small test on fee calculations (#91)

* small test on fee calculations

* cargo fmt and small fix for missing .env file

src/main.rs

@@ -52,8 +52,11 @@ async fn main() -> Result<()> {
 
 #[cfg(test)]
 mod tests {
+    use dotenvy::dotenv;
     use mostro_core::order::NewOrder;
     use mostro_core::Message;
+    use std::env::var;
+    use std::time::{SystemTime, UNIX_EPOCH};
 
     #[test]
     fn test_order_deserialize_serialize() {
@@ -78,4 +81,41 @@ mod tests {
         let message = Message::from_json(sample_message).unwrap();
         assert!(!message.verify());
     }
+
+    #[test]
+    fn test_fee_rounding() {
+        dotenv().ok();
+        let fee = var("FEE")
+            .unwrap_or(0.003.to_string())
+            .parse::<f64>()
+            .unwrap_or(0.003)
+            / 2.0;
+
+        let mut amt = SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .unwrap()
+            .subsec_micros() as i64;
+
+        // Test 1000 "random" amounts
+        for _i in 1..=1000 {
+            let fee_calculated = fee * amt as f64;
+            let rounded_fee = fee_calculated.round();
+            // Seller side
+            let seller_total_amt = rounded_fee as i64 + amt;
+            assert_eq!(amt, seller_total_amt - rounded_fee as i64);
+            // Buyer side
+
+            let buyer_total_amt = amt - rounded_fee as i64;
+            assert_eq!(amt, buyer_total_amt + rounded_fee as i64);
+
+            let nonce = SystemTime::now()
+                .duration_since(UNIX_EPOCH)
+                .unwrap()
+                .subsec_millis() as i64;
+
+            amt %= 100_000_i64;
+            amt *= (rounded_fee as i64) % 100_i64;
+            amt += nonce;
+        }
+    }
 }