Merge branch 'libcore_nofp'HEADmaster

3 files changed, 430 insertions, 103 deletions

diff --git a/libcollections/btree/map.rs b/libcollections/btree/map.rs
index 20ef273..29f3e4b 100644
--- a/libcollections/btree/map.rs
+++ b/libcollections/btree/map.rs
@@ -286,7 +286,7 @@ pub struct Values<'a, K: 'a, V: 'a> {
 }
 
 /// A mutable iterator over a BTreeMap's values.
-#[unstable(feature = "map_values_mut", reason = "recently added", issue = "32551")]
+#[stable(feature = "map_values_mut", since = "1.10.0")]
 pub struct ValuesMut<'a, K: 'a, V: 'a> {
     inner: IterMut<'a, K, V>,
 }
@@ -842,13 +842,13 @@ impl<K: Ord, V> BTreeMap<K, V> {
             // Check if right-most child is underfull.
             let mut last_edge = internal.last_edge();
             let right_child_len = last_edge.reborrow().descend().len();
-            if right_child_len < node::CAPACITY / 2 {
+            if right_child_len < node::MIN_LEN {
                 // We need to steal.
                 let mut last_kv = match last_edge.left_kv() {
                     Ok(left) => left,
                     Err(_) => unreachable!(),
                 };
-                last_kv.bulk_steal_left(node::CAPACITY/2 - right_child_len);
+                last_kv.bulk_steal_left(node::MIN_LEN - right_child_len);
                 last_edge = last_kv.right_edge();
             }
 
@@ -856,6 +856,174 @@ impl<K: Ord, V> BTreeMap<K, V> {
             cur_node = last_edge.descend();
         }
     }
+
+    /// Splits the collection into two at the given key. Returns everything after the given key,
+    /// including the key.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(btree_split_off)]
+    /// use std::collections::BTreeMap;
+    ///
+    /// let mut a = BTreeMap::new();
+    /// a.insert(1, "a");
+    /// a.insert(2, "b");
+    /// a.insert(3, "c");
+    /// a.insert(17, "d");
+    /// a.insert(41, "e");
+    ///
+    /// let b = a.split_off(&3);
+    ///
+    /// assert_eq!(a.len(), 2);
+    /// assert_eq!(b.len(), 3);
+    ///
+    /// assert_eq!(a[&1], "a");
+    /// assert_eq!(a[&2], "b");
+    ///
+    /// assert_eq!(b[&3], "c");
+    /// assert_eq!(b[&17], "d");
+    /// assert_eq!(b[&41], "e");
+    /// ```
+    #[unstable(feature = "btree_split_off",
+               reason = "recently added as part of collections reform 2",
+               issue = "19986")]
+    pub fn split_off<Q: ?Sized + Ord>(&mut self, key: &Q) -> Self where K: Borrow<Q> {
+        if self.is_empty() {
+            return Self::new();
+        }
+
+        let total_num = self.len();
+
+        let mut right = Self::new();
+        for _ in 0..(self.root.as_ref().height()) {
+            right.root.push_level();
+        }
+
+        {
+            let mut left_node = self.root.as_mut();
+            let mut right_node = right.root.as_mut();
+
+            loop {
+                let mut split_edge = match search::search_node(left_node, key) {
+                    // key is going to the right tree
+                    Found(handle) => handle.left_edge(),
+                    GoDown(handle) => handle
+                };
+
+                split_edge.move_suffix(&mut right_node);
+
+                match (split_edge.force(), right_node.force()) {
+                    (Internal(edge), Internal(node)) => {
+                        left_node = edge.descend();
+                        right_node = node.first_edge().descend();
+                    }
+                    (Leaf(_), Leaf(_)) => { break; },
+                    _ => { unreachable!(); }
+                }
+            }
+        }
+
+        self.fix_right_border();
+        right.fix_left_border();
+
+        if self.root.as_ref().height() < right.root.as_ref().height() {
+            self.recalc_length();
+            right.length = total_num - self.len();
+        } else {
+            right.recalc_length();
+            self.length = total_num - right.len();
+        }
+
+        right
+    }
+
+    /// Calculates the number of elements if it is incorrect.
+    fn recalc_length(&mut self) {
+        fn dfs<K, V>(node: NodeRef<marker::Immut, K, V, marker::LeafOrInternal>) -> usize {
+            let mut res = node.len();
+
+            if let Internal(node) = node.force() {
+                let mut edge = node.first_edge();
+                loop {
+                    res += dfs(edge.reborrow().descend());
+                    match edge.right_kv() {
+                        Ok(right_kv) => { edge = right_kv.right_edge(); },
+                        Err(_) => { break; }
+                    }
+                }
+            }
+
+            res
+        }
+
+        self.length = dfs(self.root.as_ref());
+    }
+
+    /// Removes empty levels on the top.
+    fn fix_top(&mut self) {
+        loop {
+            {
+                let node = self.root.as_ref();
+                if node.height() == 0 || node.len() > 0 {
+                    break;
+                }
+            }
+            self.root.pop_level();
+        }
+    }
+
+    fn fix_right_border(&mut self) {
+        self.fix_top();
+
+        {
+            let mut cur_node = self.root.as_mut();
+
+            while let Internal(node) = cur_node.force() {
+                let mut last_kv = node.last_kv();
+
+                if last_kv.can_merge() {
+                    cur_node = last_kv.merge().descend();
+                } else {
+                    let right_len = last_kv.reborrow().right_edge().descend().len();
+                    // `MINLEN + 1` to avoid readjust if merge happens on the next level.
+                    if right_len < node::MIN_LEN + 1 {
+                        last_kv.bulk_steal_left(node::MIN_LEN + 1 - right_len);
+                    }
+                    cur_node = last_kv.right_edge().descend();
+                }
+            }
+        }
+
+        self.fix_top();
+    }
+
+    /// The symmetric clone of `fix_right_border`.
+    fn fix_left_border(&mut self) {
+        self.fix_top();
+
+        {
+            let mut cur_node = self.root.as_mut();
+
+            while let Internal(node) = cur_node.force() {
+                let mut first_kv = node.first_kv();
+
+                if first_kv.can_merge() {
+                    cur_node = first_kv.merge().descend();
+                } else {
+                    let left_len = first_kv.reborrow().left_edge().descend().len();
+                    if left_len < node::MIN_LEN + 1 {
+                        first_kv.bulk_steal_right(node::MIN_LEN + 1 - left_len);
+                    }
+                    cur_node = first_kv.left_edge().descend();
+                }
+            }
+        }
+
+        self.fix_top();
+    }
 }
 
