histbuf: Implement DoubleEndedIterator for OldestOrdered

Introduce new helper methods next_index() and prev_index() for
cleaner iteration over underlying slice of HistoryBuffer.
Use them to rewrite OldestOrdered Iterator implementation
with a bit more abstraction and extend the struct with
DoubleEndedIterator.

Signed-off-by: Witold Lipieta <witek103@gmail.com>

src/histbuf.rs

@@ -283,7 +283,8 @@ impl<T, const N: usize> HistoryBuffer<T, N> {
         }
     }
 
-    /// Returns an iterator for iterating over the buffer from oldest to newest.
+    /// Returns double ended iterator for iterating over the buffer from
+    /// the oldest to the newest and back.
     ///
     /// # Examples
     ///
@@ -298,19 +299,49 @@ impl<T, const N: usize> HistoryBuffer<T, N> {
     /// }
     /// ```
     pub fn oldest_ordered(&self) -> OldestOrdered<'_, T, N> {
-        if self.filled {
-            OldestOrdered {
-                buf: self,
-                cur: self.write_at,
-                wrapped: false,
-            }
+        OldestOrdered {
+            buf: self,
+            next: self.oldest_index(),
+            back: self.recent_index(),
+            crossed: false,
+        }
+    }
+
+    /// Returns index of the value written after given index in the underlying
+    /// slice. If the index of the most recent value is given this returns
+    /// index of the oldest value in the buffer.
+    fn next_index(&self, current: usize) -> Option<usize> {
+        let upper_bound = if self.filled {
+            self.capacity()
         } else {
-            // special case: act like we wrapped already to handle empty buffer.
-            OldestOrdered {
-                buf: self,
-                cur: 0,
-                wrapped: true,
-            }
+            self.write_at
+        };
+
+        if current >= upper_bound {
+            None
+        } else if current == upper_bound - 1 {
+            Some(0)
+        } else {
+            Some(current + 1)
+        }
+    }
+
+    /// Returns index of the value written before given index in the underlying
+    /// slice. If the index of the oldest value is given this returns index of
+    /// the most recent value in the buffer.
+    fn prev_index(&self, current: usize) -> Option<usize> {
+        let upper_bound = if self.filled {
+            self.capacity()
+        } else {
+            self.write_at
+        };
+
+        if current >= upper_bound {
+            None
+        } else if current == 0 {
+            Some(upper_bound - 1)
+        } else {
+            Some(current - 1)
         }
     }
 }
@@ -403,31 +434,51 @@ where
     }
 }
 
-/// An iterator on the underlying buffer ordered from oldest data to newest
+/// Double ended iterator on the underlying buffer ordered from the oldest data
+/// to the newest
 #[derive(Clone)]
 pub struct OldestOrdered<'a, T, const N: usize> {
     buf: &'a HistoryBuffer<T, N>,
-    cur: usize,
-    wrapped: bool,
+    next: Option<usize>,
+    back: Option<usize>,
+    crossed: bool,
 }
 
 impl<'a, T, const N: usize> Iterator for OldestOrdered<'a, T, N> {
     type Item = &'a T;
 
     fn next(&mut self) -> Option<&'a T> {
-        if self.cur == self.buf.len() && self.buf.filled {
-            // roll-over
-            self.cur = 0;
-            self.wrapped = true;
+        let cur = self.next?;
+
+        if self.crossed {
+            return None;
         }
 
-        if self.cur == self.buf.write_at && self.wrapped {
+        if self.next == self.back {
+            self.crossed = true;
+        }
+
+        self.next = self.buf.next_index(cur);
+
+        Some(&self.buf[cur])
+    }
+}
+
+impl<'a, T, const N: usize> DoubleEndedIterator for OldestOrdered<'a, T, N> {
+    fn next_back(&mut self) -> Option<Self::Item> {
+        let cur = self.back?;
+
+        if self.crossed {
             return None;
         }
 
-        let item = &self.buf[self.cur];
-        self.cur += 1;
-        Some(item)
+        if self.next == self.back {
+            self.crossed = true;
+        }
+
+        self.back = self.buf.prev_index(cur);
+
+        Some(&self.buf[cur])
     }
 }
 
@@ -603,18 +654,28 @@ mod tests {
         let mut iter = buffer.oldest_ordered();
         assert_eq!(iter.next(), None);
         assert_eq!(iter.next(), None);
+        assert_eq!(iter.next_back(), None);
+        assert_eq!(iter.next_back(), None);
 
         // test on a un-filled buffer
         let mut buffer: HistoryBuffer<u8, 6> = HistoryBuffer::new();
         buffer.extend([1, 2, 3]);
         assert_eq!(buffer.len(), 3);
         assert_eq_iter(buffer.oldest_ordered(), &[1, 2, 3]);
+        assert_eq_iter(buffer.oldest_ordered().rev(), &[3, 2, 1]);
+        let mut iter = buffer.oldest_ordered();
+        assert_eq!(iter.next(), Some(&1));
+        assert_eq!(iter.next_back(), Some(&3));
+        assert_eq!(iter.next_back(), Some(&2));
+        assert_eq!(iter.next_back(), None);
+        assert_eq!(iter.next(), None);
 
         // test on a filled buffer
         let mut buffer: HistoryBuffer<u8, 6> = HistoryBuffer::new();
         buffer.extend([0, 0, 0, 1, 2, 3, 4, 5, 6]);
         assert_eq!(buffer.len(), 6);
         assert_eq_iter(buffer.oldest_ordered(), &[1, 2, 3, 4, 5, 6]);
+        assert_eq_iter(buffer.oldest_ordered().rev(), &[6, 5, 4, 3, 2, 1]);
 
         // comprehensive test all cases
         for n in 0..50 {
@@ -625,6 +686,10 @@ mod tests {
                 buffer.oldest_ordered().copied(),
                 n.saturating_sub(N as u8)..n,
             );
+            assert_eq_iter(
+                buffer.oldest_ordered().rev().copied(),
+                (n.saturating_sub(N as u8)..n).rev(),
+            );
         }
     }