From 0f789aad2b3cfc0b0925b726295200267130e69d Mon Sep 17 00:00:00 2001 From: Mazdak Farrokhzad Date: Mon, 12 Feb 2018 08:05:46 +0100 Subject: [PATCH] add core::iter::repeat_with --- src/libcore/iter/sources.rs | 104 ++++++++++++++++++++++++++++++++++++ src/libcore/tests/iter.rs | 44 +++++++++++++++ 2 files changed, 148 insertions(+) diff --git a/src/libcore/iter/sources.rs b/src/libcore/iter/sources.rs index b05a893e661..980f3fc7443 100644 --- a/src/libcore/iter/sources.rs +++ b/src/libcore/iter/sources.rs @@ -57,6 +57,12 @@ unsafe impl TrustedLen for Repeat {} /// /// [`take`]: trait.Iterator.html#method.take /// +/// If the element type of the iterator you need does not implement `Clone`, +/// or if you do not want to keep the repeated element in memory, you can +/// instead use the [`repeat_with`] function. +/// +/// [`repeat_with`]: fn.repeat_with.html +/// /// # Examples /// /// Basic usage: @@ -99,6 +105,104 @@ pub fn repeat(elt: T) -> Repeat { Repeat{element: elt} } +/// An iterator that repeats elements of type `A` endlessly by +/// applying the provided closure `F: FnMut() -> A`. +/// +/// This `struct` is created by the [`repeat_with`] function. +/// See its documentation for more. +/// +/// [`repeat_with`]: fn.repeat_with.html +#[unstable(feature = "iterator_repeat_with", issue = "0")] +pub struct RepeatWith { + repeater: F +} + +#[unstable(feature = "iterator_repeat_with", issue = "0")] +impl A> Iterator for RepeatWith { + type Item = A; + + #[inline] + fn next(&mut self) -> Option { Some((self.repeater)()) } + + #[inline] + fn size_hint(&self) -> (usize, Option) { (usize::MAX, None) } +} + +#[unstable(feature = "iterator_repeat_with", issue = "0")] +impl A> DoubleEndedIterator for RepeatWith { + #[inline] + fn next_back(&mut self) -> Option { self.next() } +} + +#[unstable(feature = "fused", issue = "35602")] +impl A> FusedIterator for RepeatWith {} + +#[unstable(feature = "trusted_len", issue = "37572")] +unsafe impl A> TrustedLen for RepeatWith {} + +/// Creates a new that repeats elements of type `A` endlessly by +/// applying the provided closure, the repeater, `F: FnMut() -> A`. +/// +/// The `repeat_with()` function calls the repeater over and over and over and +/// over and over and 🔁. +/// +/// Infinite iterators like `repeat_with()` are often used with adapters like +/// [`take`], in order to make them finite. +/// +/// [`take`]: trait.Iterator.html#method.take +/// +/// If the element type of the iterator you need implements `Clone`, and +/// it is OK to keep the source element in memory, you should instead use +/// the [`repeat`] function. +/// +/// [`repeat`]: fn.repeat.html +/// +/// # Examples +/// +/// Basic usage: +/// +/// ``` +/// use std::iter; +/// +/// // let's assume we have some value of a type that is not `Clone` +/// // or which don't want to have in memory just yet because it is expensive: +/// #[derive(PartialEq, Debug)] +/// struct Expensive; +/// +/// // a particular value forever: +/// let mut things = iter::repeat_with(|| Expensive); +/// +/// assert_eq!(Some(Expensive), things.next()); +/// assert_eq!(Some(Expensive), things.next()); +/// assert_eq!(Some(Expensive), things.next()); +/// assert_eq!(Some(Expensive), things.next()); +/// assert_eq!(Some(Expensive), things.next()); +/// ``` +/// +/// Using mutation and going finite: +/// +/// ```rust +/// use std::iter; +/// +/// // From the zeroth to the third power of two: +/// let mut curr = 1; +/// let mut pow2 = iter::repeat_with(|| { let tmp = curr; curr *= 2; tmp }) +/// .take(4); +/// +/// assert_eq!(Some(1), pow2.next()); +/// assert_eq!(Some(2), pow2.next()); +/// assert_eq!(Some(4), pow2.next()); +/// assert_eq!(Some(8), pow2.next()); +/// +/// // ... and now we're done +/// assert_eq!(None, pow2.next()); +/// ``` +#[inline] +#[unstable(feature = "iterator_repeat_with", issue = "0")] +pub fn repeat_with A>(repeater: F) -> RepeatWith { + RepeatWith { repeater } +} + /// An iterator that yields nothing. /// /// This `struct` is created by the [`empty`] function. See its documentation for more. diff --git a/src/libcore/tests/iter.rs b/src/libcore/tests/iter.rs index b2a5243d5e6..ca5318d198e 100644 --- a/src/libcore/tests/iter.rs +++ b/src/libcore/tests/iter.rs @@ -1549,6 +1549,50 @@ fn test_repeat_take_collect() { assert_eq!(v, vec![42, 42, 42]); } +#[test] +fn test_repeat_with() { + struct NotClone(usize); + let mut it = repeat_with(|| NotClone(42)); + assert_eq!(it.next(), Some(NotClone(42))); + assert_eq!(it.next(), Some(NotClone(42))); + assert_eq!(it.next(), Some(NotClone(42))); + assert_eq!(repeat_with(|| NotClone(42)).size_hint(), (usize::MAX, None)); +} + +#[test] +fn test_repeat_with_rev() { + let mut curr = 1; + let mut pow2 = repeat_with(|| { let tmp = curr; curr *= 2; tmp }) + .rev().take(4); + assert_eq!(it.next(), Some(1)); + assert_eq!(it.next(), Some(2)); + assert_eq!(it.next(), Some(4)); + assert_eq!(it.next(), Some(8)); + assert_eq!(it.next(), None); +} + +#[test] +fn test_repeat_with_take() { + let mut it = repeat_with(|| 42).take(3); + assert_eq!(it.next(), Some(42)); + assert_eq!(it.next(), Some(42)); + assert_eq!(it.next(), Some(42)); + assert_eq!(it.next(), None); + is_trusted_len(repeat_with(|| 42).take(3)); + assert_eq!(repeat_with(|| 42).take(3).size_hint(), (3, Some(3))); + assert_eq!(repeat_with(|| 42).take(0).size_hint(), (0, Some(0))); + assert_eq!(repeat_with(|| 42).take(usize::MAX).size_hint(), + (usize::MAX, Some(usize::MAX))); +} + +#[test] +fn test_repeat_take_collect() { + let mut curr = 1; + let v: Vec<_> = repeat_with(|| { let tmp = curr; curr *= 2; tmp }) + .take(5).collect(); + assert_eq!(v, vec![1, 2, 4, 8, 16]); +} + #[test] fn test_fuse() { let mut it = 0..3;