doubly-linked-list: Test len in more places

I simply thought we could increase the thoroughness without being an
undue burden on readability or student completion.

This is limited to testing len in additional places in tests that were
already testing len or is_empty.
It hasn't added len in tests that didn't already have either of those
two.

Author: petertseng

exercises/doubly-linked-list/tests/doubly-linked-list.rs

@@ -30,6 +30,7 @@ fn basics_single_element_back() {
 
     assert_eq!(list.pop_back(), Some(5));
 
+    assert_eq!(list.len(), 0);
     assert!(list.is_empty());
 }
 
@@ -39,10 +40,11 @@ fn basics_push_pop_at_back() {
     let mut list: LinkedList<i32> = LinkedList::new();
     for i in 0..10 {
         list.push_back(i);
+        assert_eq!(list.len(), i as usize + 1);
         assert!(!list.is_empty());
     }
-    assert_eq!(list.len(), 10);
     for i in (0..10).rev() {
+        assert_eq!(list.len(), i as usize + 1);
         assert!(!list.is_empty());
         assert_eq!(i, list.pop_back().unwrap());
     }
@@ -62,6 +64,7 @@ fn basics_single_element_front() {
 
     assert_eq!(list.pop_front(), Some(5));
 
+    assert_eq!(list.len(), 0);
     assert!(list.is_empty());
 }
 
@@ -71,10 +74,11 @@ fn basics_push_pop_at_front() {
     let mut list: LinkedList<i32> = LinkedList::new();
     for i in 0..10 {
         list.push_front(i);
+        assert_eq!(list.len(), i as usize + 1);
         assert!(!list.is_empty());
     }
-    assert_eq!(list.len(), 10);
     for i in (0..10).rev() {
+        assert_eq!(list.len(), i as usize + 1);
         assert!(!list.is_empty());
         assert_eq!(i, list.pop_front().unwrap());
     }
@@ -89,12 +93,15 @@ fn basics_push_front_pop_back() {
     let mut list: LinkedList<i32> = LinkedList::new();
     for i in 0..10 {
         list.push_front(i);
+        assert_eq!(list.len(), i as usize + 1);
         assert!(!list.is_empty());
     }
     for i in 0..10 {
+        assert_eq!(list.len(), 10 - i as usize);
         assert!(!list.is_empty());
         assert_eq!(i, list.pop_back().unwrap());
     }
+    assert_eq!(list.len(), 0);
     assert!(list.is_empty());
 }
 
@@ -104,12 +111,15 @@ fn basics_push_back_pop_front() {
     let mut list: LinkedList<i32> = LinkedList::new();
     for i in 0..10 {
         list.push_back(i);
+        assert_eq!(list.len(), i as usize + 1);
         assert!(!list.is_empty());
     }
     for i in 0..10 {
+        assert_eq!(list.len(), 10 - i as usize);
         assert!(!list.is_empty());
         assert_eq!(i, list.pop_front().unwrap());
     }
+    assert_eq!(list.len(), 0);
     assert!(list.is_empty());
 }