fix: losing precision on small amounts

packages/app/src/systems/Core/utils/convertToUsd.test.ts

@@ -23,4 +23,8 @@ describe('Convert to USD', () => {
     );
     expect(formatted).toBe('$10.00');
   });
+  it('should not lose precision when dealing with really small amounts', () => {
+    const { formatted } = convertToUsd(bn(1), DECIMAL_FUEL, MOCK_ETH_RATE);
+    expect(formatted).toBe('$0.000002');
+  });
 });

packages/app/src/systems/Core/utils/convertToUsd.ts

@@ -1,6 +1,7 @@
 import { type BN, bn } from 'fuels';
 
 const DEFAULT_MIN_PRECISION = 2;
+const EXTRA_PRECISION_DIGITS = 10;
 
 export function convertToUsd(
   amount: BN,
@@ -9,27 +10,23 @@ export function convertToUsd(
 ): { value: number; formatted: string } {
   if (!rate) return { value: 0, formatted: '$0' };
 
-  const precisionFactor = 10 ** DEFAULT_MIN_PRECISION;
-
-  // Used for the output fixed-point
+  // Use a higher extra precision multiplier to preserve small fractions.
+  const extraPrecision = 10 ** EXTRA_PRECISION_DIGITS;
+  // This is used for the fixed output decimals.
   const outFactor = 10 ** DEFAULT_MIN_PRECISION;
 
-  // Scaled rate. Instead of rate * outFactor (which is what we did before), we use an extra multiplier to keep the fractional part.
-  // Since BN handles numbers as integers, we need to round the result to avoid losing precision on smaller values.
+  // Scale the rate to fixed-point.
   const rateFixed = Math.round(rate * outFactor);
 
-  // We multiply the numerator by an extra precision factor in order to not lose the fractional part during division
-  // Our intended fixed-point integer `(representing value * (precisionFactor * outFactor))` is (amount * rateFixed * precisionFactor) / (10^decimals)
-  // Essentially ` (amount/10^decimals) * rate ` but in fixed-point arithmetic.
-  // And then we recover the actual value by dividing by (precisionFactor * outFactor).
-  const numerator = amount.mul(bn(rateFixed)).mul(bn(precisionFactor));
+  // Multiply the numerator by the extra precision factor to avoid truncation.
+  // Numerator represents: amount * rateFixed * extraPrecision.
+  const numerator = amount.mul(bn(rateFixed)).mul(bn(extraPrecision));
   const denominator = bn(10).pow(bn(decimals));
   const scaledUsdBN = numerator.div(denominator);
 
-  // Convert to a number:
-  // Here scaledUsdBN represents value * (precisionFactor * outFactor)
-  // so we must divide by (precisionFactor * outFactor) to get the actual value.
-  const value = Number(scaledUsdBN.toString()) / (precisionFactor * outFactor);
+  // scaledUsdBN now represents the value multiplied by (extraPrecision * outFactor)
+  // Recover the actual value by dividing by that factor.
+  const value = Number(scaledUsdBN.toString()) / (extraPrecision * outFactor);
 
   if (value === 0) return { value, formatted: '$0' };