 impl<'a, K: 'a, V: 'a> IntoIterator for &'a BTreeMap<K, V> {
@@ -1144,7 +1312,7 @@ impl<'a, K, V> Iterator for Range<'a, K, V> {
     }
 }
 
-#[unstable(feature = "map_values_mut", reason = "recently added", issue = "32551")]
+#[stable(feature = "map_values_mut", since = "1.10.0")]
 impl<'a, K, V> Iterator for ValuesMut<'a, K, V> {
     type Item = &'a mut V;
 
@@ -1157,14 +1325,14 @@ impl<'a, K, V> Iterator for ValuesMut<'a, K, V> {
     }
 }
 
-#[unstable(feature = "map_values_mut", reason = "recently added", issue = "32551")]
+#[stable(feature = "map_values_mut", since = "1.10.0")]
 impl<'a, K, V> DoubleEndedIterator for ValuesMut<'a, K, V> {
     fn next_back(&mut self) -> Option<&'a mut V> {
         self.inner.next_back().map(|(_, v)| v)
     }
 }
 
-#[unstable(feature = "map_values_mut", reason = "recently added", issue = "32551")]
+#[stable(feature = "map_values_mut", since = "1.10.0")]
 impl<'a, K, V> ExactSizeIterator for ValuesMut<'a, K, V> {
     fn len(&self) -> usize {
         self.inner.len()
@@ -1575,7 +1743,6 @@ impl<K, V> BTreeMap<K, V> {
     /// Basic usage:
     ///
     /// ```
-    /// # #![feature(map_values_mut)]
     /// use std::collections::BTreeMap;
     ///
     /// let mut a = BTreeMap::new();
@@ -1590,8 +1757,8 @@ impl<K, V> BTreeMap<K, V> {
     /// assert_eq!(values, [String::from("hello!"),
     ///                     String::from("goodbye!")]);
     /// ```
-    #[unstable(feature = "map_values_mut", reason = "recently added", issue = "32551")]
-    pub fn values_mut<'a>(&'a mut self) -> ValuesMut<'a, K, V> {
+    #[stable(feature = "map_values_mut", since = "1.10.0")]
+    pub fn values_mut(&mut self) -> ValuesMut<K, V> {
         ValuesMut { inner: self.iter_mut() }
     }
 
