Add DoubleEndedIterator implementation for Unique and UniqueBy

Author: futpib

src/unique_impl.rs

@@ -76,6 +76,22 @@ impl<I, V, F> Iterator for UniqueBy<I, V, F>
     }
 }
 
+impl<I, V, F> DoubleEndedIterator for UniqueBy<I, V, F>
+    where I: DoubleEndedIterator,
+          V: Eq + Hash,
+          F: FnMut(&I::Item) -> V
+{
+    fn next_back(&mut self) -> Option<I::Item> {
+        while let Some(v) = self.iter.next_back() {
+            let key = (self.f)(&v);
+            if self.used.insert(key, ()).is_none() {
+                return Some(v);
+            }
+        }
+        None
+    }
+}
+
 impl<I> Iterator for Unique<I>
     where I: Iterator,
           I::Item: Eq + Hash + Clone
@@ -104,6 +120,22 @@ impl<I> Iterator for Unique<I>
     }
 }
 
+impl<I> DoubleEndedIterator for Unique<I>
+    where I: DoubleEndedIterator,
+          I::Item: Eq + Hash + Clone
+{
+    fn next_back(&mut self) -> Option<I::Item> {
+        while let Some(v) = self.iter.iter.next_back() {
+            if let Entry::Vacant(entry) = self.iter.used.entry(v) {
+                let elt = entry.key().clone();
+                entry.insert(());
+                return Some(elt);
+            }
+        }
+        None
+    }
+}
+
 /// An iterator adapter to filter out duplicate elements.
 ///
 /// See [`.unique()`](../trait.Itertools.html#method.unique) for more information.

tests/test_std.rs

@@ -59,16 +59,26 @@ fn unique_by() {
     let xs = ["aaa", "bbbbb", "aa", "ccc", "bbbb", "aaaaa", "cccc"];
     let ys = ["aaa", "bbbbb", "ccc"];
     it::assert_equal(ys.iter(), xs.iter().unique_by(|x| x[..2].to_string()));
+    it::assert_equal(ys.iter(), xs.iter().rev().unique_by(|x| x[..2].to_string()).rev());
+    let ys_rev = ["cccc", "aaaaa", "bbbb"];
+    it::assert_equal(ys_rev.iter(), xs.iter().unique_by(|x| x[..2].to_string()).rev());
 }
 
 #[test]
 fn unique() {
     let xs = [0, 1, 2, 3, 2, 1, 3];
     let ys = [0, 1, 2, 3];
     it::assert_equal(ys.iter(), xs.iter().unique());
+    it::assert_equal(ys.iter(), xs.iter().rev().unique().rev());
+    let ys_rev = [3, 1, 2, 0];
+    it::assert_equal(ys_rev.iter(), xs.iter().unique().rev());
+
     let xs = [0, 1];
     let ys = [0, 1];
     it::assert_equal(ys.iter(), xs.iter().unique());
+    it::assert_equal(ys.iter(), xs.iter().rev().unique().rev());
+    let ys_rev = [1, 0];
+    it::assert_equal(ys_rev.iter(), xs.iter().unique().rev());
 }
 
 #[test]