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

use std::iter::Peekable;
use crate::PutBack;
#[cfg(feature = "use_alloc")]
use crate::PutBackN;

/// An iterator that allows peeking at an element before deciding to accept it.
///
/// See [`.peeking_take_while()`](crate::Itertools::peeking_take_while)
/// for more information.
///
/// This is implemented by peeking adaptors like peekable and put back,
/// but also by a few iterators that can be peeked natively, like the slice’s
/// by reference iterator (`std::slice::Iter`).
pub trait PeekingNext : Iterator {
    /// Pass a reference to the next iterator element to the closure `accept`;
    /// if `accept` returns true, return it as the next element,
    /// else None.
    fn peeking_next<F>(&mut self, accept: F) -> Option<Self::Item>
        where F: FnOnce(&Self::Item) -> bool;
}

impl<I> PeekingNext for Peekable<I>
    where I: Iterator,
{
    fn peeking_next<F>(&mut self, accept: F) -> Option<Self::Item>
        where F: FnOnce(&Self::Item) -> bool
    {
        if let Some(r) = self.peek() {
            if !accept(r) {
                return None;
            }
        }
        self.next()
    }
}

impl<I> PeekingNext for PutBack<I>
    where I: Iterator,
{
    fn peeking_next<F>(&mut self, accept: F) -> Option<Self::Item>
        where F: FnOnce(&Self::Item) -> bool
    {
        if let Some(r) = self.next() {
            if !accept(&r) {
                self.put_back(r);
                return None;
            }
            Some(r)
        } else {
            None
        }
    }
}

#[cfg(feature = "use_alloc")]
impl<I> PeekingNext for PutBackN<I>
    where I: Iterator,
{
    fn peeking_next<F>(&mut self, accept: F) -> Option<Self::Item>
        where F: FnOnce(&Self::Item) -> bool
    {
        if let Some(r) = self.next() {
            if !accept(&r) {
                self.put_back(r);
                return None;
            }
            Some(r)
        } else {
            None
        }
    }
}

/// An iterator adaptor that takes items while a closure returns `true`.
///
/// See [`.peeking_take_while()`](crate::Itertools::peeking_take_while)
/// for more information.
#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
pub struct PeekingTakeWhile<'a, I: 'a, F>
    where I: Iterator,
{
    iter: &'a mut I,
    f: F,
}

impl<'a, I: 'a, F> std::fmt::Debug for PeekingTakeWhile<'a, I, F>
where
    I: Iterator + std::fmt::Debug,
{
    debug_fmt_fields!(PeekingTakeWhile, iter);
}

/// Create a `PeekingTakeWhile`
pub fn peeking_take_while<I, F>(iter: &mut I, f: F) -> PeekingTakeWhile<I, F>
    where I: Iterator,
{
    PeekingTakeWhile {
        iter,
        f,
    }
}

impl<'a, I, F> Iterator for PeekingTakeWhile<'a, I, F>
    where I: PeekingNext,
          F: FnMut(&I::Item) -> bool,

{
    type Item = I::Item;
    fn next(&mut self) -> Option<Self::Item> {
        self.iter.peeking_next(&mut self.f)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (0, self.iter.size_hint().1)
    }
}

// Some iterators are so lightweight we can simply clone them to save their
// state and use that for peeking.
macro_rules! peeking_next_by_clone {
    ([$($typarm:tt)*] $type_:ty) => {
        impl<$($typarm)*> PeekingNext for $type_ {
            fn peeking_next<F>(&mut self, accept: F) -> Option<Self::Item>
                where F: FnOnce(&Self::Item) -> bool
            {
                let saved_state = self.clone();
                if let Some(r) = self.next() {
                    if !accept(&r) {
                        *self = saved_state;
                    } else {
                        return Some(r)
                    }
                }
                None
            }
        }
    }
}

peeking_next_by_clone! { ['a, T] ::std::slice::Iter<'a, T> }
peeking_next_by_clone! { ['a] ::std::str::Chars<'a> }
peeking_next_by_clone! { ['a] ::std::str::CharIndices<'a> }
peeking_next_by_clone! { ['a] ::std::str::Bytes<'a> }
peeking_next_by_clone! { ['a, T] ::std::option::Iter<'a, T> }
peeking_next_by_clone! { ['a, T] ::std::result::Iter<'a, T> }
peeking_next_by_clone! { [T] ::std::iter::Empty<T> }
#[cfg(feature = "use_alloc")]
peeking_next_by_clone! { ['a, T] alloc::collections::linked_list::Iter<'a, T> }
#[cfg(feature = "use_alloc")]
peeking_next_by_clone! { ['a, T] alloc::collections::vec_deque::Iter<'a, T> }

// cloning a Rev has no extra overhead; peekable and put backs are never DEI.
peeking_next_by_clone! { [I: Clone + PeekingNext + DoubleEndedIterator]
                         ::std::iter::Rev<I> }