changes

src/main.rs

@@ -817,19 +817,33 @@ impl PartialEq for Order {
         }
     }
 
-
+//
+// If Some(shadow) then peek should return a reference to shadow, and pop should replace shadow with v[next[0]]
+//
 struct OrderQueue {
   v: Vec<Order>,
+  shadowing: bool,
+  shadow: Option<Order>,
+  next: Vec<usize>,
   order_finder: HashMap<usize,usize>, // Maps OrderIDs to locations in v
   }
 
+
 impl OrderQueue {
     fn new()->Self {
         OrderQueue {
             v: Vec::new(),
+            shadowing: false,
+            shadow: None,
+            next: Vec::new(),
             order_finder:HashMap::new(),
             }
         }
+    fn start_shadow(&mut self) {
+        assert!(self.shadowing==false);
+        self.shadowing=true;
+        self.shadow=if self.v.len()>0 { Some(self.v[0].clone()) } else { None }
+        }
     fn peek(&mut self) -> &mut Order {
         self.v.first_mut().unwrap()
         }