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Reject loan offers that can never amortise

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At high rate and long duration the rounded instalment converges to C*i from
above and lands exactly on the first month's interest, leaving zero principal:
at 99 % the frontier is 294 months for the maximum capital. The schedule would
never terminate.

Checking the first month suffices: once principal is positive the outstanding
capital decreases, so interest decreases and principal grows. The check must
stay after the bounds checks, since it calls monthly_repayment() and that
function's expect() is justified by them.

creating_loan_offer_with_limit_inputs_works asserted that the three maxima
combined were valid, which this invariant makes false. The valid domain is no
longer a box. It is replaced by the corner that actually maximises the
instalment, duration 1, at 1 082 499 999,99 EUR against 82 500 000,00 EUR for
the corner it tested before.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
Marcel Enguehard 2026-08-25 11:40:35 +02:00
parent 8d00f6a936
commit f1ba015cc3
1 changed files with 29 additions and 11 deletions
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40
crates/immo-core/src/loan_offer.rs
View file

@ -18,6 +18,8 @@ pub enum LoanOfferError {
MaxAnnualNominalRateExceeded(Decimal),
#[error("loan duration in months should be lower than {MAX_LOAN_DURATION_IN_MONTHS}: {0}")]
MaxLoanDurationExceeded(u64),
#[error("loan cannot amortise")]
LoanCannotAmortise,
}
/// 1b€ is a pretty expensive house...
@ -59,11 +61,21 @@ impl LoanOffer {
} else if borrowed_capital > MAX_BORROWED_CAPITAL {
Err(LoanOfferError::MaxBorrowedCapitalExceeded(borrowed_capital))
} else {
Ok(LoanOffer {
let loan_offer = LoanOffer {
borrowed_capital,
annual_nominal_rate,
loan_duration_in_months,
})
};
// Check that the first month is not "only interests", otherwise the loan cannot be reimbursed
// /!\ Must be kept after all the previous verifications otherwise monthly_repayment() can panic
if loan_offer.monthly_repayment().as_decimal()
> (borrowed_capital.as_decimal() * loan_offer.proportional_monthly_interest_rate())
.round_dp(2)
{
Ok(loan_offer)
} else {
Err(LoanOfferError::LoanCannotAmortise)
}
}
}
@ -98,7 +110,7 @@ mod tests {
#[test]
fn creating_loan_offer_with_negative_interest_rate_returns_negative_interest_rate_error() {
let ret = LoanOffer::new(Euros::from_cents_as_i64(123), Decimal::new(-45, 2), 5);
let ret = LoanOffer::new(Euros::from_cents_as_i64(123), Decimal::new(-45, 2), 60);
assert_eq!(
ret,
Err(LoanOfferError::NegativeInterestRate(Decimal::new(-45, 2)))
@ -177,6 +189,16 @@ mod tests {
);
}
#[test]
fn creating_loan_offer_that_cannot_amortise_returns_loan_cannot_amortise_error() {
let ret = LoanOffer::new(
Euros::from_cents_as_i64(439_000_00),
Decimal::new(99, 2),
480,
);
assert_eq!(ret, Err(LoanOfferError::LoanCannotAmortise));
}
#[test]
fn creating_loan_offer_with_correct_inputs_works() {
let ret = LoanOffer::new(Euros::from_cents_as_i64(10), Decimal::new(45, 2), 15);
@ -191,12 +213,8 @@ mod tests {
}
#[test]
fn creating_loan_offer_with_limit_inputs_works() {
let ret = LoanOffer::new(
MAX_BORROWED_CAPITAL,
MAX_ANNUAL_NOMINAL_RATE,
MAX_LOAN_DURATION_IN_MONTHS,
);
fn creating_loan_offer_with_max_repayment_works() {
let ret = LoanOffer::new(MAX_BORROWED_CAPITAL, MAX_ANNUAL_NOMINAL_RATE, 1);
assert!(ret.is_ok());
// Should not panic - checks that expect assertion is valid
ret.unwrap().monthly_repayment();
@ -219,11 +237,11 @@ mod tests {
fn conversion_to_annual_nominal_rate_in_percent_works() {
let offer = LoanOffer::new(
Euros::from_cents_as_i64(150_000_00),
Decimal::new(4, 4),
Decimal::new(4, 2),
300,
)
.unwrap();
assert_eq!(offer.annual_nominal_rate_in_percent(), Decimal::new(4, 2));
assert_eq!(offer.annual_nominal_rate_in_percent(), Decimal::new(400, 2));
}
#[test]