library/core/src/iter/sources/repeat_with.rs

use crate::fmt;
use crate::iter::{FusedIterator, TrustedLen};
use crate::ops::Try;

/// 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 again.
///
/// Infinite iterators like `repeat_with()` are often used with adapters like
/// [`Iterator::take()`], in order to make them finite.
///
/// 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.
///
/// An iterator produced by `repeat_with()` is not a [`DoubleEndedIterator`].
/// If you need `repeat_with()` to return a [`DoubleEndedIterator`],
/// please open a GitHub issue explaining your use case.
///
/// [`repeat()`]: crate::iter::repeat
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// use std::iter;
///
/// // let's assume we have some value of a type that is not `Clone`
/// // or which we 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]
#[stable(feature = "iterator_repeat_with", since = "1.28.0")]
pub fn repeat_with<A, F: FnMut() -> A>(repeater: F) -> RepeatWith<F> {
    RepeatWith { repeater }
}

/// 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.
#[derive(Copy, Clone)]
#[stable(feature = "iterator_repeat_with", since = "1.28.0")]
pub struct RepeatWith<F> {
    repeater: F,
}

#[stable(feature = "iterator_repeat_with_debug", since = "1.68.0")]
impl<F> fmt::Debug for RepeatWith<F> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("RepeatWith").finish_non_exhaustive()
    }
}

#[stable(feature = "iterator_repeat_with", since = "1.28.0")]
impl<A, F: FnMut() -> A> Iterator for RepeatWith<F> {
    type Item = A;

    #[inline]
    fn next(&mut self) -> Option<A> {
        Some((self.repeater)())
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        (usize::MAX, None)
    }

    #[inline]
    fn try_fold<Acc, Fold, R>(&mut self, mut init: Acc, mut fold: Fold) -> R
    where
        Fold: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>,
    {
        // This override isn't strictly needed, but avoids the need to optimize
        // away the `next`-always-returns-`Some` and emphasizes that the `?`
        // is the only way to exit the loop.

        loop {
            let item = (self.repeater)();
            init = fold(init, item)?;
        }
    }
}

#[stable(feature = "iterator_repeat_with", since = "1.28.0")]
impl<A, F: FnMut() -> A> FusedIterator for RepeatWith<F> {}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<A, F: FnMut() -> A> TrustedLen for RepeatWith<F> {}