fix(stdlib): prevent precision loss in GasFees.mul() for large bigint values

yarn-project/stdlib/src/gas/gas_fees.test.ts

@@ -0,0 +1,61 @@
+import { GasFees } from './gas_fees.js';
+
+/** Helper: ceiling division for bigints, matching the fix's behavior. */
+function ceilDiv(a: bigint, b: bigint): bigint {
+  return (a + b - 1n) / b;
+}
+
+describe('GasFees.mul', () => {
+  it('multiplies by an integer number scalar', () => {
+    const fees = new GasFees(100n, 200n);
+    const result = fees.mul(2);
+    expect(result).toEqual(new GasFees(200n, 400n));
+  });
+
+  it('multiplies by a bigint scalar', () => {
+    const fees = new GasFees(100n, 200n);
+    const result = fees.mul(3n);
+    expect(result).toEqual(new GasFees(300n, 600n));
+  });
+
+  it('multiplies by a non-integer scalar', () => {
+    const fees = new GasFees(100n, 200n);
+    const result = fees.mul(1.5);
+    expect(result).toEqual(new GasFees(150n, 300n));
+  });
+
+  it('applies ceiling to non-integer scalar results', () => {
+    const fees = new GasFees(1n, 1n);
+    const result = fees.mul(1.5);
+    // 1 * 1.5 = 1.5, ceil(1.5) = 2
+    expect(result).toEqual(new GasFees(2n, 2n));
+  });
+
+  it('returns a clone when scalar is 1', () => {
+    const fees = new GasFees(100n, 200n);
+    const result = fees.mul(1);
+    expect(result).toEqual(fees);
+    expect(result).not.toBe(fees); // should be a new instance (clone)
+  });
+
+  it('preserves precision for values above 2^53 (regression test)', () => {
+    const bigValue = 2n ** 64n;
+    const fees = new GasFees(bigValue, bigValue);
+    const result = fees.mul(1.5);
+
+    // 1.5 = 3/2, so the exact result is ceilDiv(2^64 * 3, 2)
+    const expected = ceilDiv(bigValue * 3n, 2n);
+    expect(result.feePerDaGas).toEqual(expected);
+    expect(result.feePerL2Gas).toEqual(expected);
+  });
+
+  it('preserves precision for values near 2^128', () => {
+    const bigValue = 2n ** 128n - 1n; // max UInt128
+    const fees = new GasFees(bigValue, bigValue);
+    const result = fees.mul(1.5);
+
+    const expected = ceilDiv(bigValue * 3n, 2n);
+    expect(result.feePerDaGas).toEqual(expected);
+    expect(result.feePerL2Gas).toEqual(expected);
+  });
+});

yarn-project/stdlib/src/gas/gas_fees.ts

@@ -15,6 +15,14 @@ import { z } from 'zod';
 import type { UInt128 } from '../types/shared.js';
 import type { GasDimensions } from './gas.js';
 
+/** Ceiling division for non-negative bigints. */
+function ceilDiv(a: bigint, b: bigint): bigint {
+  return (a + b - 1n) / b;
+}
+
+/** Precision scale for converting fractional scalars to integer numerators (18 decimal digits). */
+const FRACTIONAL_PRECISION = 10n ** 18n;
+
 /** Gas prices for each dimension. */
 export class GasFees {
   public readonly feePerDaGas: UInt128;
@@ -60,9 +68,10 @@ export class GasFees {
       const s = BigInt(scalar);
       return new GasFees(this.feePerDaGas * s, this.feePerL2Gas * s);
     } else {
+      const numerator = BigInt(Math.round(scalar * Number(FRACTIONAL_PRECISION)));
       return new GasFees(
-        BigInt(Math.ceil(Number(this.feePerDaGas) * scalar)),
-        BigInt(Math.ceil(Number(this.feePerL2Gas) * scalar)),
+        ceilDiv(this.feePerDaGas * numerator, FRACTIONAL_PRECISION),
+        ceilDiv(this.feePerL2Gas * numerator, FRACTIONAL_PRECISION),
       );
     }
   }