1use crate::fmt;
2use crate::iter::FusedIterator;
3
4/// Creates an iterator which, starting from an initial item,
5/// computes each successive item from the preceding one.
6///
7/// This iterator stores an optional item (`Option<T>`) and a successor closure (`impl FnMut(&T) -> Option<T>`).
8/// Its `next` method returns the stored optional item and
9/// if it is `Some(val)` calls the stored closure on `&val` to compute and store its successor.
10/// The iterator will apply the closure successively to the stored option's value until the option is `None`.
11/// This also means that once the stored option is `None` it will remain `None`,
12/// as the closure will not be called again, so the created iterator is a [`FusedIterator`].
13/// The iterator's items will be the initial item and all of its successors as calculated by the successor closure.
14///
15/// ```
16/// use std::iter::successors;
17///
18/// let powers_of_10 = successors(Some(1_u16), |n| n.checked_mul(10));
19/// assert_eq!(powers_of_10.collect::<Vec<_>>(), &[1, 10, 100, 1_000, 10_000]);
20/// ```
21#[stable(feature = "iter_successors", since = "1.34.0")]
22pub fn successors<T, F>(first: Option<T>, succ: F) -> Successors<T, F>
23where
24 F: FnMut(&T) -> Option<T>,
25{
26 // If this function returned `impl Iterator<Item=T>`
27 // it could be based on `from_fn` and not need a dedicated type.
28 // However having a named `Successors<T, F>` type allows it to be `Clone` when `T` and `F` are.
29 Successors { next: first, succ }
30}
31
32/// An iterator which, starting from an initial item,
33/// computes each successive item from the preceding one.
34///
35/// This `struct` is created by the [`iter::successors()`] function.
36/// See its documentation for more.
37///
38/// [`iter::successors()`]: successors
39#[derive(Clone)]
40#[stable(feature = "iter_successors", since = "1.34.0")]
41pub struct Successors<T, F> {
42 next: Option<T>,
43 succ: F,
44}
45
46#[stable(feature = "iter_successors", since = "1.34.0")]
47impl<T, F> Iterator for Successors<T, F>
48where
49 F: FnMut(&T) -> Option<T>,
50{
51 type Item = T;
52
53 #[inline]
54 fn next(&mut self) -> Option<Self::Item> {
55 let item: T = self.next.take()?;
56 self.next = (self.succ)(&item);
57 Some(item)
58 }
59
60 #[inline]
61 fn size_hint(&self) -> (usize, Option<usize>) {
62 if self.next.is_some() { (1, None) } else { (0, Some(0)) }
63 }
64}
65
66#[stable(feature = "iter_successors", since = "1.34.0")]
67impl<T, F> FusedIterator for Successors<T, F> where F: FnMut(&T) -> Option<T> {}
68
69#[stable(feature = "iter_successors", since = "1.34.0")]
70impl<T: fmt::Debug, F> fmt::Debug for Successors<T, F> {
71 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
72 f.debug_struct("Successors").field(name:"next", &self.next).finish()
73 }
74}
75

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