auto merge of #19946 : cgaebel/rust/hashmap-drain-iter, r=gankro

It is useful to move all the elements out of a hashmap without deallocating
the underlying buffer. It came up in IRC, and this patch implements it as
`drain`.

r? @Gankro
cc: @frankmcsherry

3 files changed, 157 insertions, 10 deletions

diff --git a/src/libstd/collections/hash/map.rs b/src/libstd/collections/hash/map.rs
index d068c4610be..0b04edf6776 100644
--- a/src/libstd/collections/hash/map.rs
+++ b/src/libstd/collections/hash/map.rs
@@ -982,6 +982,35 @@ impl<K: Eq + Hash<S>, V, S, H: Hasher<S>> HashMap<K, V, H> {
     #[unstable = "matches collection reform specification, waiting for dust to settle"]
     pub fn is_empty(&self) -> bool { self.len() == 0 }
 
+    /// Clears the map, returning all key-value pairs as an iterator. Keeps the
+    /// allocated memory for reuse.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use std::collections::HashMap;
+    ///
+    /// let mut a = HashMap::new();
+    /// a.insert(1u, "a");
+    /// a.insert(2u, "b");
+    ///
+    /// for (k, v) in a.drain().take(1) {
+    ///     assert!(k == 1 || k == 2);
+    ///     assert!(v == "a" || v == "b");
+    /// }
+    ///
+    /// assert!(a.is_empty());
+    /// ```
+    #[inline]
+    #[unstable = "matches collection reform specification, waiting for dust to settle"]
+    pub fn drain(&mut self) -> Drain<K, V> {
+        fn last_two<A, B, C>((_, b, c): (A, B, C)) -> (B, C) { (b, c) }
+
+        Drain {
+            inner: self.table.drain().map(last_two),
+        }
+    }
+
     /// Clears the map, removing all key-value pairs. Keeps the allocated memory
     /// for reuse.
     ///
@@ -996,16 +1025,9 @@ impl<K: Eq + Hash<S>, V, S, H: Hasher<S>> HashMap<K, V, H> {
     /// assert!(a.is_empty());
     /// ```
     #[unstable = "matches collection reform specification, waiting for dust to settle"]
+    #[inline]
     pub fn clear(&mut self) {
-        let cap = self.table.capacity();
-        let mut buckets = Bucket::first(&mut self.table);
-
-        while buckets.index() != cap {
-            buckets = match buckets.peek() {
-                Empty(b)   => b.next(),
-                Full(full) => full.take().0.next(),
-            };
-        }
+        self.drain();
     }
 
     /// Deprecated: Renamed to `get`.
@@ -1306,6 +1328,16 @@ pub struct Values<'a, K: 'a, V: 'a> {
     inner: Map<(&'a K, &'a V), &'a V, Entries<'a, K, V>, fn((&'a K, &'a V)) -> &'a V>
 }
 
+/// HashMap drain iterator
+pub struct Drain<'a, K: 'a, V: 'a> {
+    inner: iter::Map<
+        (SafeHash, K, V),
+        (K, V),
+        table::Drain<'a, K, V>,
+        fn((SafeHash, K, V)) -> (K, V),
+    >
+}
+
 /// A view into a single occupied location in a HashMap
 pub struct OccupiedEntry<'a, K:'a, V:'a> {
     elem: FullBucket<K, V, &'a mut RawTable<K, V>>,
@@ -1360,6 +1392,17 @@ impl<'a, K, V> Iterator<&'a V> for Values<'a, K, V> {
     #[inline] fn size_hint(&self) -> (uint, Option<uint>) { self.inner.size_hint() }
 }
 
+impl<'a, K: 'a, V: 'a> Iterator<(K, V)> for Drain<'a, K, V> {
+    #[inline]
+    fn next(&mut self) -> Option<(K, V)> {
+        self.inner.next()
+    }
+    #[inline]
+    fn size_hint(&self) -> (uint, Option<uint>) {
+        self.inner.size_hint()
+    }
+}
+
 impl<'a, K, V> OccupiedEntry<'a, K, V> {
     /// Gets a reference to the value in the entry
     pub fn get(&self) -> &V {
diff --git a/src/libstd/collections/hash/set.rs b/src/libstd/collections/hash/set.rs
index 71cc4a1e5a6..ee701b5473d 100644
--- a/src/libstd/collections/hash/set.rs
+++ b/src/libstd/collections/hash/set.rs
@@ -21,7 +21,7 @@ use iter::{Iterator, IteratorExt, FromIterator, Map, FilterMap, Chain, Repeat, Z
 use option::Option::{Some, None, mod};
 use result::Result::{Ok, Err};
 
-use super::map::{HashMap, MoveEntries, Keys, INITIAL_CAPACITY};
+use super::map::{mod, HashMap, MoveEntries, Keys, INITIAL_CAPACITY};
 
 // FIXME(conventions): implement BitOr, BitAnd, BitXor, and Sub
 
@@ -420,6 +420,14 @@ impl<T: Eq + Hash<S>, S, H: Hasher<S>> HashSet<T, H> {
     #[unstable = "matches collection reform specification, waiting for dust to settle"]
     pub fn is_empty(&self) -> bool { self.map.len() == 0 }
 
