new get_highest_priority function:

- created fn
- created tests
- created small doc

src/orderbook.rs

@@ -410,6 +410,16 @@ where
         result
     }
 
+    /// Returns the `OrderID` of the next to be fulfilled order by side
+    pub fn get_highest_priority_order(&self, side: Side) -> Option<OrderID> {
+        let shared_order = match side {
+            Side::Buy => self.buy_side.get_highest_priority(),
+            Side::Sell => self.sell_side.get_highest_priority()
+        };
+
+        shared_order.map(|o| o.borrow().id)
+    }
+
     fn get_priority(&mut self) -> u64 {
         self.priority += 1;
         self.priority

tests/test.rs

@@ -537,6 +537,44 @@ fn general1() {
     );
 }
 
+#[test]
+fn get_highest_priority_order1() {
+    let mut ob = OrderBook::new();
+
+    assert_eq!(
+        ob.process_limit_order(1, Side::Buy, Decimal::from(20), Decimal::from(5))
+            .unwrap()
+            .len(),
+        0
+    );
+    assert_eq!(
+        ob.process_limit_order(2, Side::Buy, Decimal::from(15), Decimal::from(3))
+            .unwrap()
+            .len(),
+        0
+    );
+    assert_eq!(
+        ob.process_limit_order(3, Side::Sell, Decimal::from(75), Decimal::from(10))
+            .unwrap()
+            .len(),
+        0
+    );
+    assert_eq!(
+        ob.process_limit_order(4, Side::Sell, Decimal::from(50), Decimal::from(4))
+            .unwrap()
+            .len(),
+        0
+    );
+
+    assert_eq!(ob.get_highest_priority_order(Side::Buy), Some(1));
+    assert_eq!(ob.get_highest_priority_order(Side::Sell), Some(4));
+
+    _ = ob.cancel_order(3);
+    _ = ob.cancel_order(4);
+
+    assert_eq!(ob.get_highest_priority_order(Side::Sell), None);
+}
+
 #[test]
 fn print() {
     let mut ob = OrderBook::new();