BTreeMap::merge optimized

library/alloc/src/collections/btree/map.rs

@@ -1240,6 +1240,161 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
         )
     }
 
+    /// Moves all elements from `other` into `self`, leaving `other` empty.
+    ///
+    /// If a key from `other` is already present in `self`, then the `conflict`
+    /// closure is used to return a value to `self`. The `conflict`
+    /// closure takes in a borrow of `self`'s key, `self`'s value, and `other`'s value
+    /// in that order.
+    ///
+    /// An example of why one might use this method over [`append`]
+    /// is to combine `self`'s value with `other`'s value when their keys conflict.
+    ///
+    /// Similar to [`insert`], though, the key is not overwritten,
+    /// which matters for types that can be `==` without being identical.
+    ///
+    ///
+    /// [`insert`]: BTreeMap::insert
+    /// [`append`]: BTreeMap::append
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(btree_merge)]
+    /// use std::collections::BTreeMap;
+    ///
+    /// let mut a = BTreeMap::new();
+    /// a.insert(1, String::from("a"));
+    /// a.insert(2, String::from("b"));
+    /// a.insert(3, String::from("c")); // Note: Key (3) also present in b.
+    ///
+    /// let mut b = BTreeMap::new();
+    /// b.insert(3, String::from("d")); // Note: Key (3) also present in a.
+    /// b.insert(4, String::from("e"));
+    /// b.insert(5, String::from("f"));
+    ///
+    /// // concatenate a's value and b's value
+    /// a.merge(b, |_, a_val, b_val| {
+    ///     format!("{a_val}{b_val}")
+    /// });
+    ///
+    /// assert_eq!(a.len(), 5); // all of b's keys in a
+    ///
+    /// assert_eq!(a[&1], "a");
+    /// assert_eq!(a[&2], "b");
+    /// assert_eq!(a[&3], "cd"); // Note: "c" has been combined with "d".
+    /// assert_eq!(a[&4], "e");
+    /// assert_eq!(a[&5], "f");
+    /// ```
+    #[unstable(feature = "btree_merge", issue = "152152")]
+    pub fn merge(&mut self, mut other: Self, mut conflict: impl FnMut(&K, V, V) -> V)
+    where
+        K: Ord,
+        A: Clone,
+    {
+        // Do we have to append anything at all?
+        if other.is_empty() {
+            return;
+        }
+
+        // We can just swap `self` and `other` if `self` is empty.
+        if self.is_empty() {
+            mem::swap(self, &mut other);
+            return;
+        }
+
+        let mut other_iter = other.into_iter();
+        let (first_other_key, first_other_val) = other_iter.next().unwrap();
+
+        // find the first gap that has the smallest key greater than the first key from other
+        let mut self_cursor = self.lower_bound_mut(Bound::Included(&first_other_key));
+
+        if let Some((self_key, self_val)) = self_cursor.peek_next() {
+            match K::cmp(&first_other_key, self_key) {
+                Ordering::Equal => {
+                    // SAFETY: We read in self_val's and hand it over to our conflict function
+                    // which will always return a value that we can use to overwrite what's
+                    // in self_val
+                    unsafe {
+                        let val = ptr::read(self_val);
+                        let next_val = (conflict)(self_key, val, first_other_val);
+                        ptr::write(self_val, next_val);
+                    }
+                }
+                Ordering::Less =>
+                // SAFETY: we know our other_key's ordering is less than self_key,
+                // so inserting before will guarantee sorted order
+                unsafe {
+                    self_cursor.insert_before_unchecked(first_other_key, first_other_val);
+                },
+                Ordering::Greater => {
+                    unreachable!("cursor's peek_next would return None in this case");
+                }
+            }
+        } else {
+            // SAFETY: if we reach here, that means our cursor has reached
+            // the end of self BTreeMap, (other_key is greater than all the
+            // previous self BTreeMap keys) so we just insert other_key here
+            // at the end of the CursorMut
+            unsafe {
+                self_cursor.insert_after_unchecked(first_other_key, first_other_val);
+            }
+        }
+
+        for (other_key, other_val) in other_iter {
+            if self_cursor.peek_next().is_some() {
+                loop {
+                    let self_entry = self_cursor.peek_next();
+                    if let Some((self_key, self_val)) = self_entry {
+                        match K::cmp(&other_key, self_key) {
+                            Ordering::Equal => {
+                                // SAFETY: We read in self_val's and hand it over to our conflict function
+                                // which will always return a value that we can use to overwrite what's
+                                // in self_val
+                                unsafe {
+                                    let val = ptr::read(self_val);
+                                    let next_val = (conflict)(self_key, val, other_val);
+                                    ptr::write(self_val, next_val);
+                                }
+                                break;
+                            }
+                            Ordering::Less => {
+                                // SAFETY: we know our other_key's ordering is less than self_key,
+                                // so inserting before will guarantee sorted order
+                                unsafe {
+                                    self_cursor.insert_before_unchecked(other_key, other_val);
+                                }
+                                break;
+                            }
+                            Ordering::Greater => {
+                                self_cursor.next();
+                            }
+                        }
+                    } else {
+                        // SAFETY: if we reach here, that means our cursor has reached
+                        // the end of self BTreeMap, (other_key is greater than all the
+                        // previous self BTreeMap keys) so we just insert other_key here
+                        // at the end of the Cursor
+                        unsafe {
+                            self_cursor.insert_after_unchecked(other_key, other_val);
+                        }
+                        self_cursor.next();
+                        break;
+                    }
+                }
+            } else {
+                // SAFETY: if we reach here, that means our cursor has reached
+                // the end of self BTreeMap, (other_key is greater than all the
+                // previous self BTreeMap keys) so we just insert the rest of
+                // other_keys here at the end of CursorMut
+                unsafe {
+                    self_cursor.insert_after_unchecked(other_key, other_val);
+                }
+                self_cursor.next();
+            }
+        }
+    }
+
     /// Constructs a double-ended iterator over a sub-range of elements in the map.
     /// The simplest way is to use the range syntax `min..max`, thus `range(min..max)` will
     /// yield elements from min (inclusive) to max (exclusive).