@@ -1654,12 +1821,21 @@ impl<'a, K: Ord, V> Entry<'a, K, V> {
             Vacant(entry) => entry.insert(default()),
         }
     }
+
+    /// Returns a reference to this entry's key.
+    #[stable(feature = "map_entry_keys", since = "1.10.0")]
+    pub fn key(&self) -> &K {
+        match *self {
+            Occupied(ref entry) => entry.key(),
+            Vacant(ref entry) => entry.key(),
+        }
+    }
 }
 
 impl<'a, K: Ord, V> VacantEntry<'a, K, V> {
     /// Gets a reference to the key that would be used when inserting a value
     /// through the VacantEntry.
-    #[unstable(feature = "map_entry_keys", issue = "32281")]
+    #[stable(feature = "map_entry_keys", since = "1.10.0")]
     pub fn key(&self) -> &K {
         &self.key
     }
@@ -1709,7 +1885,7 @@ impl<'a, K: Ord, V> VacantEntry<'a, K, V> {
 
 impl<'a, K: Ord, V> OccupiedEntry<'a, K, V> {
     /// Gets a reference to the key in the entry.
-    #[unstable(feature = "map_entry_keys", issue = "32281")]
+    #[stable(feature = "map_entry_keys", since = "1.10.0")]
     pub fn key(&self) -> &K {
         self.handle.reborrow().into_kv().0
     }
diff --git a/libcollections/btree/node.rs b/libcollections/btree/node.rs
index ca1cf6b..e9bc291 100644
--- a/libcollections/btree/node.rs
+++ b/libcollections/btree/node.rs
@@ -51,6 +51,7 @@ use core::slice;
 use boxed::Box;
 
 const B: usize = 6;
+pub const MIN_LEN: usize = B - 1;
 pub const CAPACITY: usize = 2 * B - 1;
 
 /// The underlying representation of leaf nodes. Note that it is often unsafe to actually store
@@ -413,6 +414,19 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
         let len = self.len();
         Handle::new_edge(self, len)
     }
+
+    /// Note that `self` must be nonempty.
+    pub fn first_kv(self) -> Handle<Self, marker::KV> {
+        debug_assert!(self.len() > 0);
+        Handle::new_kv(self, 0)
+    }
+
+    /// Note that `self` must be nonempty.
+    pub fn last_kv(self) -> Handle<Self, marker::KV> {
+        let len = self.len();
+        debug_assert!(len > 0);
+        Handle::new_kv(self, len - 1)
+    }
 }
 
 impl<K, V> NodeRef<marker::Owned, K, V, marker::Leaf> {
@@ -602,6 +616,17 @@ impl<'a, K, V> NodeRef<marker::Mut<'a>, K, V, marker::Internal> {
         }
     }
 
+    fn correct_childrens_parent_links(&mut self, first: usize, after_last: usize) {
+        for i in first..after_last {
+            Handle::new_edge(unsafe { self.reborrow_mut() }, i).correct_parent_link();
+        }
+    }
+
+    fn correct_all_childrens_parent_links(&mut self) {
+        let len = self.len();
+        self.correct_childrens_parent_links(0, len + 1);
+    }
+
     /// Adds a key/value pair and an edge to go to the left of that pair to
     /// the beginning of the node.
     pub fn push_front(&mut self, key: K, val: V, edge: Root<K, V>) {
@@ -623,11 +648,8 @@ impl<'a, K, V> NodeRef<marker::Mut<'a>, K, V, marker::Internal> {
 
             self.as_leaf_mut().len += 1;
 
-            for i in 0..self.len()+1 {
-                Handle::new_edge(self.reborrow_mut(), i).correct_parent_link();
-            }
+            self.correct_all_childrens_parent_links();
         }
-
     }
 }
 
@@ -696,6 +718,13 @@ impl<'a, K, V> NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal> {
             (key, val, edge)
         }
     }
+
+    fn into_kv_pointers_mut(mut self) -> (*mut K, *mut V) {
+        (
+            self.keys_mut().as_mut_ptr(),
+            self.vals_mut().as_mut_ptr()
+        )
+    }
 }
 
 impl<BorrowType, K, V> NodeRef<BorrowType, K, V, marker::LeafOrInternal> {
@@ -1275,105 +1304,155 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::
     }
 
