Reference
referenceUses Decimal at full precision and rounds only the fee and the final destination amount.
03 · FX transfers
Quote transfers with currency-specific minor units
Quote a currency transfer with the fee taken in the source currency and each currency's own decimal places.
What the hardened grader checks
- JPY has 0 decimal places and KWD has 3
- Cross rates via USD are multiplied without rounding the middle leg
- The fee is deducted in the source currency before conversion
- Exact half-unit ties round to even
- Unknown pairs raise RateUnavailable
Solutions
Alternative
alt_validExact Fraction scaling and integer half-even rounding instead of Decimal amount arithmetic.
wrong_fee_after_conversion
Rounds destination fees after conversion instead of source fees before it.
wrong_round_cross_leg
Rounds the intermediate USD amount in a cross quote.
wrong_two_decimals
Assumes every currency has two decimal places.
Held-out solutions
weak_validationExpect: rejectObserved: rejectedAccepts bool amounts and fees over 10000 bps, violating the stated integer and fee range rules.
infers_inverseExpect: rejectObserved: rejectedInfers inverse rates, violating the direct-or-USD-only routing rule.
cross_rate_6dpExpect: rejectObserved: rejectedRounds the cross product to six decimals, violating full precision with no intermediate rounding.
default_context_prec28Expect: rejectObserved: rejectedTruncates the cross rate at the default Decimal precision, violating the full product rate_used rule.
Tests × solutions
Each row groups a test’s parameterized cases; any failing case marks the row as Fail. * marks a wrong solution’s declared must_fail test.
| Test | Starter | Reference | Alternative | fee after conversion | round cross leg | two decimals |
|---|---|---|---|---|---|---|
| Jpy and kwd units | Fail | Pass | Pass | Fail | Pass | Fail * |
| Cross has no intermediate rounding | Fail | Pass | Pass | Pass | Fail * | Pass |
| Source fee before conversion | Fail | Pass | Pass | Fail * | Pass | Pass |
| Discovered half even ties | Pass | Pass | Pass | Pass | Pass | Pass |
| Unavailable and zero | Pass | Pass | Pass | Pass | Pass | Pass |
| Inverse only is unavailable | Pass | Pass | Pass | Pass | Pass | Pass |
| Full precision cross rate | Fail | Pass | Pass | Fail | Fail | Pass |
| Invalid amount and fee | Pass | Pass | Pass | Pass | Pass | Pass |
| Exact scaling and monotonicity | Fail | Pass | Pass | Fail | Pass | Fail |
| Zero fee roundtrip | Pass | Pass | Pass | Pass | Pass | Pass |
Naive grader
| Test | Starter | Reference | Alternative | fee after conversion | round cross leg | two decimals |
|---|---|---|---|---|---|---|
| Direct cad usd | Fail | Pass | Pass | Pass | Pass | Pass |
| Ticket example cross quote | Fail | Pass | Pass | Pass | Pass | Pass |
| Ticket example zero decimal currency | Fail | Pass | Pass | Pass | Pass | Fail |
| Unknown route rejected | Pass | Pass | Pass | Pass | Pass | Pass |
The ticket
Ticket: quote a synthetic currency transfer
fx.quote.quote_transfer(amount_minor:int,src:str,dst:str,rates:dict[tuple[str,str],Decimal],fee_bps:int) returns frozen Quote(fee_minor:int,converted_minor:int,rate_used:Decimal,path:tuple[str,...]). Expose RateUnavailable(ValueError) for unavailable currencies/pairs.
Minor units: CAD/USD/EUR/GBP=2, JPY=0, KWD/BHD=3. Rates are positive finite Decimals in destination major units per source major unit. All money values returned are int minor units. Amount is nonnegative int, never bool; fee_bps integer in [0,10000]; invalid values -> ValueError. Inputs use distinct src/dst; an unsupported currency -> RateUnavailable.
- Fee in source minor units = amount_minor × fee_bps / 10000, rounded HALF_EVEN to integer. Deduct this fee BEFORE conversion. Returned fee_minor is this source fee.
- Use direct
(src,dst)rate if present, path(src,dst). Otherwise require both(src,"USD")and("USD",dst), multiply at full precision, path(src,"USD",dst). Do not infer inverses or other routes. Missing required rates -> RateUnavailable. Direct rate takes precedence. - Convert net source minor units to source major units, apply the full rate, then scale to destination minor units. Round ONLY the final destination amount HALF_EVEN to integer. Do not round the USD leg of a cross quote. rate_used is the full direct/product Decimal.
Worked examples. (a) Cross quote: rates = {("GBP","USD"): Decimal("1.25"), ("USD","CAD"): Decimal("1.40")}, quote_transfer(20000, "GBP", "CAD", rates, 50): fee = 20000 x 50 / 10000 = 100 (1.00 GBP); net 19900 = 199.00 GBP; rate_used = 1.25 x 1.40 = 1.7500; 199.00 x 1.7500 = 348.25 CAD, so Quote(100, 34825, Decimal("1.7500"), ("GBP","USD","CAD")). (b) Zero-decimal currency: quote_transfer(5000, "CAD", "JPY", {("CAD","JPY"): Decimal("110")}, 0): 50.00 CAD x 110 = 5500 JPY, so Quote(0, 5500, Decimal("110"), ("CAD","JPY")).
Do not mutate rates. Zero amount returns zero amounts while still resolving the declared route. No actual currency market or customer is represented. Run python -m pytest tests -q from starter.