Banker's rounding (round-half-to-even) for interest calculations

Standard Math.round() always rounds 0.5 upward, introducing a systematic positive bias in interest calculations. Over millions of compounding periods this bias is non-trivial and legally relevant — financial regulators and ISO 25010 functional-correctness standards expect rounding to be demonstrably unbiased. Banker's rounding (round-half-to-even) is the industry standard because it distributes rounding error evenly across the dataset. CWE-682 covers this class of incorrect calculation leading to incorrect financial results.

Replace bare Math.round() calls in financial calculation logic with a banker's rounding implementation or a library that supports ROUND_HALF_EVEN. Centralize this in src/lib/finance.ts.

// src/lib/finance.ts
import Decimal from 'decimal.js';

export function roundToNearestCent(value: number): number {
  return new Decimal(value)
    .toDecimalPlaces(2, Decimal.ROUND_HALF_EVEN)
    .toNumber();
}

Search for all Math.round, .toFixed, and Math.ceil calls in src/ and replace with roundToNearestCent() for any interest, APY, or fee calculation.

• ID: finserv-form-validation.currency-amount.bankers-rounding

• Severity: high

• What to look for: Count all rounding call sites in the codebase (search for Math.round, Math.floor, Math.ceil, .toFixed, toDecimalPlaces, ROUND_HALF_EVEN). For each, classify whether it uses banker's rounding or standard rounding. Quote the actual rounding expression found (e.g., Math.round(interest * 100) or Decimal.ROUND_HALF_EVEN). Report the ratio: "X of Y rounding call sites use banker's rounding."

• Pass criteria: At least 100% of interest, APY, and fee rounding call sites use banker's rounding (round-half-to-even) or a financial library (decimal.js, big.js, dinero.js) with ROUND_HALF_EVEN. No more than 0 financial rounding sites use Math.round(). Report even on pass: "X rounding call sites found, all using banker's rounding."

• Fail criteria: Any financial rounding site uses standard Math.round() (round-half-up), or no rounding strategy is documented for calculations that produce fractional cents.

• Must not pass when: A financial library is imported but rounding mode is not explicitly set to ROUND_HALF_EVEN — default modes vary by library.

• Skip (N/A) when: The project has no interest, APY, or fee calculations (0 rounding call sites found in financial logic after searching src/, lib/, utils/).

• Detail on fail: Describe the rounding issue. Example: "Interest calculation uses Math.round() (round-half-up). At scale, this biases results upward. Should use banker's rounding." or "APY calculation has no rounding strategy documented"

• Cross-reference: The Calculation Accuracy category's interest-rounding-consistency check verifies rounding is applied consistently across all locations.

• Remediation: Banker's rounding eliminates systematic bias. In src/lib/finance.ts or equivalent calculation utility:

  Using decimal.js in src/lib/finance.ts:

  import Decimal from 'decimal.js';
  const interestRate = new Decimal('0.045'); // 4.5% APY
  const principal = new Decimal('1000.00');
  const interest = principal.times(interestRate).toDecimalPlaces(2, Decimal.ROUND_HALF_EVEN);
  

  Manual banker's rounding in src/lib/finance.ts:

  function bankersRound(value) {
    const shifted = value * 100;
    const floor = Math.floor(shifted);
    const frac = shifted - floor;
    if (frac < 0.5) return floor;
    if (frac > 0.5) return floor + 1;
    // Exactly 0.5: round to nearest even
    return floor % 2 === 0 ? floor : floor + 1;
  }
  const roundedCents = bankersRound(15.505); // Returns 1551 (15.51)