From 9a19b6c8bdf50ba56f38c23985cc640775bdf9fe Mon Sep 17 00:00:00 2001
From: Gabriel Comte <gabriel.comte@gmail.com>
Date: Mon, 30 Mar 2026 09:44:35 +0200
Subject: [PATCH] Add mock-based unit tests for ExchangeRateProvider

Tests cover: correct inverse calculation, caching behavior (fetch_api
called only once), missing currency panic, zero rate producing infinity,
and multi-currency lookups. All run offline without hitting the API.
---
 src/fiat_rates/exchange_rate_provider.rs | 99 ++++++++++++++++++++++++
 1 file changed, 99 insertions(+)

diff --git a/src/fiat_rates/exchange_rate_provider.rs b/src/fiat_rates/exchange_rate_provider.rs
index e4dadda..cb236f0 100644
--- a/src/fiat_rates/exchange_rate_provider.rs
+++ b/src/fiat_rates/exchange_rate_provider.rs
@@ -23,3 +23,102 @@ impl<T: ExchangeRateApiConsumer> ExchangeRateProvider<T> {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use std::sync::atomic::{AtomicUsize, Ordering};
+
+    struct MockApiConsumer {
+        fetch_count: &'static AtomicUsize,
+    }
+
+    impl ExchangeRateApiConsumer for MockApiConsumer {
+        fn fetch_api(&self) -> HashMap<Fiat, f64> {
+            self.fetch_count.fetch_add(1, Ordering::SeqCst);
+            let mut rates = HashMap::new();
+            rates.insert(Fiat::USD, 50_000.0);
+            rates.insert(Fiat::EUR, 45_000.0);
+            rates.insert(Fiat::JPY, 7_500_000.0);
+            rates
+        }
+    }
+
+    fn mock_provider_with_data(rates: HashMap<Fiat, f64>) -> ExchangeRateProvider<MockApiConsumer> {
+        static UNUSED: AtomicUsize = AtomicUsize::new(0);
+        ExchangeRateProvider {
+            data_source: MockApiConsumer {
+                fetch_count: &UNUSED,
+            },
+            data: Some(rates),
+        }
+    }
+
+    fn mock_provider_with_fetch(
+        counter: &'static AtomicUsize,
+    ) -> ExchangeRateProvider<MockApiConsumer> {
+        ExchangeRateProvider {
+            data_source: MockApiConsumer {
+                fetch_count: counter,
+            },
+            data: None,
+        }
+    }
+
+    #[test]
+    fn btc_value_returns_inverse_of_rate() {
+        let mut rates = HashMap::new();
+        rates.insert(Fiat::USD, 50_000.0);
+        let mut provider = mock_provider_with_data(rates);
+
+        let btc_value = provider.btc_value(&Fiat::USD);
+        assert!((btc_value - 1.0 / 50_000.0).abs() < f64::EPSILON);
+    }
+
+    #[test]
+    fn data_is_cached_after_first_fetch() {
+        static COUNTER: AtomicUsize = AtomicUsize::new(0);
+        let mut provider = mock_provider_with_fetch(&COUNTER);
+
+        assert!(provider.data.is_none());
+        provider.btc_value(&Fiat::USD);
+        assert!(provider.data.is_some());
+        assert_eq!(COUNTER.load(Ordering::SeqCst), 1);
+
+        // Second call uses cached data — fetch_api not called again
+        provider.btc_value(&Fiat::EUR);
+        assert_eq!(COUNTER.load(Ordering::SeqCst), 1);
+    }
+
+    #[test]
+    #[should_panic(expected = "called `Option::unwrap()` on a `None` value")]
+    fn missing_currency_panics() {
+        let rates = HashMap::new();
+        let mut provider = mock_provider_with_data(rates);
+        provider.btc_value(&Fiat::USD);
+    }
+
+    #[test]
+    fn zero_rate_produces_infinity() {
+        let mut rates = HashMap::new();
+        rates.insert(Fiat::USD, 0.0);
+        let mut provider = mock_provider_with_data(rates);
+
+        let btc_value = provider.btc_value(&Fiat::USD);
+        assert!(btc_value.is_infinite());
+    }
+
+    #[test]
+    fn multiple_currencies_return_correct_values() {
+        let mut rates = HashMap::new();
+        rates.insert(Fiat::USD, 50_000.0);
+        rates.insert(Fiat::EUR, 45_000.0);
+        rates.insert(Fiat::JPY, 7_500_000.0);
+        let mut provider = mock_provider_with_data(rates);
+
+        assert!((provider.btc_value(&Fiat::USD) - 1.0 / 50_000.0).abs() < f64::EPSILON);
+        assert!((provider.btc_value(&Fiat::EUR) - 1.0 / 45_000.0).abs() < f64::EPSILON);
+        assert!((provider.btc_value(&Fiat::JPY) - 1.0 / 7_500_000.0).abs() < f64::EPSILON);
+    }
+}