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Introduce Vec::resize_with method (see rust-lang#41758)
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djc committed Apr 3, 2018
1 parent 135f334 commit da0ceef
Showing 1 changed file with 57 additions and 7 deletions.
64 changes: 57 additions & 7 deletions src/liballoc/vec.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1306,6 +1306,49 @@ impl<T> Vec<T> {
}
other
}

/// Resizes the `Vec` in-place so that `len` is equal to `new_len`.
///
/// If `new_len` is greater than `len`, the `Vec` is extended by the
/// difference, with each additional slot filled with the result of
/// calling the closure `f`. The return values from `f` will end up
/// in the `Vec` in the order they have been generated.
///
/// If `new_len` is less than `len`, the `Vec` is simply truncated.
///
/// This method uses a closure to create new values on every push. If
/// you'd rather [`Clone`] a given value, use [`resize`]. If you want
/// to use the [`Default`] trait to generate values, you can pass
/// [`Default::default()`] as the second argument..
///
/// # Examples
///
/// ```
/// #![feature(vec_resize_with)]
///
/// let mut vec = vec![1, 2, 3];
/// vec.resize_with(5, Default::default);
/// assert_eq!(vec, [1, 2, 3, 0, 0]);
///
/// let mut vec = vec![];
/// let mut p = 1;
/// vec.resize_with(4, || { p *= 2; p });
/// assert_eq!(vec, [2, 4, 8, 16]);
/// ```
///
/// [`resize`]: #method.resize
/// [`Clone`]: ../../std/clone/trait.Clone.html
#[unstable(feature = "vec_resize_with", issue = "41758")]
pub fn resize_with<F>(&mut self, new_len: usize, f: F)
where F: FnMut() -> T
{
let len = self.len();
if new_len > len {
self.extend_with(new_len - len, ExtendFunc(f));
} else {
self.truncate(new_len);
}
}
}

impl<T: Clone> Vec<T> {
Expand All @@ -1316,8 +1359,8 @@ impl<T: Clone> Vec<T> {
/// If `new_len` is less than `len`, the `Vec` is simply truncated.
///
/// This method requires [`Clone`] to be able clone the passed value. If
/// you'd rather create a value with [`Default`] instead, see
/// [`resize_default`].
/// you need more flexibility (or want to rely on [`Default`] instead of
/// [`Clone`]), use [`resize_with`].
///
/// # Examples
///
Expand All @@ -1333,7 +1376,7 @@ impl<T: Clone> Vec<T> {
///
/// [`Clone`]: ../../std/clone/trait.Clone.html
/// [`Default`]: ../../std/default/trait.Default.html
/// [`resize_default`]: #method.resize_default
/// [`resize_with`]: #method.resize_with
#[stable(feature = "vec_resize", since = "1.5.0")]
pub fn resize(&mut self, new_len: usize, value: T) {
let len = self.len();
Expand Down Expand Up @@ -1412,24 +1455,31 @@ impl<T: Default> Vec<T> {

// This code generalises `extend_with_{element,default}`.
trait ExtendWith<T> {
fn next(&self) -> T;
fn next(&mut self) -> T;
fn last(self) -> T;
}

struct ExtendElement<T>(T);
impl<T: Clone> ExtendWith<T> for ExtendElement<T> {
fn next(&self) -> T { self.0.clone() }
fn next(&mut self) -> T { self.0.clone() }
fn last(self) -> T { self.0 }
}

struct ExtendDefault;
impl<T: Default> ExtendWith<T> for ExtendDefault {
fn next(&self) -> T { Default::default() }
fn next(&mut self) -> T { Default::default() }
fn last(self) -> T { Default::default() }
}

struct ExtendFunc<F>(F);
impl<T, F: FnMut() -> T> ExtendWith<T> for ExtendFunc<F> {
fn next(&mut self) -> T { (self.0)() }
fn last(mut self) -> T { (self.0)() }
}

impl<T> Vec<T> {
/// Extend the vector by `n` values, using the given generator.
fn extend_with<E: ExtendWith<T>>(&mut self, n: usize, value: E) {
fn extend_with<E: ExtendWith<T>>(&mut self, n: usize, mut value: E) {
self.reserve(n);

unsafe {
Expand Down

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