     /// This does stealing similar to `steal_left` but steals multiple elements at once.
-    pub fn bulk_steal_left(&mut self, n: usize) {
+    pub fn bulk_steal_left(&mut self, count: usize) {
         unsafe {
-            // Get raw pointers to left child's keys, values and edges.
-            let (left_len, left_k, left_v, left_e) = {
-                let mut left = self.reborrow_mut().left_edge().descend();
-
-                (left.len(),
-                 left.keys_mut().as_mut_ptr(),
-                 left.vals_mut().as_mut_ptr(),
-                 match left.force() {
-                     ForceResult::Leaf(_) => None,
-                     ForceResult::Internal(mut i) => Some(i.as_internal_mut().edges.as_mut_ptr()),
-                 })
-            };
-
-            // Get raw pointers to right child's keys, values and edges.
-            let (right_len, right_k, right_v, right_e) = {
-                let mut right = self.reborrow_mut().right_edge().descend();
-
-                (right.len(),
-                 right.keys_mut().as_mut_ptr(),
-                 right.vals_mut().as_mut_ptr(),
-                 match right.force() {
-                     ForceResult::Leaf(_) => None,
-                     ForceResult::Internal(mut i) => Some(i.as_internal_mut().edges.as_mut_ptr()),
-                 })
-            };
-
-            // Get raw pointers to parent's key and value.
-            let (parent_k, parent_v) = {
-                let kv = self.reborrow_mut().into_kv_mut();
-                (kv.0 as *mut K, kv.1 as *mut V)
-            };
+            let mut left_node = ptr::read(self).left_edge().descend();
+            let left_len = left_node.len();
+            let mut right_node = ptr::read(self).right_edge().descend();
+            let right_len = right_node.len();
 
             // Make sure that we may steal safely.
-            debug_assert!(right_len + n <= CAPACITY);
-            debug_assert!(left_len >= n);
-
-            // Make room for stolen elements in right child.
-            ptr::copy(right_k,
-                      right_k.offset(n as isize),
-                      right_len);
-            ptr::copy(right_v,
-                      right_v.offset(n as isize),
-                      right_len);
-            if let Some(edges) = right_e {
-                ptr::copy(edges,
-                          edges.offset(n as isize),
-                          right_len+1);
+            debug_assert!(right_len + count <= CAPACITY);
+            debug_assert!(left_len >= count);
+
+            let new_left_len = left_len - count;
+
+            // Move data.
+            {
+                let left_kv = left_node.reborrow_mut().into_kv_pointers_mut();
+                let right_kv = right_node.reborrow_mut().into_kv_pointers_mut();
+                let parent_kv = {
+                    let kv = self.reborrow_mut().into_kv_mut();
+                    (kv.0 as *mut K, kv.1 as *mut V)
+                };
+
+                // Make room for stolen elements in the right child.
+                ptr::copy(right_kv.0,
+                          right_kv.0.offset(count as isize),
+                          right_len);
+                ptr::copy(right_kv.1,
+                          right_kv.1.offset(count as isize),
+                          right_len);
+
+                // Move elements from the left child to the right one.
+                move_kv(left_kv, new_left_len + 1, right_kv, 0, count - 1);
+
+                // Move parent's key/value pair to the right child.
+                move_kv(parent_kv, 0, right_kv, count - 1, 1);
+
+                // Move the left-most stolen pair to the parent.
+                move_kv(left_kv, new_left_len, parent_kv, 0, 1);
             }
 
-            // Move elements from the left child to the right one.
-            let left_ind = (left_len - n) as isize;
-            ptr::copy_nonoverlapping(left_k.offset(left_ind + 1),
-                                     right_k,
-                                     n - 1);
-            ptr::copy_nonoverlapping(left_v.offset(left_ind + 1),
-                                     right_v,
-                                     n - 1);
-            match (left_e, right_e) {
-                (Some(left), Some(right)) => {
-                    ptr::copy_nonoverlapping(left.offset(left_ind + 1),
-                                             right,
-                                             n);
+            left_node.reborrow_mut().as_leaf_mut().len -= count as u16;
+            right_node.reborrow_mut().as_leaf_mut().len += count as u16;
+
+            match (left_node.force(), right_node.force()) {
+                (ForceResult::Internal(left), ForceResult::Internal(mut right)) => {
+                    // Make room for stolen edges.
+                    let right_edges = right.reborrow_mut().as_internal_mut().edges.as_mut_ptr();
+                    ptr::copy(right_edges,
+                              right_edges.offset(count as isize),
+                              right_len + 1);
+                    right.correct_childrens_parent_links(count, count + right_len + 1);
+
+                    move_edges(left, new_left_len + 1, right, 0, count);
                 },
-                (Some(_), None) => unreachable!(),
-                (None, Some(_)) => unreachable!(),
-                (None, None) => {},
+                (ForceResult::Leaf(_), ForceResult::Leaf(_)) => { }
+                _ => { unreachable!(); }
             }
+        }
+    }
 
