1use std::fmt;
2use std::iter::{Fuse, FusedIterator, Peekable};
3
4/// An iterator adaptor that wraps each element in an [`Position`].
5///
6/// Iterator element type is `(Position, I::Item)`.
7///
8/// See [`.with_position()`](crate::Itertools::with_position) for more information.
9#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
10pub struct WithPosition<I>
11where
12 I: Iterator,
13{
14 handled_first: bool,
15 peekable: Peekable<Fuse<I>>,
16}
17
18impl<I> fmt::Debug for WithPosition<I>
19where
20 I: Iterator,
21 Peekable<Fuse<I>>: fmt::Debug,
22{
23 debug_fmt_fields!(WithPosition, handled_first, peekable);
24}
25
26impl<I> Clone for WithPosition<I>
27where
28 I: Clone + Iterator,
29 I::Item: Clone,
30{
31 clone_fields!(handled_first, peekable);
32}
33
34/// Create a new `WithPosition` iterator.
35pub fn with_position<I>(iter: I) -> WithPosition<I>
36where
37 I: Iterator,
38{
39 WithPosition {
40 handled_first: false,
41 peekable: iter.fuse().peekable(),
42 }
43}
44
45/// The first component of the value yielded by `WithPosition`.
46/// Indicates the position of this element in the iterator results.
47///
48/// See [`.with_position()`](crate::Itertools::with_position) for more information.
49#[derive(Copy, Clone, Debug, PartialEq, Eq)]
50pub enum Position {
51 /// This is the first element.
52 First,
53 /// This is neither the first nor the last element.
54 Middle,
55 /// This is the last element.
56 Last,
57 /// This is the only element.
58 Only,
59}
60
61impl<I: Iterator> Iterator for WithPosition<I> {
62 type Item = (Position, I::Item);
63
64 fn next(&mut self) -> Option<Self::Item> {
65 match self.peekable.next() {
66 Some(item) => {
67 if !self.handled_first {
68 // Haven't seen the first item yet, and there is one to give.
69 self.handled_first = true;
70 // Peek to see if this is also the last item,
71 // in which case tag it as `Only`.
72 match self.peekable.peek() {
73 Some(_) => Some((Position::First, item)),
74 None => Some((Position::Only, item)),
75 }
76 } else {
77 // Have seen the first item, and there's something left.
78 // Peek to see if this is the last item.
79 match self.peekable.peek() {
80 Some(_) => Some((Position::Middle, item)),
81 None => Some((Position::Last, item)),
82 }
83 }
84 }
85 // Iterator is finished.
86 None => None,
87 }
88 }
89
90 fn size_hint(&self) -> (usize, Option<usize>) {
91 self.peekable.size_hint()
92 }
93
94 fn fold<B, F>(mut self, mut init: B, mut f: F) -> B
95 where
96 F: FnMut(B, Self::Item) -> B,
97 {
98 if let Some(mut head) = self.peekable.next() {
99 if !self.handled_first {
100 // The current head is `First` or `Only`,
101 // it depends if there is another item or not.
102 match self.peekable.next() {
103 Some(second) => {
104 let first = std::mem::replace(&mut head, second);
105 init = f(init, (Position::First, first));
106 }
107 None => return f(init, (Position::Only, head)),
108 }
109 }
110 // Have seen the first item, and there's something left.
111 init = self.peekable.fold(init, |acc, mut item| {
112 std::mem::swap(&mut head, &mut item);
113 f(acc, (Position::Middle, item))
114 });
115 // The "head" is now the last item.
116 init = f(init, (Position::Last, head));
117 }
118 init
119 }
120}
121
122impl<I> ExactSizeIterator for WithPosition<I> where I: ExactSizeIterator {}
123
124impl<I: Iterator> FusedIterator for WithPosition<I> {}
125