/rust/registry/src/index.crates.io-1949cf8c6b5b557f/itertools-0.12.1/src/with_position.rs

Source
use std::fmt;
use std::iter::{Fuse, FusedIterator, Peekable};

/// An iterator adaptor that wraps each element in an [`Position`].
///
/// Iterator element type is `(Position, I::Item)`.
///
/// See [`.with_position()`](crate::Itertools::with_position) for more information.
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
pub struct WithPosition<I>
where
    I: Iterator,
{
    handled_first: bool,
    peekable: Peekable<Fuse<I>>,
}

impl<I> fmt::Debug for WithPosition<I>
where
    I: Iterator,
    Peekable<Fuse<I>>: fmt::Debug,
{
    debug_fmt_fields!(WithPosition, handled_first, peekable);
}

impl<I> Clone for WithPosition<I>
where
    I: Clone + Iterator,
    I::Item: Clone,
{
    clone_fields!(handled_first, peekable);
}

/// Create a new `WithPosition` iterator.
pub fn with_position<I>(iter: I) -> WithPosition<I>
where
    I: Iterator,
{
    WithPosition {
        handled_first: false,
        peekable: iter.fuse().peekable(),
    }
}

/// The first component of the value yielded by `WithPosition`.
/// Indicates the position of this element in the iterator results.
///
/// See [`.with_position()`](crate::Itertools::with_position) for more information.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum Position {
    /// This is the first element.
    First,
    /// This is neither the first nor the last element.
    Middle,
    /// This is the last element.
    Last,
    /// This is the only element.
    Only,
}

impl<I: Iterator> Iterator for WithPosition<I> {
    type Item = (Position, I::Item);

    fn next(&mut self) -> Option<Self::Item> {
        match self.peekable.next() {
            Some(item) => {
                if !self.handled_first {
                    // Haven't seen the first item yet, and there is one to give.
                    self.handled_first = true;
                    // Peek to see if this is also the last item,
                    // in which case tag it as `Only`.
                    match self.peekable.peek() {
                        Some(_) => Some((Position::First, item)),
                        None => Some((Position::Only, item)),
                    }
                } else {
                    // Have seen the first item, and there's something left.
                    // Peek to see if this is the last item.
                    match self.peekable.peek() {
                        Some(_) => Some((Position::Middle, item)),
                        None => Some((Position::Last, item)),
                    }
                }
            }
            // Iterator is finished.
            None => None,
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.peekable.size_hint()
    }

    fn fold<B, F>(mut self, mut init: B, mut f: F) -> B
    where
        F: FnMut(B, Self::Item) -> B,
    {
        if let Some(mut head) = self.peekable.next() {
            if !self.handled_first {
                // The current head is `First` or `Only`,
                // it depends if there is another item or not.
                match self.peekable.next() {
                    Some(second) => {
                        let first = std::mem::replace(&mut head, second);
                        init = f(init, (Position::First, first));
                    }
                    None => return f(init, (Position::Only, head)),
                }
            }
            // Have seen the first item, and there's something left.
            init = self.peekable.fold(init, |acc, mut item| {
                std::mem::swap(&mut head, &mut item);
                f(acc, (Position::Middle, item))
            });
            // The "head" is now the last item.
            init = f(init, (Position::Last, head));
        }
        init
    }
}

impl<I> ExactSizeIterator for WithPosition<I> where I: ExactSizeIterator {}

impl<I: Iterator> FusedIterator for WithPosition<I> {}

Line	Count	Source
1		use std::fmt;
2		use 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"]
10		pub struct WithPosition<I>
11		where
12		I: Iterator,
13		{
14		handled_first: bool,
15		peekable: Peekable<Fuse<I>>,
16		}
17
18		impl<I> fmt::Debug for WithPosition<I>
19		where
20		I: Iterator,
21		Peekable<Fuse<I>>: fmt::Debug,
22		{
23		debug_fmt_fields!(WithPosition, handled_first, peekable);
24		}
25
26		impl<I> Clone for WithPosition<I>
27		where
28		I: Clone + Iterator,
29		I::Item: Clone,
30		{
31		clone_fields!(handled_first, peekable);
32		}
33
34		/// Create a new `WithPosition` iterator.
35	0	pub fn with_position<I>(iter: I) -> WithPosition<I>
36	0	where
37	0	I: Iterator,
38		{
39	0	WithPosition {
40	0	handled_first: false,
41	0	peekable: iter.fuse().peekable(),
42	0	}
43	0	}
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)]
50		pub 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
61		impl<I: Iterator> Iterator for WithPosition<I> {
62		type Item = (Position, I::Item);
63
64	0	fn next(&mut self) -> Option<Self::Item> {
65	0	match self.peekable.next() {
66	0	Some(item) => {
67	0	if !self.handled_first {
68		// Haven't seen the first item yet, and there is one to give.
69	0	self.handled_first = true;
70		// Peek to see if this is also the last item,
71		// in which case tag it as `Only`.
72	0	match self.peekable.peek() {
73	0	Some(_) => Some((Position::First, item)),
74	0	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	0	match self.peekable.peek() {
80	0	Some(_) => Some((Position::Middle, item)),
81	0	None => Some((Position::Last, item)),
82		}
83		}
84		}
85		// Iterator is finished.
86	0	None => None,
87		}
88	0	}
89
90	0	fn size_hint(&self) -> (usize, Option<usize>) {
91	0	self.peekable.size_hint()
92	0	}
93
94	0	fn fold<B, F>(mut self, mut init: B, mut f: F) -> B
95	0	where
96	0	F: FnMut(B, Self::Item) -> B,
97		{
98	0	if let Some(mut head) = self.peekable.next() {
99	0	if !self.handled_first {
100		// The current head is `First` or `Only`,
101		// it depends if there is another item or not.
102	0	match self.peekable.next() {
103	0	Some(second) => {
104	0	let first = std::mem::replace(&mut head, second);
105	0	init = f(init, (Position::First, first));
106	0	}
107	0	None => return f(init, (Position::Only, head)),
108		}
109	0	}
110		// Have seen the first item, and there's something left.
111	0	init = self.peekable.fold(init, \|acc, mut item\| {
112	0	std::mem::swap(&mut head, &mut item);
113	0	f(acc, (Position::Middle, item))
114	0	});
115		// The "head" is now the last item.
116	0	init = f(init, (Position::Last, head));
117	0	}
118	0	init
119	0	}
120		}
121
122		impl<I> ExactSizeIterator for WithPosition<I> where I: ExactSizeIterator {}
123
124		impl<I: Iterator> FusedIterator for WithPosition<I> {}

Coverage Report

Created: 2025-10-28 08:03