-            // Copy parent key/value pair to right child.
-            ptr::copy_nonoverlapping(parent_k,
-                                     right_k.offset(n as isize - 1),
-                                     1);
-            ptr::copy_nonoverlapping(parent_v,
-                                     right_v.offset(n as isize - 1),
-                                     1);
-            // Copy left-most stolen pair to parent.
-            ptr::copy_nonoverlapping(left_k.offset(left_ind),
-                                     parent_k,
-                                     1);
-            ptr::copy_nonoverlapping(left_v.offset(left_ind),
-                                     parent_v,
-                                     1);
-
-            // Fix lengths of left and right child and parent pointers in children of the right
-            // child.
-            self.reborrow_mut().left_edge().descend().as_leaf_mut().len -= n as u16;
-            let mut right = self.reborrow_mut().right_edge().descend();
-            right.as_leaf_mut().len += n as u16;
-            if let ForceResult::Internal(mut node) = right.force() {
-                for i in 0..(right_len+n+1) {
-                    Handle::new_edge(node.reborrow_mut(), i as usize).correct_parent_link();
-                }
+    /// The symmetric clone of `bulk_steal_left`.
+    pub fn bulk_steal_right(&mut self, count: usize) {
+        unsafe {
+            let mut left_node = ptr::read(self).left_edge().descend();
+            let left_len = left_node.len();
+            let mut right_node = ptr::read(self).right_edge().descend();
+            let right_len = right_node.len();
+
+            // Make sure that we may steal safely.
+            debug_assert!(left_len + count <= CAPACITY);
+            debug_assert!(right_len >= count);
+
+            let new_right_len = right_len - count;
+
+            // Move data.
+            {
+                let left_kv = left_node.reborrow_mut().into_kv_pointers_mut();
+                let right_kv = right_node.reborrow_mut().into_kv_pointers_mut();
+                let parent_kv = {
+                    let kv = self.reborrow_mut().into_kv_mut();
+                    (kv.0 as *mut K, kv.1 as *mut V)
+                };
+
+                // Move parent's key/value pair to the left child.
+                move_kv(parent_kv, 0, left_kv, left_len, 1);
+
+                // Move elements from the right child to the left one.
+                move_kv(right_kv, 0, left_kv, left_len + 1, count - 1);
+
+                // Move the right-most stolen pair to the parent.
+                move_kv(right_kv, count - 1, parent_kv, 0, 1);
+
+                // Fix right indexing
+                ptr::copy(right_kv.0.offset(count as isize),
+                          right_kv.0,
+                          new_right_len);
+                ptr::copy(right_kv.1.offset(count as isize),
+                          right_kv.1,
+                          new_right_len);
+            }
+
+            left_node.reborrow_mut().as_leaf_mut().len += count as u16;
+            right_node.reborrow_mut().as_leaf_mut().len -= count as u16;
+
+            match (left_node.force(), right_node.force()) {
+                (ForceResult::Internal(left), ForceResult::Internal(mut right)) => {
+                    move_edges(right.reborrow_mut(), 0, left, left_len + 1, count);
+
+                    // Fix right indexing.
+                    let right_edges = right.reborrow_mut().as_internal_mut().edges.as_mut_ptr();
+                    ptr::copy(right_edges.offset(count as isize),
+                              right_edges,
+                              new_right_len + 1);
+                    right.correct_childrens_parent_links(0, new_right_len + 1);
+                },
+                (ForceResult::Leaf(_), ForceResult::Leaf(_)) => { }
+                _ => { unreachable!(); }
             }
         }
     }
 }
 
