Add LoanOffer with validating constructor

LoanOffer::new is the only way in: fields are private and the constructor
rejects a negative rate, a zero duration, and a non-positive capital. A zero
rate is accepted on purpose — the PTZ (prêt à taux zéro) is a real French
instrument.

Add thiserror for the typed error, as required for immo-core. Rate and
duration take no upper bound for now.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Cargo.lock

@@ -174,6 +174,7 @@ name = "immo-core"
 version = "0.1.0"
 dependencies = [
  "rust_decimal",
+ "thiserror",
 ]
 
 [[package]]
@@ -492,6 +493,26 @@ version = "1.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "55937e1799185b12863d447f42597ed69d9928686b8d88a1df17376a097d8369"
 
+[[package]]
+name = "thiserror"
+version = "2.0.19"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "09a43598840e33d5b0331f38c5e30d13bb11c11210a4b58f0d9b18a5a5eefcd9"
+dependencies = [
+ "thiserror-impl",
+]
+
+[[package]]
+name = "thiserror-impl"
+version = "2.0.19"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "43cbfe0cf76104d42a574802844187e84a305e531ed54455f11fbde0f10541cd"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 3.0.3",
+]
+
 [[package]]
 name = "tinyvec"
 version = "1.12.0"

crates/immo-core/Cargo.toml

@@ -6,6 +6,7 @@ rust-version.workspace = true
 
 [dependencies]
 rust_decimal = "1.42.1"
+thiserror = "2.0.19"
 
 [lints.clippy]
 inconsistent_digit_grouping = "allow"

crates/immo-core/src/lib.rs

@@ -1 +1,2 @@
 pub mod euros;
+pub mod loan_offer;

crates/immo-core/src/loan_offer.rs

@@ -0,0 +1,113 @@
+use rust_decimal::Decimal;
+use thiserror::Error;
+
+use crate::euros::Euros;
+
+#[derive(Debug, PartialEq, Eq, Error, Clone)]
+pub enum LoanOfferError {
+    #[error("loan cannot have a negative interest rate: {0}%")]
+    NegativeInterestRate(Decimal),
+    #[error("loan duration cannot be zero")]
+    ZeroLoanDuration,
+    #[error("borrowed capital should be strictly positive: {0}")]
+    NonPositiveBorrowedCapital(Euros),
+}
+
+#[derive(Debug, PartialEq, Eq)]
+pub struct LoanOffer {
+    borrowed_capital: Euros,
+    annual_rate_in_percent: Decimal,
+    loan_duration_in_months: u64,
+}
+
+impl LoanOffer {
+    pub fn new(
+        borrowed_capital: Euros,
+        annual_rate_in_percent: Decimal,
+        loan_duration_in_months: u64,
+    ) -> Result<Self, LoanOfferError> {
+        if annual_rate_in_percent < Decimal::ZERO {
+            Err(LoanOfferError::NegativeInterestRate(annual_rate_in_percent))
+        } else if loan_duration_in_months == 0 {
+            Err(LoanOfferError::ZeroLoanDuration)
+        } else if borrowed_capital <= Euros::from_cents(0) {
+            Err(LoanOfferError::NonPositiveBorrowedCapital(borrowed_capital))
+        } else {
+            Ok(LoanOffer {
+                borrowed_capital,
+                annual_rate_in_percent,
+                loan_duration_in_months,
+            })
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn creating_loan_offer_with_negative_interest_rate_returns_negative_interest_rate_error() {
+        let ret = LoanOffer::new(Euros::from_cents(123), Decimal::new(-45, 2), 5);
+        assert_eq!(
+            ret,
+            Err(LoanOfferError::NegativeInterestRate(Decimal::new(-45, 2)))
+        );
+    }
+
+    #[test]
+    fn creating_loan_offer_with_null_loan_duration_returns_zero_loan_duration_error() {
+        let ret = LoanOffer::new(Euros::from_cents(123), Decimal::new(45, 2), 0);
+        assert_eq!(ret, Err(LoanOfferError::ZeroLoanDuration));
+    }
+
+    #[test]
+    fn creating_loan_offer_with_negative_borrowed_capital_returns_non_positive_borrowed_capital_error()
+     {
+        let ret = LoanOffer::new(Euros::from_cents(-1), Decimal::new(45, 2), 15);
+        assert_eq!(
+            ret,
+            Err(LoanOfferError::NonPositiveBorrowedCapital(
+                Euros::from_cents(-1)
+            ))
+        );
+    }
+
+    #[test]
+    fn creating_loan_offer_with_null_borrowed_capital_returns_non_positive_borrowed_capital_error()
+    {
+        let ret = LoanOffer::new(Euros::from_cents(0), Decimal::new(45, 2), 15);
+        assert_eq!(
+            ret,
+            Err(LoanOfferError::NonPositiveBorrowedCapital(
+                Euros::from_cents(0)
+            ))
+        );
+    }
+
+    #[test]
+    fn creating_loan_offer_with_correct_inputs_works() {
+        let ret = LoanOffer::new(Euros::from_cents(10), Decimal::new(45, 2), 15);
+        assert_eq!(
+            ret,
+            Ok(LoanOffer {
+                borrowed_capital: Euros::from_cents(10),
+                annual_rate_in_percent: Decimal::new(45, 2),
+                loan_duration_in_months: 15
+            })
+        );
+    }
+
+    #[test]
+    fn creating_loan_offer_with_null_interest_rate_works() {
+        let ret = LoanOffer::new(Euros::from_cents(10), Decimal::ZERO, 15);
+        assert_eq!(
+            ret,
+            Ok(LoanOffer {
+                borrowed_capital: Euros::from_cents(10),
+                annual_rate_in_percent: Decimal::ZERO,
+                loan_duration_in_months: 15
+            })
+        );
+    }
+}