Add a range argument to vec.extract_if

2 files changed, 46 insertions, 20 deletions

diff --git a/library/alloc/src/vec/extract_if.rs b/library/alloc/src/vec/extract_if.rs
index 72d51e89044..f722d89d400 100644
--- a/library/alloc/src/vec/extract_if.rs
+++ b/library/alloc/src/vec/extract_if.rs
@@ -1,4 +1,4 @@
-use core::{ptr, slice};
+use core::{ops::{Range, RangeBounds}, ptr, slice};
 
 use super::Vec;
 use crate::alloc::{Allocator, Global};
@@ -14,7 +14,7 @@ use crate::alloc::{Allocator, Global};
 /// #![feature(extract_if)]
 ///
 /// let mut v = vec![0, 1, 2];
-/// let iter: std::vec::ExtractIf<'_, _, _> = v.extract_if(|x| *x % 2 == 0);
+/// let iter: std::vec::ExtractIf<'_, _, _> = v.extract_if(.., |x| *x % 2 == 0);
 /// ```
 #[unstable(feature = "extract_if", reason = "recently added", issue = "43244")]
 #[derive(Debug)]
@@ -30,6 +30,8 @@ pub struct ExtractIf<
     pub(super) vec: &'a mut Vec<T, A>,
     /// The index of the item that will be inspected by the next call to `next`.
     pub(super) idx: usize,
+    /// Elements at and beyond this point will be retained. Must be equal or smaller than `old_len`.
+    pub(super) end: usize,
     /// The number of items that have been drained (removed) thus far.
     pub(super) del: usize,
     /// The original length of `vec` prior to draining.
@@ -38,10 +40,21 @@ pub struct ExtractIf<
     pub(super) pred: F,
 }
 
-impl<T, F, A: Allocator> ExtractIf<'_, T, F, A>
+impl<'a, T, F, A: Allocator> ExtractIf<'a, T, F, A>
 where
     F: FnMut(&mut T) -> bool,
 {
+    pub(super) fn new<R: RangeBounds<usize>>(vec: &'a mut Vec<T, A>, pred: F, range: R) -> Self {
+        let old_len = vec.len();
+        let Range { start, end } = slice::range(range, ..old_len);
+
+        // Guard against the vec getting leaked (leak amplification)
+        unsafe {
+            vec.set_len(0);
+        }
+        ExtractIf { vec, idx: start, del: 0, end, old_len, pred }
+    }
+
     /// Returns a reference to the underlying allocator.
     #[unstable(feature = "allocator_api", issue = "32838")]
     #[inline]
@@ -59,7 +72,7 @@ where
 
     fn next(&mut self) -> Option<T> {
         unsafe {
-            while self.idx < self.old_len {
+            while self.idx < self.end {
                 let i = self.idx;
                 let v = slice::from_raw_parts_mut(self.vec.as_mut_ptr(), self.old_len);
                 let drained = (self.pred)(&mut v[i]);
@@ -82,7 +95,7 @@ where
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
-        (0, Some(self.old_len - self.idx))
+        (0, Some(self.end - self.idx))
     }
 }
 
diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs
index 7e7a8ff72c7..5f4b85b58a9 100644
--- a/library/alloc/src/vec/mod.rs
+++ b/library/alloc/src/vec/mod.rs
@@ -3615,12 +3615,15 @@ impl<T, A: Allocator> Vec<T, A> {
         Splice { drain: self.drain(range), replace_with: replace_with.into_iter() }
     }
 
-    /// Creates an iterator which uses a closure to determine if an element should be removed.
+    /// Creates an iterator which uses a closure to determine if element in the range should be removed.
     ///
     /// If the closure returns true, then the element is removed and yielded.
     /// If the closure returns false, the element will remain in the vector and will not be yielded
     /// by the iterator.
     ///
+    /// Only elements that fall in the provided range are considered for extraction, but any elements
+    /// after the range will still have to be moved if any element has been extracted.
+    ///
     /// If the returned `ExtractIf` is not exhausted, e.g. because it is dropped without iterating
     /// or the iteration short-circuits, then the remaining elements will be retained.
     /// Use [`retain`] with a negated predicate if you do not need the returned iterator.
@@ -3630,10 +3633,12 @@ impl<T, A: Allocator> Vec<T, A> {
     /// Using this method is equivalent to the following code:
     ///
     /// ```
+    /// # use std::cmp::min;
     /// # let some_predicate = |x: &mut i32| { *x == 2 || *x == 3 || *x == 6 };
     /// # let mut vec = vec![1, 2, 3, 4, 5, 6];
-    /// let mut i = 0;
-    /// while i < vec.len() {
+    /// # let range = 1..4;
+    /// let mut i = range.start;
+    /// while i < min(vec.len(), range.end) {
     ///     if some_predicate(&mut vec[i]) {
     ///         let val = vec.remove(i);
     ///         // your code here
@@ -3648,8 +3653,12 @@ impl<T, A: Allocator> Vec<T, A> {
     /// But `extract_if` is easier to use. `extract_if` is also more efficient,
     /// because it can backshift the elements of the array in bulk.
     ///
-    /// Note that `extract_if` also lets you mutate every element in the filter closure,
-    /// regardless of whether you choose to keep or remove it.
+    /// Note that `extract_if` also lets you mutate the elements passed to the filter closure,
+    /// regardless of whether you choose to keep or remove them.
+    ///
+    /// # Panics
+    ///
+    /// If `range` is out of bounds.
     ///
     /// # Examples
     ///
@@ -3659,25 +3668,29 @@ impl<T, A: Allocator> Vec<T, A> {
     /// #![feature(extract_if)]
     /// let mut numbers = vec![1, 2, 3, 4, 5, 6, 8, 9, 11, 13, 14, 15];
     ///
-    /// let evens = numbers.extract_if(|x| *x % 2 == 0).collect::<Vec<_>>();
+    /// let evens = numbers.extract_if(.., |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]);
     /// ```
+    ///
+    /// Using the range argument to only process a part of the vector:
+    ///
+    /// ```
+    /// #![feature(extract_if)]
+    /// let mut items = vec![0, 0, 0, 0, 0, 0, 0, 1, 2, 1, 2, 1, 2];
+    /// let ones = items.extract_if(7.., |x| *x == 1).collect::<Vec<_>>();
+    /// assert_eq!(items, vec![0, 0, 0, 0, 0, 0, 0, 2, 2, 2]);
+    /// assert_eq!(ones.len(), 3);
+    /// ```
     #[unstable(feature = "extract_if", reason = "recently added", issue = "43244")]
-    pub fn extract_if<F>(&mut self, filter: F) -> ExtractIf<'_, T, F, A>
+    pub fn extract_if<F, R>(&mut self, range: R, filter: F) -> ExtractIf<'_, T, F, A>
     where
         F: FnMut(&mut T) -> bool,
+        R: RangeBounds<usize>,
     {
-        let old_len = self.len();
-
-        // Guard against us getting leaked (leak amplification)
-        unsafe {
-            self.set_len(0);
-        }
-
-        ExtractIf { vec: self, idx: 0, del: 0, old_len, pred: filter }
+        ExtractIf::new(self, filter, range)
     }
 }