1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
use crate::size_hint;
use crate::PeekingNext;
use alloc::collections::VecDeque;
use std::iter::Fuse;

/// See [`peek_nth()`] for more information.
#[derive(Clone, Debug)]
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
pub struct PeekNth<I>
where
    I: Iterator,
{
    iter: Fuse<I>,
    buf: VecDeque<I::Item>,
}

/// A drop-in replacement for [`std::iter::Peekable`] which adds a `peek_nth`
/// method allowing the user to `peek` at a value several iterations forward
/// without advancing the base iterator.
///
/// This differs from `multipeek` in that subsequent calls to `peek` or
/// `peek_nth` will always return the same value until `next` is called
/// (making `reset_peek` unnecessary).
pub fn peek_nth<I>(iterable: I) -> PeekNth<I::IntoIter>
where
    I: IntoIterator,
{
    PeekNth {
        iter: iterable.into_iter().fuse(),
        buf: VecDeque::new(),
    }
}

impl<I> PeekNth<I>
where
    I: Iterator,
{
    /// Works exactly like the `peek` method in `std::iter::Peekable`
    pub fn peek(&mut self) -> Option<&I::Item> {
        self.peek_nth(0)
    }

    /// Works exactly like the `peek_mut` method in `std::iter::Peekable`
    pub fn peek_mut(&mut self) -> Option<&mut I::Item> {
        self.peek_nth_mut(0)
    }

    /// Returns a reference to the `nth` value without advancing the iterator.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use itertools::peek_nth;
    ///
    /// let xs = vec![1, 2, 3];
    /// let mut iter = peek_nth(xs.into_iter());
    ///
    /// assert_eq!(iter.peek_nth(0), Some(&1));
    /// assert_eq!(iter.next(), Some(1));
    ///
    /// // The iterator does not advance even if we call `peek_nth` multiple times
    /// assert_eq!(iter.peek_nth(0), Some(&2));
    /// assert_eq!(iter.peek_nth(1), Some(&3));
    /// assert_eq!(iter.next(), Some(2));
    ///
    /// // Calling `peek_nth` past the end of the iterator will return `None`
    /// assert_eq!(iter.peek_nth(1), None);
    /// ```
    pub fn peek_nth(&mut self, n: usize) -> Option<&I::Item> {
        let unbuffered_items = (n + 1).saturating_sub(self.buf.len());

        self.buf.extend(self.iter.by_ref().take(unbuffered_items));

        self.buf.get(n)
    }

    /// Returns a mutable reference to the `nth` value without advancing the iterator.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use itertools::peek_nth;
    ///
    /// let xs = vec![1, 2, 3, 4, 5];
    /// let mut iter = peek_nth(xs.into_iter());
    ///
    /// assert_eq!(iter.peek_nth_mut(0), Some(&mut 1));
    /// assert_eq!(iter.next(), Some(1));
    ///
    /// // The iterator does not advance even if we call `peek_nth_mut` multiple times
    /// assert_eq!(iter.peek_nth_mut(0), Some(&mut 2));
    /// assert_eq!(iter.peek_nth_mut(1), Some(&mut 3));
    /// assert_eq!(iter.next(), Some(2));
    ///
    /// // Peek into the iterator and set the value behind the mutable reference.
    /// if let Some(p) = iter.peek_nth_mut(1) {
    ///     assert_eq!(*p, 4);
    ///     *p = 9;
    /// }
    ///
    /// // The value we put in reappears as the iterator continues.
    /// assert_eq!(iter.next(), Some(3));
    /// assert_eq!(iter.next(), Some(9));
    ///
    /// // Calling `peek_nth_mut` past the end of the iterator will return `None`
    /// assert_eq!(iter.peek_nth_mut(1), None);
    /// ```
    pub fn peek_nth_mut(&mut self, n: usize) -> Option<&mut I::Item> {
        let unbuffered_items = (n + 1).saturating_sub(self.buf.len());

        self.buf.extend(self.iter.by_ref().take(unbuffered_items));

        self.buf.get_mut(n)
    }

    /// Works exactly like the `next_if` method in `std::iter::Peekable`
    pub fn next_if(&mut self, func: impl FnOnce(&I::Item) -> bool) -> Option<I::Item> {
        match self.next() {
            Some(item) if func(&item) => Some(item),
            Some(item) => {
                self.buf.push_front(item);
                None
            }
            _ => None,
        }
    }

    /// Works exactly like the `next_if_eq` method in `std::iter::Peekable`
    pub fn next_if_eq<T>(&mut self, expected: &T) -> Option<I::Item>
    where
        T: ?Sized,
        I::Item: PartialEq<T>,
    {
        self.next_if(|next| next == expected)
    }
}

impl<I> Iterator for PeekNth<I>
where
    I: Iterator,
{
    type Item = I::Item;

    fn next(&mut self) -> Option<Self::Item> {
        self.buf.pop_front().or_else(|| self.iter.next())
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        size_hint::add_scalar(self.iter.size_hint(), self.buf.len())
    }
}

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

impl<I> PeekingNext for PeekNth<I>
where
    I: Iterator,
{
    fn peeking_next<F>(&mut self, accept: F) -> Option<Self::Item>
    where
        F: FnOnce(&Self::Item) -> bool,
    {
        self.peek().filter(|item| accept(item))?;
        self.next()
    }
}