feat(tax_deduction_estimator): configure decimal rounding policies (#… - #590
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#590 made half-even the default rounding policy for calculateTaxDeduction and estimateBracketTax, which would silently change the tax amount existing callers (routes, formatForDbStorage, CSV export, bracket estimates) compute on halves and above-half fractions. Keep "truncate" (plain bigint division, the pre-#590 behaviour) as the default for both functions and roundIntegerDivision; the new policies stay available as an explicit opt-in. Also fix roundIntegerDivision's "truncate" for negative numerators, which rounded toward negative infinity like "floor"; it now rounds toward zero, and "floor" keeps rounding down. Update #590's tests: the default-policy test now asserts truncation, the half-even cases pass the policy explicitly, and a new case covers the default and truncate vs floor on negatives. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01PJM5SuF3hAuKGMpPBz7Rwr
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Description
Resolves #449
This PR configures consistent decimal rounding policies and remainder validation within
tax_deduction_estimator. It introduces configurable rounding protocols (defaulting to Banker's rounding / round-to-nearest-even) for division operations and ensures intermediate division remainders are accurately preserved and validated.Key Changes
src/utils/tax_deduction_estimator.ts):RoundingPolicysupporting"half-even"/"round-to-nearest-even"(default),"half-up","truncate","ceil", and"floor".roundIntegerDivision(numerator, divisor, mode)helper for integer-based rounding arithmetic.calculateTaxDeductionandestimateBracketTaxto accept rounding policies via parameter or options object ({ roundingMode }).ERROR_CODES.INVALID_ROUNDING_MODE(TAX_ESTIMATOR_INVALID_ROUNDING_MODE) for invalid policy rejection.roundingModeintoDbPrecisionSchema(for DB formatting) andTaxCsvExportOptions(for CSV exports).verifyTaxDeductionRemainder(and aliasvalidateTaxDeductionRemainder) to assert that intermediate division remainders are not dropped or lost (grossAmount * taxRate === unroundedTax * taxScale + remainder) and total funds are conserved (grossAmount === netAmount + taxAmount).__tests__/tax_deduction_estimator.test.ts):roundIntegerDivisionacross all rounding policies.Verification & Testing
npx tsc --noEmitwith 0 errors.npm run build.__tests__/tax_deduction_estimator.test.tsand 100 tests in__tests__/tax_bracket_estimator.test.ts.Closes #449
Closes #448
Closes #447
Closes #446