + effective.frac;

if (spotScaled === 0n) return "0.00";
// Issue #1011 — normalize both values to their common scale before
// comparing, so that an integer spot price ("1") and a fractional
// effective price ("0.91") are not compared at different scales.
const commonScale = spot.scale > effective.scale ? spot.scale : effective.scale;
const spotCommon = spotScaled * (commonScale / spot.scale);
const effectiveCommon = effectiveScaled * (commonScale / effective.scale);

if (spotCommon === 0n) return "0.00";

// (spot - effective) / spot * 100 with 4 decimal places of precision
const SCALE = 10000n;
const numerator = (spotScaled - effectiveScaled) * SCALE * 100n;
const denominator = spotScaled;
const numerator = (spotCommon - effectiveCommon) * SCALE * 100n;
const denominator = spotCommon;

if (numerator <= 0n) return "0.00";

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