Add Vec::drain_filter

src/liballoc/vec.rs

@@ -1960,6 +1960,65 @@ impl<T> Vec<T> {
         }
     }
 
+    /// Creates an iterator which uses a closure to determine if an element should be removed.
+    ///
+    /// If the closure returns true, then the element is removed and yielded.
+    /// If the closure returns false, it will try again, and call the closure
+    /// on the next element, seeing if it passes the test.
+    ///
+    /// Using this method is equivalent to the following code:
+    ///
+    /// ```
+    /// # let some_predicate = |x: &mut i32| { *x == 2 };
+    /// # let mut vec = vec![1, 2, 3, 4, 5];
+    /// let mut i = 0;
+    /// while i != vec.len() {
+    ///     if some_predicate(&mut vec[i]) {
+    ///         let val = vec.remove(i);
+    ///         // your code here
+    ///     }
+    ///     i += 1;
+    /// }
+    /// ```
+    ///
+    /// But `drain_filter` is easier to use. `drain_filter` is also more efficient,
+    /// because it can backshift the elements of the array in bulk.
+    ///
+    /// Note that `drain_filter` also lets you mutate every element in the filter closure,
+    /// regardless of whether you choose to keep or remove it.
+    ///
+    ///
+    /// # Examples
+    ///
+    /// Splitting an array into evens and odds, reusing the original allocation:
+    ///
+    /// ```
+    /// #![feature(drain_filter)]
+    /// let mut numbers = vec![1, 2, 3, 4, 5, 6, 8, 9, 11, 13, 14, 15];
+    ///
+    /// let evens = numbers.drain_filter(|x| *x % 2 == 0).collect::<Vec<_>>();
+    /// let odds = numbers;
+    ///
+    /// assert_eq!(evens, vec![2, 4, 6, 8, 14]);
+    /// assert_eq!(odds, vec![1, 3, 5, 9, 11, 13, 15]);
+    /// ```
+    #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
+    pub fn drain_filter<F>(&mut self, filter: F) -> DrainFilter<T, F>
+        where F: FnMut(&mut T) -> bool,
+    {
+        let old_len = self.len();
+
+        // Guard against us getting leaked (leak amplification)
+        unsafe { self.set_len(0); }
+
+        DrainFilter {
+            vec: self,
+            idx: 0,
+            del: 0,
+            old_len,
+            pred: filter,
+        }
+    }
 }
 
 #[stable(feature = "extend_ref", since = "1.2.0")]
@@ -2596,3 +2655,57 @@ impl<'a, T> Drain<'a, T> {
         self.tail_start = new_tail_start;
     }
 }
+
+/// An iterator produced by calling `drain_filter` on Vec.
+#[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
+#[derive(Debug)]
+pub struct DrainFilter<'a, T: 'a, F>
+    where F: FnMut(&mut T) -> bool,
+{
+    vec: &'a mut Vec<T>,
+    idx: usize,
+    del: usize,
+    old_len: usize,
+    pred: F,
+}
+
+#[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
+impl<'a, T, F> Iterator for DrainFilter<'a, T, F>
+    where F: FnMut(&mut T) -> bool,
+{
+    type Item = T;
+
+    fn next(&mut self) -> Option<T> {
+        unsafe {
+            while self.idx != self.old_len {
+                let i = self.idx;
+                self.idx += 1;
+                let v = slice::from_raw_parts_mut(self.vec.as_mut_ptr(), self.old_len);
+                if (self.pred)(&mut v[i]) {
+                    self.del += 1;
+                    return Some(ptr::read(&v[i]));
+                } else if self.del > 0 {
+                    v.swap(i - self.del, i);
+                }
+            }
+            None
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(self.old_len - self.idx))
+    }
+}
+
+#[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
+impl<'a, T, F> Drop for DrainFilter<'a, T, F>
+    where F: FnMut(&mut T) -> bool,
+{
+    fn drop(&mut self) {
+        for _ in self.by_ref() { }
+
+        unsafe {
+            self.vec.set_len(self.old_len - self.del);
+        }
+    }
+}