iter: Implement .fold() for .chain()

Chain can do something interesting here where it passes on the fold
into its inner iterators.

The lets the underlying iterator's custom fold() be used, and skips the
regular chain logic in next.

2 files changed, 31 insertions, 0 deletions

diff --git a/src/libcore/iter/mod.rs b/src/libcore/iter/mod.rs
index 2c3b8864a11..df4f5e5c576 100644
--- a/src/libcore/iter/mod.rs
+++ b/src/libcore/iter/mod.rs
@@ -550,6 +550,25 @@ impl<A, B> Iterator for Chain<A, B> where
         }
     }
 
+    fn fold<Acc, F>(self, init: Acc, mut f: F) -> Acc
+        where F: FnMut(Acc, Self::Item) -> Acc,
+    {
+        let mut accum = init;
+        match self.state {
+            ChainState::Both | ChainState::Front => {
+                accum = self.a.fold(accum, &mut f);
+            }
+            _ => { }
+        }
+        match self.state {
+            ChainState::Both | ChainState::Back => {
+                accum = self.b.fold(accum, &mut f);
+            }
+            _ => { }
+        }
+        accum
+    }
+
     #[inline]
     fn nth(&mut self, mut n: usize) -> Option<A::Item> {
         match self.state {
diff --git a/src/libcoretest/iter.rs b/src/libcoretest/iter.rs
index 27eb25537f3..58b6444ef88 100644
--- a/src/libcoretest/iter.rs
+++ b/src/libcoretest/iter.rs
@@ -985,6 +985,18 @@ fn test_empty() {
     assert_eq!(it.next(), None);
 }
 
+#[test]
+fn test_chain_fold() {
+    let xs = [1, 2, 3];
+    let ys = [1, 2, 0];
+
+    let mut iter = xs.iter().chain(&ys);
+    iter.next();
+    let mut result = Vec::new();
+    iter.fold((), |(), &elt| result.push(elt));
+    assert_eq!(&[2, 3, 1, 2, 0], &result[..]);
+}
+
 #[bench]
 fn bench_rposition(b: &mut Bencher) {
     let it: Vec<usize> = (0..300).collect();