itertools/
with_position.rs

1use std::iter::{Fuse,Peekable, FusedIterator};
2
3/// An iterator adaptor that wraps each element in an [`Position`].
4///
5/// Iterator element type is `(Position, I::Item)`.
6///
7/// See [`.with_position()`](crate::Itertools::with_position) for more information.
8#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
9pub struct WithPosition<I>
10    where I: Iterator,
11{
12    handled_first: bool,
13    peekable: Peekable<Fuse<I>>,
14}
15
16impl<I> Clone for WithPosition<I>
17    where I: Clone + Iterator,
18          I::Item: Clone,
19{
20    clone_fields!(handled_first, peekable);
21}
22
23/// Create a new `WithPosition` iterator.
24pub fn with_position<I>(iter: I) -> WithPosition<I>
25    where I: Iterator,
26{
27    WithPosition {
28        handled_first: false,
29        peekable: iter.fuse().peekable(),
30    }
31}
32
33/// The first component of the value yielded by `WithPosition`.
34/// Indicates the position of this element in the iterator results.
35///
36/// See [`.with_position()`](crate::Itertools::with_position) for more information.
37#[derive(Copy, Clone, Debug, PartialEq)]
38pub enum Position {
39    /// This is the first element.
40    First,
41    /// This is neither the first nor the last element.
42    Middle,
43    /// This is the last element.
44    Last,
45    /// This is the only element.
46    Only,
47}
48
49impl<I: Iterator> Iterator for WithPosition<I> {
50    type Item = (Position, I::Item);
51
52    fn next(&mut self) -> Option<Self::Item> {
53        match self.peekable.next() {
54            Some(item) => {
55                if !self.handled_first {
56                    // Haven't seen the first item yet, and there is one to give.
57                    self.handled_first = true;
58                    // Peek to see if this is also the last item,
59                    // in which case tag it as `Only`.
60                    match self.peekable.peek() {
61                        Some(_) => Some((Position::First, item)),
62                        None => Some((Position::Only, item)),
63                    }
64                } else {
65                    // Have seen the first item, and there's something left.
66                    // Peek to see if this is the last item.
67                    match self.peekable.peek() {
68                        Some(_) => Some((Position::Middle, item)),
69                        None => Some((Position::Last, item)),
70                    }
71                }
72            }
73            // Iterator is finished.
74            None => None,
75        }
76    }
77
78    fn size_hint(&self) -> (usize, Option<usize>) {
79        self.peekable.size_hint()
80    }
81}
82
83impl<I> ExactSizeIterator for WithPosition<I>
84    where I: ExactSizeIterator,
85{ }
86
87impl<I: Iterator> FusedIterator for WithPosition<I> 
88{}
itertools/with_position.rs

itertools/
with_position.rs
