add history tests

src/history.rs

@@ -12,9 +12,30 @@ impl History {
     }
 
     pub fn look_back(&self, idx: usize) -> Option<&str> {
-        if idx > self.events.len() {
+        if idx == 0 || idx > self.events.len() {
             return None;
         }
         Some(&self.events[self.events.len() - idx])
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn new_history() {
+        let mut history = History::new();
+        assert_eq!(history.look_back(1), None);
+        assert_eq!(history.look_back(0), None);
+
+        history.add("Hello");
+        assert_eq!(history.look_back(1), Some("Hello"));
+        assert_eq!(history.look_back(0), None);
+
+        history.add("Hello again");
+        assert_eq!(history.look_back(2), Some("Hello"));
+        assert_eq!(history.look_back(1), Some("Hello again"));
+        assert_eq!(history.look_back(0), None);
+    }
+}