diff --git a/src/libcore/iter/mod.rs b/src/libcore/iter/mod.rs index 29b62c901f310..d929d1d65a918 100644 --- a/src/libcore/iter/mod.rs +++ b/src/libcore/iter/mod.rs @@ -333,6 +333,8 @@ pub use self::range::Step; #[stable(feature = "rust1", since = "1.0.0")] pub use self::sources::{Repeat, repeat}; +#[unstable(feature = "iterator_repeat_with", issue = "48169")] +pub use self::sources::{RepeatWith, repeat_with}; #[stable(feature = "iter_empty", since = "1.2.0")] pub use self::sources::{Empty, empty}; #[stable(feature = "iter_once", since = "1.2.0")] diff --git a/src/libcore/iter/sources.rs b/src/libcore/iter/sources.rs index b05a893e66104..3e9d799c08948 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 +105,115 @@ 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 +#[derive(Copy, Clone, Debug)] +#[unstable(feature = "iterator_repeat_with", issue = "48169")] +pub struct RepeatWith { + repeater: F +} + +#[unstable(feature = "iterator_repeat_with", issue = "48169")] +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 = "48169")] +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 iterator 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 +/// +/// An iterator produced by `repeat_with()` is a `DoubleEndedIterator`. +/// It is important to not that reversing `repeat_with(f)` will produce +/// the exact same sequence as the non-reversed iterator. In other words, +/// `repeat_with(f).rev().collect::>()` is equivalent to +/// `repeat_with(f).collect::>()`. +/// +/// # Examples +/// +/// Basic usage: +/// +/// ``` +/// #![feature(iterator_repeat_with)] +/// +/// 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 +/// #![feature(iterator_repeat_with)] +/// +/// 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 = "48169")] +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/lib.rs b/src/libcore/lib.rs index 59a296c2a762c..447e144bf0fd6 100644 --- a/src/libcore/lib.rs +++ b/src/libcore/lib.rs @@ -92,6 +92,7 @@ #![feature(unwind_attributes)] #![feature(doc_spotlight)] #![feature(rustc_const_unstable)] +#![feature(iterator_repeat_with)] #[prelude_import] #[allow(unused)] diff --git a/src/libcore/tests/iter.rs b/src/libcore/tests/iter.rs index d8c9dcd866486..f91c919d7447d 100644 --- a/src/libcore/tests/iter.rs +++ b/src/libcore/tests/iter.rs @@ -1592,6 +1592,51 @@ fn test_repeat_take_collect() { assert_eq!(v, vec![42, 42, 42]); } +#[test] +fn test_repeat_with() { + #[derive(PartialEq, Debug)] + 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!(pow2.next(), Some(1)); + assert_eq!(pow2.next(), Some(2)); + assert_eq!(pow2.next(), Some(4)); + assert_eq!(pow2.next(), Some(8)); + assert_eq!(pow2.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_with_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; diff --git a/src/libcore/tests/lib.rs b/src/libcore/tests/lib.rs index 0d55e6eeeb1ce..3e901a9d442ce 100644 --- a/src/libcore/tests/lib.rs +++ b/src/libcore/tests/lib.rs @@ -27,6 +27,7 @@ #![feature(iterator_try_fold)] #![feature(iter_rfind)] #![feature(iter_rfold)] +#![feature(iterator_repeat_with)] #![feature(nonzero)] #![feature(pattern)] #![feature(range_is_empty)]