+unsafe fn move_kv<K, V>(
+    source: (*mut K, *mut V), source_offset: usize,
+    dest: (*mut K, *mut V), dest_offset: usize,
+    count: usize)
+{
+    ptr::copy_nonoverlapping(source.0.offset(source_offset as isize),
+                             dest.0.offset(dest_offset as isize),
+                             count);
+    ptr::copy_nonoverlapping(source.1.offset(source_offset as isize),
+                             dest.1.offset(dest_offset as isize),
+                             count);
+}
+
+// Source and destination must have the same height.
+unsafe fn move_edges<K, V>(
+    mut source: NodeRef<marker::Mut, K, V, marker::Internal>, source_offset: usize,
+    mut dest: NodeRef<marker::Mut, K, V, marker::Internal>, dest_offset: usize,
+    count: usize)
+{
+    let source_ptr = source.as_internal_mut().edges.as_mut_ptr();
+    let dest_ptr = dest.as_internal_mut().edges.as_mut_ptr();
+    ptr::copy_nonoverlapping(source_ptr.offset(source_offset as isize),
+                             dest_ptr.offset(dest_offset as isize),
+                             count);
+    dest.correct_childrens_parent_links(dest_offset, dest_offset + count);
+}
+
 impl<BorrowType, K, V, HandleType>
         Handle<NodeRef<BorrowType, K, V, marker::LeafOrInternal>, HandleType> {
 
@@ -1397,6 +1476,41 @@ impl<BorrowType, K, V, HandleType>
     }
 }
 
+impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal>, marker::Edge> {
+    /// Move the suffix after `self` from one node to another one. `right` must be empty.
+    /// The first edge of `right` remains unchanged.
+    pub fn move_suffix(&mut self,
+            right: &mut NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal>) {
+        unsafe {
+            let left_new_len = self.idx;
+            let mut left_node = self.reborrow_mut().into_node();
+
+            let right_new_len = left_node.len() - left_new_len;
+            let mut right_node = right.reborrow_mut();
+
+            debug_assert!(right_node.len() == 0);
+            debug_assert!(left_node.height == right_node.height);
+
+            let left_kv = left_node.reborrow_mut().into_kv_pointers_mut();
+            let right_kv = right_node.reborrow_mut().into_kv_pointers_mut();
+
+
+            move_kv(left_kv, left_new_len, right_kv, 0, right_new_len);
+
+            left_node.reborrow_mut().as_leaf_mut().len = left_new_len as u16;
+            right_node.reborrow_mut().as_leaf_mut().len = right_new_len as u16;
+
+            match (left_node.force(), right_node.force()) {
+                (ForceResult::Internal(left), ForceResult::Internal(right)) => {
+                    move_edges(left, left_new_len + 1, right, 1, right_new_len);
+                },
+                (ForceResult::Leaf(_), ForceResult::Leaf(_)) => { }
+                _ => { unreachable!(); }
+            }
+        }
+    }
+}
+
 pub enum ForceResult<Leaf, Internal> {
     Leaf(Leaf),
     Internal(Internal)
diff --git a/libcollections/btree/set.rs b/libcollections/btree/set.rs
index 5419d7a..765595b 100644
--- a/libcollections/btree/set.rs
+++ b/libcollections/btree/set.rs
@@ -477,9 +477,9 @@ impl<T: Ord> BTreeSet<T> {
 
     /// Adds a value to the set.
     ///
-    /// If the set did not have a value present, `true` is returned.
+    /// If the set did not have this value present, `true` is returned.
     ///
-    /// If the set did have this key present, `false` is returned, and the
+    /// If the set did have this value present, `false` is returned, and the
     /// entry is not updated. See the [module-level documentation] for more.
     ///
     /// [module-level documentation]: index.html#insert-and-complex-keys
@@ -580,6 +580,43 @@ impl<T: Ord> BTreeSet<T> {
     pub fn append(&mut self, other: &mut Self) {
         self.map.append(&mut other.map);
     }
+
+    /// Splits the collection into two at the given key. Returns everything after the given key,
+    /// including the key.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(btree_split_off)]
+    /// use std::collections::BTreeMap;
+    ///
+    /// let mut a = BTreeMap::new();
+    /// a.insert(1, "a");
+    /// a.insert(2, "b");
+    /// a.insert(3, "c");
+    /// a.insert(17, "d");
+    /// a.insert(41, "e");
+    ///
+    /// let b = a.split_off(&3);
+    ///
+    /// assert_eq!(a.len(), 2);
+    /// assert_eq!(b.len(), 3);
+    ///
+    /// assert_eq!(a[&1], "a");
+    /// assert_eq!(a[&2], "b");
+    ///
+    /// assert_eq!(b[&3], "c");
+    /// assert_eq!(b[&17], "d");
+    /// assert_eq!(b[&41], "e");
+    /// ```
+    #[unstable(feature = "btree_split_off",
+               reason = "recently added as part of collections reform 2",
+               issue = "19986")]
+    pub fn split_off<Q: ?Sized + Ord>(&mut self, key: &Q) -> Self where T: Borrow<Q> {
+        BTreeSet { map: self.map.split_off(key) }
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]