library/alloc/src/collections/btree/map/tests.rs

@@ -1,9 +1,9 @@
 use core::assert_matches;
-use std::iter;
 use std::ops::Bound::{Excluded, Included, Unbounded};
 use std::panic::{AssertUnwindSafe, catch_unwind};
 use std::sync::atomic::AtomicUsize;
 use std::sync::atomic::Ordering::SeqCst;
+use std::{cmp, iter};
 
 use super::*;
 use crate::boxed::Box;
@@ -2128,6 +2128,76 @@ create_append_test!(test_append_239, 239);
 #[cfg(not(miri))] // Miri is too slow
 create_append_test!(test_append_1700, 1700);
 
+macro_rules! create_merge_test {
+    ($name:ident, $len:expr) => {
+        #[test]
+        fn $name() {
+            let mut a = BTreeMap::new();
+            for i in 0..8 {
+                a.insert(i, i);
+            }
+
+            let mut b = BTreeMap::new();
+            for i in 5..$len {
+                b.insert(i, 2 * i);
+            }
+
+            a.merge(b, |_, a_val, b_val| a_val + b_val);
+
+            assert_eq!(a.len(), cmp::max($len, 8));
+
+            for i in 0..cmp::max($len, 8) {
+                if i < 5 {
+                    assert_eq!(a[&i], i);
+                } else {
+                    if i < cmp::min($len, 8) {
+                        assert_eq!(a[&i], i + 2 * i);
+                    } else if i >= $len {
+                        assert_eq!(a[&i], i);
+                    } else {
+                        assert_eq!(a[&i], 2 * i);
+                    }
+                }
+            }
+
+            a.check();
+            assert_eq!(
+                a.remove(&($len - 1)),
+                if $len >= 5 && $len < 8 {
+                    Some(($len - 1) + 2 * ($len - 1))
+                } else {
+                    Some(2 * ($len - 1))
+                }
+            );
+            assert_eq!(a.insert($len - 1, 20), None);
+            a.check();
+        }
+    };
+}
+
+// These are mostly for testing the algorithm that "fixes" the right edge after insertion.
+// Single node, merge conflicting key values.
+create_merge_test!(test_merge_7, 7);
+// Single node.
+create_merge_test!(test_merge_9, 9);
+// Two leafs that don't need fixing.
+create_merge_test!(test_merge_17, 17);
+// Two leafs where the second one ends up underfull and needs stealing at the end.
+create_merge_test!(test_merge_14, 14);
+// Two leafs where the second one ends up empty because the insertion finished at the root.
+create_merge_test!(test_merge_12, 12);
+// Three levels; insertion finished at the root.
+create_merge_test!(test_merge_144, 144);
+// Three levels; insertion finished at leaf while there is an empty node on the second level.
+create_merge_test!(test_merge_145, 145);
+// Tests for several randomly chosen sizes.
+create_merge_test!(test_merge_170, 170);
+create_merge_test!(test_merge_181, 181);
+#[cfg(not(miri))] // Miri is too slow
+create_merge_test!(test_merge_239, 239);
+#[cfg(not(miri))] // Miri is too slow
+create_merge_test!(test_merge_1700, 1700);
+
 #[test]
 #[cfg_attr(not(panic = "unwind"), ignore = "test requires unwinding support")]
 fn test_append_drop_leak() {
@@ -2615,3 +2685,19 @@ fn test_id_based_append() {
 
     assert_eq!(lhs.pop_first().unwrap().0.name, "lhs_k".to_string());
 }
+
+#[test]
+fn test_id_based_merge() {
+    let mut lhs = BTreeMap::new();
+    let mut rhs = BTreeMap::new();
+
+    lhs.insert(IdBased { id: 0, name: "lhs_k".to_string() }, "1".to_string());
+    rhs.insert(IdBased { id: 0, name: "rhs_k".to_string() }, "2".to_string());
+
+    lhs.merge(rhs, |_, mut lhs_val, rhs_val| {
+        lhs_val.push_str(&rhs_val);
+        lhs_val
+    });
+
+    assert_eq!(lhs.pop_first().unwrap().0.name, "lhs_k".to_string());
+}

library/alloctests/tests/lib.rs

@@ -1,5 +1,6 @@
 #![feature(allocator_api)]
 #![feature(binary_heap_pop_if)]
+#![feature(btree_merge)]
 #![feature(const_heap)]
 #![feature(deque_extend_front)]
 #![feature(iter_array_chunks)]