From d3019d16b992105d5948fae4a4fb17337ccac177 Mon Sep 17 00:00:00 2001
From: Yuki Okushi <huyuumi.dev@gmail.com>
Date: Mon, 5 Aug 2019 02:44:08 +0900
Subject: Implement nth_back for ChunksExactMut

---
 src/libcore/slice/mod.rs   | 16 ++++++++++++++++
 src/libcore/tests/slice.rs | 19 +++++++++++++++++++
 2 files changed, 35 insertions(+)

(limited to 'src/libcore')

diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs
index b06511cad97..a777400b97a 100644
--- a/src/libcore/slice/mod.rs
+++ b/src/libcore/slice/mod.rs
@@ -4612,6 +4612,22 @@ impl<'a, T> DoubleEndedIterator for ChunksExactMut<'a, T> {
             Some(tail)
         }
     }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        let len = self.len();
+        if n >= len {
+            self.v = &mut [];
+            None
+        } else {
+            let start = (len - 1 - n) * self.chunk_size;
+            let end = start + self.chunk_size;
+            let (temp, _tail) = mem::replace(&mut self.v, &mut []).split_at_mut(end);
+            let (head, nth_back) = temp.split_at_mut(start);
+            self.v = head;
+            Some(nth_back)
+        }
+    }
 }
 
 #[stable(feature = "chunks_exact", since = "1.31.0")]
diff --git a/src/libcore/tests/slice.rs b/src/libcore/tests/slice.rs
index b046c3ce5ce..342f9231f07 100644
--- a/src/libcore/tests/slice.rs
+++ b/src/libcore/tests/slice.rs
@@ -374,6 +374,25 @@ fn test_chunks_exact_mut_nth() {
     assert_eq!(c2.next(), None);
 }
 
+#[test]
+fn test_chunks_exact_mut_nth_back() {
+    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
+    let mut c = v.chunks_exact_mut(2);
+    assert_eq!(c.nth_back(1).unwrap(), &[2, 3]);
+    assert_eq!(c.next().unwrap(), &[0, 1]);
+    assert_eq!(c.next(), None);
+
+    let v2: &mut [i32] = &mut [0, 1, 2, 3, 4];
+    let mut c2 = v2.chunks_exact_mut(3);
+    assert_eq!(c2.nth_back(0).unwrap(), &[0, 1, 2]);
+    assert_eq!(c2.next(), None);
+    assert_eq!(c2.next_back(), None);
+
+    let v3: &mut [i32] = &mut [0, 1, 2, 3, 4];
+    let mut c3 = v3.chunks_exact_mut(10);
+    assert_eq!(c3.nth_back(0), None);
+}
+
 #[test]
 fn test_chunks_exact_mut_last() {
     let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
-- 
cgit 1.4.1-3-g733a5


From ec54340756f325324f4b710105a708da1cf26564 Mon Sep 17 00:00:00 2001
From: Tim Vermeulen <tvermeulen@me.com>
Date: Sun, 18 Aug 2019 21:47:23 +0200
Subject: Fix bug in iter::Chain::size_hint

---
 src/libcore/iter/adapters/chain.rs | 22 ++++++++++-------
 src/libcore/tests/iter.rs          | 48 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 62 insertions(+), 8 deletions(-)

(limited to 'src/libcore')

diff --git a/src/libcore/iter/adapters/chain.rs b/src/libcore/iter/adapters/chain.rs
index 0b9f7f6b609..c9612596b1b 100644
--- a/src/libcore/iter/adapters/chain.rs
+++ b/src/libcore/iter/adapters/chain.rs
@@ -173,17 +173,23 @@ impl<A, B> Iterator for Chain<A, B> where
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let (a_lower, a_upper) = self.a.size_hint();
-        let (b_lower, b_upper) = self.b.size_hint();
+        match self.state {
+            ChainState::Both => {
+                let (a_lower, a_upper) = self.a.size_hint();
+                let (b_lower, b_upper) = self.b.size_hint();
 
-        let lower = a_lower.saturating_add(b_lower);
+                let lower = a_lower.saturating_add(b_lower);
 
-        let upper = match (a_upper, b_upper) {
-            (Some(x), Some(y)) => x.checked_add(y),
-            _ => None
-        };
+                let upper = match (a_upper, b_upper) {
+                    (Some(x), Some(y)) => x.checked_add(y),
+                    _ => None
+                };
 
-        (lower, upper)
+                (lower, upper)
+            }
+            ChainState::Front => self.a.size_hint(),
+            ChainState::Back => self.b.size_hint(),
+        }
     }
 }
 
diff --git a/src/libcore/tests/iter.rs b/src/libcore/tests/iter.rs
index a1a27e1d538..3a4f76852a0 100644
--- a/src/libcore/tests/iter.rs
+++ b/src/libcore/tests/iter.rs
@@ -152,6 +152,54 @@ fn test_iterator_chain_find() {
     assert_eq!(iter.next(), None);
 }
 
+#[test]
+fn test_iterator_chain_size_hint() {
+    struct Iter {
+        is_empty: bool,
+    }
+
+    impl Iterator for Iter {
+        type Item = ();
+
+        // alternates between `None` and `Some(())`
+        fn next(&mut self) -> Option<Self::Item> {
+            if self.is_empty {
+                self.is_empty = false;
+                None
+            } else {
+                self.is_empty = true;
+                Some(())
+            }
+        }
+
+        fn size_hint(&self) -> (usize, Option<usize>) {
+            if self.is_empty {
+                (0, Some(0))
+            } else {
+                (1, Some(1))
+            }
+        }
+    }
+
+    impl DoubleEndedIterator for Iter {
+        fn next_back(&mut self) -> Option<Self::Item> {
+            self.next()
+        }
+    }
+
+    // this chains an iterator of length 0 with an iterator of length 1,
+    // so after calling `.next()` once, the iterator is empty and the
+    // state is `ChainState::Back`. `.size_hint()` should now disregard
+    // the size hint of the left iterator
+    let mut iter = Iter { is_empty: true }.chain(once(()));
+    assert_eq!(iter.next(), Some(()));
+    assert_eq!(iter.size_hint(), (0, Some(0)));
+
+    let mut iter = once(()).chain(Iter { is_empty: true });
+    assert_eq!(iter.next_back(), Some(()));
+    assert_eq!(iter.size_hint(), (0, Some(0)));
+}
+
 #[test]
 fn test_zip_nth() {
     let xs = [0, 1, 2, 4, 5];
-- 
cgit 1.4.1-3-g733a5