test: add fee, commission, and net-proceeds calculation tests - #574
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Zero test coverage existed for the financial math shown in the Deposit
screen. Any change to fee constants, roundToCents, projectedReturn, or
the net-proceeds formula could silently regress.
This commit adds src/lib/feeCalculations.test.ts (48 tests, all passing):
DEPOSIT_FEE_USDC constant (4 tests)
- exact value, sub-cent promise, positive/finite, smaller than min deposit
Net proceeds — amount minus fee (8 tests)
- standard 100 USDC, minimum deposit, fee-sized input, sub-fee input,
large amounts, decimal precision, formula parity with roundToCents,
idempotency check
projectedReturn — simple interest (13 tests)
- zero/negative/NaN/Infinity amounts return 0
- 1/5/10-year returns at 6%
- linear scaling with years and amount
- fractional rates (4.75%), default years=1, zero rate
- matches the three Deposit screen projections (1y/5y/10y)
roundToCents — monetary precision (7 tests)
- the classic 1.005 float trap (rounds to 1.01 not 1.00)
- 2.355 → 2.36, 1.004 → 1.00
- zero, negative, idempotency, max 2 decimal digits
formatDecimal — display precision (6 tests)
- 0 and 100 USDC at 2dp, share count at 4dp
- round-before-format, string return, trailing zeros
roundToDecimals — generic precision helper (4 tests)
- 0/3/7 decimals, 0.1+0.2 epsilon drift
Integration: full deposit preview (5 tests)
- 100 USDC / 1.0 price / 6%: verifies fee, proceeds, shares, 1y/5y/10y
- 250 USDC / 1.05 price / 5.5%
- minimum deposit yields positive net proceeds
- fee is flat (same for $10 and $10k deposits)
- proceeds scale linearly with amount
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Summary
Fee, commission, and net-proceeds calculations had zero test coverage. Any change to
DEPOSIT_FEE_USDC,roundToCents,projectedReturn, or the inline net-proceeds formula could silently break the values shown to investors in the Deposit screen.This PR adds
src/lib/feeCalculations.test.ts— 48 tests covering every financial formula in the deposit flow.What was added
src/lib/feeCalculations.test.ts(333 lines, 48 tests)DEPOSIT_FEE_USDC constantNet proceeds (amount − fee)roundToCents(n - DEPOSIT_FEE_USDC)formulaprojectedReturn — simple interestroundToCents — monetary precisionformatDecimal — display precisionroundToDecimals — generic precisionIntegration: full deposit previewKey regressions now caught
DEPOSIT_FEE_USDCis changed from0.01, the constant suite fails immediatelyroundToCentsloses theNumber.EPSILONfix, the1.005 → 1.01test failsprojectedReturnis accidentally made compound instead of simple, the linear scaling tests failWhat was tested
All 48 tests pass:
No runtime code was changed — test-only PR.
closes #447