+    /// Clears the set, returning all elements in an iterator.
+    #[inline]
+    #[unstable = "matches collection reform specification, waiting for dust to settle"]
+    pub fn drain(&mut self) -> Drain<T> {
+        fn first<A, B>((a, _): (A, B)) -> A { a }
+        Drain { iter: self.map.drain().map(first) }
+    }
+
     /// Clears the set, removing all values.
     ///
     /// # Example
@@ -626,6 +634,11 @@ pub struct SetMoveItems<K> {
     iter: Map<(K, ()), K, MoveEntries<K, ()>, fn((K, ())) -> K>
 }
 
+/// HashSet drain iterator
+pub struct Drain<'a, K: 'a> {
+    iter: Map<(K, ()), K, map::Drain<'a, K, ()>, fn((K, ())) -> K>,
+}
+
 // `Repeat` is used to feed the filter closure an explicit capture
 // of a reference to the other set
 /// Set operations iterator, used directly for intersection and difference
@@ -658,6 +671,11 @@ impl<K> Iterator<K> for SetMoveItems<K> {
     fn size_hint(&self) -> (uint, Option<uint>) { self.iter.size_hint() }
 }
 
+impl<'a, K: 'a> Iterator<K> for Drain<'a, K> {
+    fn next(&mut self) -> Option<K> { self.iter.next() }
+    fn size_hint(&self) -> (uint, Option<uint>) { self.iter.size_hint() }
+}
+
 impl<'a, T, H> Iterator<&'a T> for SetAlgebraItems<'a, T, H> {
     fn next(&mut self) -> Option<&'a T> { self.iter.next() }
     fn size_hint(&self) -> (uint, Option<uint>) { self.iter.size_hint() }
@@ -913,4 +931,41 @@ mod test_set {
         assert!(set_str == "{1, 2}" || set_str == "{2, 1}");
         assert_eq!(format!("{}", empty), "{}");
     }
+
+    #[test]
+    fn test_trivial_drain() {
+        let mut s = HashSet::<int>::new();
+        for _ in s.drain() {}
+        assert!(s.is_empty());
+        drop(s);
+
+        let mut s = HashSet::<int>::new();
+        drop(s.drain());
+        assert!(s.is_empty());
+    }
+
+    #[test]
+    fn test_drain() {
+        let mut s: HashSet<int> = range(1, 100).collect();
+
+        // try this a bunch of times to make sure we don't screw up internal state.
+        for _ in range(0i, 20) {
+            assert_eq!(s.len(), 99);
+
+            {
+                let mut last_i = 0;
+                let mut d = s.drain();
+                for (i, x) in d.by_ref().take(50).enumerate() {
+                    last_i = i;
+                    assert!(x != 0);
+                }
+                assert_eq!(last_i, 49);
+            }
+
+            for _ in s.iter() { panic!("s should be empty!"); }
+
+            // reset to try again.
+            s.extend(range(1, 100));
+        }
+    }
 }
diff --git a/src/libstd/collections/hash/table.rs b/src/libstd/collections/hash/table.rs
index da06387e9a5..115edcabca1 100644
--- a/src/libstd/collections/hash/table.rs
+++ b/src/libstd/collections/hash/table.rs
@@ -684,6 +684,19 @@ impl<K, V> RawTable<K, V> {
         }
     }
 
+    pub fn drain(&mut self) -> Drain<K, V> {
+        let RawBuckets { raw, hashes_end, .. } = self.raw_buckets();
+        // Replace the marker regardless of lifetime bounds on parameters.
+        Drain {
+            iter: RawBuckets {
+                raw: raw,
+                hashes_end: hashes_end,
+                marker: marker::ContravariantLifetime::<'static>,
+            },
+            table: self,
+        }
+    }
+
     /// Returns an iterator that copies out each entry. Used while the table
     /// is being dropped.
     unsafe fn rev_move_buckets(&mut self) -> RevMoveBuckets<K, V> {
@@ -774,6 +787,12 @@ pub struct MoveEntries<K, V> {
     iter: RawBuckets<'static, K, V>
 }
 
+/// Iterator over the entries in a table, clearing the table.
+pub struct Drain<'a, K: 'a, V: 'a> {
+    table: &'a mut RawTable<K, V>,
+    iter: RawBuckets<'static, K, V>,
+}
+
 impl<'a, K, V> Iterator<(&'a K, &'a V)> for Entries<'a, K, V> {
     fn next(&mut self) -> Option<(&'a K, &'a V)> {
         self.iter.next().map(|bucket| {
@@ -828,6 +847,36 @@ impl<K, V> Iterator<(SafeHash, K, V)> for MoveEntries<K, V> {
     }
 }
 
+impl<'a, K: 'a, V: 'a> Iterator<(SafeHash, K, V)> for Drain<'a, K, V> {
+    #[inline]
+    fn next(&mut self) -> Option<(SafeHash, K, V)> {
+        self.iter.next().map(|bucket| {
+            self.table.size -= 1;
+            unsafe {
+                (
+                    SafeHash {
+                        hash: ptr::replace(bucket.hash, EMPTY_BUCKET),
+                    },
+                    ptr::read(bucket.key as *const K),
+                    ptr::read(bucket.val as *const V)
+                )
+            }
+        })
+    }
+
+    fn size_hint(&self) -> (uint, Option<uint>) {
+        let size = self.table.size();
+        (size, Some(size))
+    }
+}
+
+#[unsafe_destructor]
+impl<'a, K: 'a, V: 'a> Drop for Drain<'a, K, V> {
+    fn drop(&mut self) {
+        for _ in *self {}
+    }
+}
+
 impl<K: Clone, V: Clone> Clone for RawTable<K, V> {
     fn clone(&self) -> RawTable<K, V> {
         unsafe {