feat: Add retain_with_break to HashSet/Table/Map

With the removal of the raw table, it is hard to implement an efficient
loop to conditionally remove/keep certain fields up to a limit. i.e. a
loop that can be aborted and does not require rehash the key for removal
of the entry.

Author: tugtugtug

src/map.rs

@@ -929,6 +929,53 @@ impl<K, V, S, A: Allocator> HashMap<K, V, S, A> {
         }
     }
 
+    /// Retains only the elements specified by the predicate and breaks the iteration when
+    /// the predicate fails. Keeps the allocated memory for reuse.
+    ///
+    /// In other words, remove all pairs `(k, v)` such that `f(&k, &mut v)` returns `Some(false)` until
+    /// `f(&k, &mut v)` returns `None`
+    /// The elements are visited in unsorted (and unspecified) order.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use hashbrown::HashMap;
+    ///
+    /// let mut map: HashMap<i32, i32> = (0..8).map(|x|(x, x*10)).collect();
+    /// assert_eq!(map.len(), 8);
+    /// let mut removed = 0;
+    /// map.retain_with_break(|&k, _| if removed < 3 {
+    ///     if k % 2 == 0 {
+    ///         Some(true)
+    ///     } else {
+    ///         removed += 1;
+    ///         Some(false)
+    ///     }
+    /// } else {
+    ///     None
+    /// });
+    ///
+    /// // We can see, that the number of elements inside map is changed and the
+    /// // length matches when we have aborted the retain with the return of `None`
+    /// assert_eq!(map.len(), 5);
+    /// ```
+    pub fn retain_with_break<F>(&mut self, mut f: F)
+    where
+        F: FnMut(&K, &mut V) -> Option<bool>,
+    {
+        // Here we only use `iter` as a temporary, preventing use-after-free
+        unsafe {
+            for item in self.table.iter() {
+                let &mut (ref key, ref mut value) = item.as_mut();
+                match f(key, value) {
+                    Some(false) => self.table.erase(item),
+                    Some(true) => continue,
+                    None => break,
+                }
+            }
+        }
+    }
+
     /// Drains elements which are true under the given predicate,
     /// and returns an iterator over the removed items.
     ///
@@ -5909,6 +5956,30 @@ mod test_map {
         assert_eq!(map[&6], 60);
     }
 
+    #[test]
+    fn test_retain_with_break() {
+        let mut map: HashMap<i32, i32> = (0..100).map(|x| (x, x * 10)).collect();
+        // looping and removing any key > 50, but stop after 40 iterations
+        let mut removed = 0;
+        map.retain_with_break(|&k, _| {
+            if removed < 40 {
+                if k > 50 {
+                    removed += 1;
+                    Some(false)
+                } else {
+                    Some(true)
+                }
+            } else {
+                None
+            }
+        });
+        assert_eq!(map.len(), 60);
+        // check nothing up to 50 is removed
+        for k in 0..=50 {
+            assert_eq!(map[&k], k * 10);
+        }
+    }
+
     #[test]
     fn test_extract_if() {
         {

src/set.rs

@@ -372,6 +372,37 @@ impl<T, S, A: Allocator> HashSet<T, S, A> {
         self.map.retain(|k, _| f(k));
     }
 
+    /// Retains only the elements specified by the predicate until the predicate returns `None`.
+    ///
+    /// In other words, remove all elements `e` such that `f(&e)` returns `Some(false)` until
+    /// `f(&e)` returns `None`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use hashbrown::HashSet;
+    ///
+    /// let xs = [1,2,3,4,5,6];
+    /// let mut set: HashSet<i32> = xs.into_iter().collect();
+    /// let mut count = 0;
+    /// set.retain_with_break(|&k| if count < 2 {
+    ///     if k % 2 == 0 {
+    ///         Some(true)
+    ///     } else {
+    ///         Some(false)
+    ///     }
+    /// } else {
+    ///     None
+    /// });
+    /// assert_eq!(set.len(), 3);
+    /// ```
+    pub fn retain_with_break<F>(&mut self, mut f: F)
+    where
+        F: FnMut(&T) -> Option<bool>,
+    {
+        self.map.retain_with_break(|k, _| f(k));
+    }
+
     /// Drains elements which are true under the given predicate,
     /// and returns an iterator over the removed items.
     ///
@@ -2980,6 +3011,30 @@ mod test_set {
         assert!(set.contains(&6));
     }
 
