Implement split_off for BTreeMap and BTreeSet (RFC 509)

7 files changed, 538 insertions, 92 deletions

diff --git a/src/libcollections/btree/map.rs b/src/libcollections/btree/map.rs
index ec2f4a9f7f0..29f3e4b1b61 100644
--- a/src/libcollections/btree/map.rs
+++ b/src/libcollections/btree/map.rs
@@ -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> {
diff --git a/src/libcollections/btree/node.rs b/src/libcollections/btree/node.rs
index ca1cf6bcc50..e9bc29118d5 100644
--- a/src/libcollections/btree/node.rs
+++ b/src/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/src/libcollections/btree/set.rs b/src/libcollections/btree/set.rs
index 3ee42499a38..765595be317 100644
--- a/src/libcollections/btree/set.rs
+++ b/src/libcollections/btree/set.rs
@@ -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")]
diff --git a/src/libcollectionstest/btree/map.rs b/src/libcollectionstest/btree/map.rs
index e19090c7599..49fce68d15e 100644
--- a/src/libcollectionstest/btree/map.rs
+++ b/src/libcollectionstest/btree/map.rs
@@ -13,6 +13,9 @@ use std::collections::Bound::{self, Excluded, Included, Unbounded};
 use std::collections::btree_map::Entry::{Occupied, Vacant};
 use std::rc::Rc;
 
+use std::iter::FromIterator;
+use super::DeterministicRng;
+
 #[test]
 fn test_basic_large() {
     let mut map = BTreeMap::new();
@@ -528,6 +531,51 @@ create_append_test!(test_append_181, 181);
 create_append_test!(test_append_239, 239);
 create_append_test!(test_append_1700, 1700);
 
+fn rand_data(len: usize) -> Vec<(u32, u32)> {
+    let mut rng = DeterministicRng::new();
+    Vec::from_iter(
+        (0..len).map(|_| (rng.next(), rng.next()))
+    )
+}
+
+#[test]
+fn test_split_off_empty_right() {
+    let mut data = rand_data(173);
+
+    let mut map = BTreeMap::from_iter(data.clone());
+    let right = map.split_off(&(data.iter().max().unwrap().0 + 1));
+
+    data.sort();
+    assert!(map.into_iter().eq(data));
+    assert!(right.into_iter().eq(None));
+}
+
+#[test]
+fn test_split_off_empty_left() {
+    let mut data = rand_data(314);
+
+    let mut map = BTreeMap::from_iter(data.clone());
+    let right = map.split_off(&data.iter().min().unwrap().0);
+
+    data.sort();
+    assert!(map.into_iter().eq(None));
+    assert!(right.into_iter().eq(data));
+}
+
+#[test]
+fn test_split_off_large_random_sorted() {
+    let mut data = rand_data(1529);
+    // special case with maximum height.
+    data.sort();
+
+    let mut map = BTreeMap::from_iter(data.clone());
+    let key = data[data.len() / 2].0;
+    let right = map.split_off(&key);
+
+    assert!(map.into_iter().eq(data.clone().into_iter().filter(|x| x.0 < key)));
+    assert!(right.into_iter().eq(data.into_iter().filter(|x| x.0 >= key)));
+}
+
 mod bench {
     use std::collections::BTreeMap;
     use std::__rand::{Rng, thread_rng};
diff --git a/src/libcollectionstest/btree/mod.rs b/src/libcollectionstest/btree/mod.rs
index 0db48f3ce9e..ea43f423b7c 100644
--- a/src/libcollectionstest/btree/mod.rs
+++ b/src/libcollectionstest/btree/mod.rs
@@ -10,3 +10,33 @@
 
 mod map;
 mod set;
+
+/// XorShiftRng
+struct DeterministicRng {
+    x: u32,
+    y: u32,
+    z: u32,
+    w: u32,
+}
+
+impl DeterministicRng {
+    fn new() -> Self {
+        DeterministicRng {
+            x: 0x193a6754,
+            y: 0xa8a7d469,
+            z: 0x97830e05,
+            w: 0x113ba7bb
+        }
+    }
+
+    fn next(&mut self) -> u32 {
+        let x = self.x;
+        let t = x ^ (x << 11);
+        self.x = self.y;
+        self.y = self.z;
+        self.z = self.w;
+        let w_ = self.w;
+        self.w = w_ ^ (w_ >> 19) ^ (t ^ (t >> 8));
+        self.w
+    }
+}
diff --git a/src/libcollectionstest/btree/set.rs b/src/libcollectionstest/btree/set.rs
index 53ccfd5b4e2..f7b647d7772 100644
--- a/src/libcollectionstest/btree/set.rs
+++ b/src/libcollectionstest/btree/set.rs
@@ -10,6 +10,9 @@
 
 use std::collections::BTreeSet;
 
+use std::iter::FromIterator;
+use super::DeterministicRng;
+
 #[test]
 fn test_clone_eq() {
   let mut m = BTreeSet::new();
@@ -289,3 +292,48 @@ fn test_append() {
     assert_eq!(a.contains(&4), true);
     assert_eq!(a.contains(&5), true);
 }
+
+fn rand_data(len: usize) -> Vec<u32> {
+    let mut rng = DeterministicRng::new();
+    Vec::from_iter(
+        (0..len).map(|_| rng.next())
+    )
+}
+
+#[test]
+fn test_split_off_empty_right() {
+    let mut data = rand_data(173);
+
+    let mut set = BTreeSet::from_iter(data.clone());
+    let right = set.split_off(&(data.iter().max().unwrap() + 1));
+
+    data.sort();
+    assert!(set.into_iter().eq(data));
+    assert!(right.into_iter().eq(None));
+}
+
+#[test]
+fn test_split_off_empty_left() {
+    let mut data = rand_data(314);
+
+    let mut set = BTreeSet::from_iter(data.clone());
+    let right = set.split_off(data.iter().min().unwrap());
+
+    data.sort();
+    assert!(set.into_iter().eq(None));
+    assert!(right.into_iter().eq(data));
+}
+
+#[test]
+fn test_split_off_large_random_sorted() {
+    let mut data = rand_data(1529);
+    // special case with maximum height.
+    data.sort();
+
+    let mut set = BTreeSet::from_iter(data.clone());
+    let key = data[data.len() / 2];
+    let right = set.split_off(&key);
+
+    assert!(set.into_iter().eq(data.clone().into_iter().filter(|x| *x < key)));
+    assert!(right.into_iter().eq(data.into_iter().filter(|x| *x >= key)));
+}
diff --git a/src/libcollectionstest/lib.rs b/src/libcollectionstest/lib.rs
index bae21f1bd9b..840a83f048b 100644
--- a/src/libcollectionstest/lib.rs
+++ b/src/libcollectionstest/lib.rs
@@ -14,6 +14,7 @@
 #![feature(binary_heap_append)]
 #![feature(box_syntax)]
 #![feature(btree_append)]
+#![feature(btree_split_off)]
 #![feature(btree_range)]
 #![feature(collections)]
 #![feature(collections_bound)]