itertools/
zip_longest.rs

1use super::size_hint;
2use std::cmp::Ordering::{Equal, Greater, Less};
3use std::iter::{Fuse, FusedIterator};
4
5use crate::either_or_both::EitherOrBoth;
6
7// ZipLongest originally written by SimonSapin,
8// and dedicated to itertools https://github.com/rust-lang/rust/pull/19283
9
10/// An iterator which iterates two other iterators simultaneously
11///
12/// This iterator is *fused*.
13///
14/// See [`.zip_longest()`](crate::Itertools::zip_longest) for more information.
15#[derive(Clone, Debug)]
16#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
17pub struct ZipLongest<T, U> {
18    a: Fuse<T>,
19    b: Fuse<U>,
20}
21
22/// Create a new `ZipLongest` iterator.
23pub fn zip_longest<T, U>(a: T, b: U) -> ZipLongest<T, U>
24where
25    T: Iterator,
26    U: Iterator,
27{
28    ZipLongest {
29        a: a.fuse(),
30        b: b.fuse(),
31    }
32}
33
34impl<T, U> Iterator for ZipLongest<T, U>
35where
36    T: Iterator,
37    U: Iterator,
38{
39    type Item = EitherOrBoth<T::Item, U::Item>;
40
41    #[inline]
42    fn next(&mut self) -> Option<Self::Item> {
43        match (self.a.next(), self.b.next()) {
44            (None, None) => None,
45            (Some(a), None) => Some(EitherOrBoth::Left(a)),
46            (None, Some(b)) => Some(EitherOrBoth::Right(b)),
47            (Some(a), Some(b)) => Some(EitherOrBoth::Both(a, b)),
48        }
49    }
50
51    #[inline]
52    fn size_hint(&self) -> (usize, Option<usize>) {
53        size_hint::max(self.a.size_hint(), self.b.size_hint())
54    }
55
56    #[inline]
57    fn fold<B, F>(self, init: B, mut f: F) -> B
58    where
59        Self: Sized,
60        F: FnMut(B, Self::Item) -> B,
61    {
62        let Self { mut a, mut b } = self;
63        let res = a.try_fold(init, |init, a| match b.next() {
64            Some(b) => Ok(f(init, EitherOrBoth::Both(a, b))),
65            None => Err(f(init, EitherOrBoth::Left(a))),
66        });
67        match res {
68            Ok(acc) => b.map(EitherOrBoth::Right).fold(acc, f),
69            Err(acc) => a.map(EitherOrBoth::Left).fold(acc, f),
70        }
71    }
72}
73
74impl<T, U> DoubleEndedIterator for ZipLongest<T, U>
75where
76    T: DoubleEndedIterator + ExactSizeIterator,
77    U: DoubleEndedIterator + ExactSizeIterator,
78{
79    #[inline]
80    fn next_back(&mut self) -> Option<Self::Item> {
81        match self.a.len().cmp(&self.b.len()) {
82            Equal => match (self.a.next_back(), self.b.next_back()) {
83                (None, None) => None,
84                (Some(a), Some(b)) => Some(EitherOrBoth::Both(a, b)),
85                // These can only happen if .len() is inconsistent with .next_back()
86                (Some(a), None) => Some(EitherOrBoth::Left(a)),
87                (None, Some(b)) => Some(EitherOrBoth::Right(b)),
88            },
89            Greater => self.a.next_back().map(EitherOrBoth::Left),
90            Less => self.b.next_back().map(EitherOrBoth::Right),
91        }
92    }
93
94    fn rfold<B, F>(self, mut init: B, mut f: F) -> B
95    where
96        F: FnMut(B, Self::Item) -> B,
97    {
98        let Self { mut a, mut b } = self;
99        let a_len = a.len();
100        let b_len = b.len();
101        match a_len.cmp(&b_len) {
102            Equal => {}
103            Greater => {
104                init = a
105                    .by_ref()
106                    .rev()
107                    .take(a_len - b_len)
108                    .map(EitherOrBoth::Left)
109                    .fold(init, &mut f)
110            }
111            Less => {
112                init = b
113                    .by_ref()
114                    .rev()
115                    .take(b_len - a_len)
116                    .map(EitherOrBoth::Right)
117                    .fold(init, &mut f)
118            }
119        }
120        a.rfold(init, |acc, item_a| {
121            f(acc, EitherOrBoth::Both(item_a, b.next_back().unwrap()))
122        })
123    }
124}
125
126impl<T, U> ExactSizeIterator for ZipLongest<T, U>
127where
128    T: ExactSizeIterator,
129    U: ExactSizeIterator,
130{
131}
132
133impl<T, U> FusedIterator for ZipLongest<T, U>
134where
135    T: Iterator,
136    U: Iterator,
137{
138}
itertools/zip_longest.rs

itertools/
zip_longest.rs
