Implement the prev button

src/main.rs

@@ -150,7 +150,8 @@ fn main() -> ! {
             tokens.next();
         }
         if down_button.update() == button::Event::PressUp {
-            todo!("To be implemented");
+            changed = true;
+            tokens.prev();
         }
         let curr_time = INTERVAL - (timestamp % INTERVAL);
         if curr_time == last_time {

src/storage.rs

@@ -59,7 +59,15 @@ impl<'a> Tokens<'a> {
     }
 
     pub fn prev(&mut self) -> Option<u16> {
-        todo!();
+        let mut index = self.current.unwrap();
+        for _ in 0..self.capacity {
+            index = (index - 1) % self.capacity;
+            if self.mem.read_byte(index * SECRET_KEY_FULL_LEN) != ENDL {
+                self.current = Some(index);
+                return Some(index);
+            }
+        }
+        None
     }
 
     pub fn read(&self, index: u16, name: &mut [u8], key: &mut [u8]) -> Result<(usize, usize), ()> {