+    #[test]
+    fn test_retain_with_break() {
+        let mut set: HashSet<i32> = (0..100).collect();
+        // looping and removing any key > 50, but stop after 40 iterations
+        let mut removed = 0;
+        set.retain_with_break(|&k| {
+            if removed < 40 {
+                if k > 50 {
+                    removed += 1;
+                    Some(false)
+                } else {
+                    Some(true)
+                }
+            } else {
+                None
+            }
+        });
+        assert_eq!(set.len(), 60);
+        // check nothing up to 50 is removed
+        for k in 0..=50 {
+            assert!(set.contains(&k));
+        }
+    }
+
     #[test]
     fn test_extract_if() {
         {

src/table.rs

@@ -870,6 +870,61 @@ where
         }
     }
 
+    /// Retains only the elements specified by the predicate until the predicate returns `None`.
+    ///
+    /// In other words, remove all elements `e` such that `f(&e)` returns `Ok(false)` until
+    /// `f(&e)` returns `None`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #[cfg(feature = "nightly")]
+    /// # fn test() {
+    /// use hashbrown::{HashTable, DefaultHashBuilder};
+    /// use std::hash::BuildHasher;
+    ///
+    /// let mut table = HashTable::new();
+    /// let hasher = DefaultHashBuilder::default();
+    /// let hasher = |val: &_| {
+    ///     use core::hash::Hasher;
+    ///     let mut state = hasher.build_hasher();
+    ///     core::hash::Hash::hash(&val, &mut state);
+    ///     state.finish()
+    /// };
+    /// let mut removed = 0;
+    /// for x in 1..=8 {
+    ///     table.insert_unique(hasher(&x), x, hasher);
+    /// }
+    /// table.retain_with_break(|&mut v| if removed < 3 {
+    ///     if v % 2 == 0 {
+    ///         Some(true)
+    ///     } else {
+    ///         removed += 1;
+    ///         Some(false)
+    ///     }
+    /// } else {
+    ///     None
+    /// });
+    /// assert_eq!(table.len(), 5);
+    /// # }
+    /// # fn main() {
+    /// #     #[cfg(feature = "nightly")]
+    /// #     test()
+    /// # }
+    /// ```
+    pub fn retain_with_break(&mut self, mut f: impl FnMut(&mut T) -> Option<bool>) {
+        // Here we only use `iter` as a temporary, preventing use-after-free
+        unsafe {
+            for item in self.raw.iter() {
+                match f(item.as_mut()) {
+                    Some(false) => self.raw.erase(item),
+                    Some(true) => continue,
+                    None => break,
+                }
+            }
+        }
+    }
+
     /// Clears the set, returning all elements in an iterator.
     ///
     /// # Examples
@@ -2372,12 +2427,49 @@ impl<T, F, A: Allocator> FusedIterator for ExtractIf<'_, T, F, A> where F: FnMut
 
 #[cfg(test)]
 mod tests {
+    use crate::DefaultHashBuilder;
+
     use super::HashTable;
 
+    use core::hash::BuildHasher;
     #[test]
     fn test_allocation_info() {
         assert_eq!(HashTable::<()>::new().allocation_size(), 0);
         assert_eq!(HashTable::<u32>::new().allocation_size(), 0);
         assert!(HashTable::<u32>::with_capacity(1).allocation_size() > core::mem::size_of::<u32>());
     }
+
+    #[test]
+    fn test_retain_with_break() {
+        let mut table = HashTable::new();
+        let hasher = DefaultHashBuilder::default();
+        let hasher = |val: &_| {
+            use core::hash::Hasher;
+            let mut state = hasher.build_hasher();
+            core::hash::Hash::hash(&val, &mut state);
+            state.finish()
+        };
+        for x in 0..100 {
+            table.insert_unique(hasher(&x), x, hasher);
+        }
+        // looping and removing any value > 50, but stop after 40 iterations
+        let mut removed = 0;
+        table.retain_with_break(|&mut v| {
+            if removed < 40 {
+                if v > 50 {
+                    removed += 1;
+                    Some(false)
+                } else {
+                    Some(true)
+                }
+            } else {
+                None
+            }
+        });
+        assert_eq!(table.len(), 60);
+        // check nothing up to 50 is removed
+        for v in 0..=50 {
+            assert_eq!(table.find(hasher(&v), |&val| val == v), Some(&v));
+        }
+    }
 }