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
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
use crate::fmt;
use crate::iter::adapters::{
    zip::try_get_unchecked, SourceIter, TrustedRandomAccess, TrustedRandomAccessNoCoerce,
};
use crate::iter::{FusedIterator, InPlaceIterable, TrustedLen};
use crate::ops::Try;

/// An iterator that maps the values of `iter` with `f`.
///
/// This `struct` is created by the [`map`] method on [`Iterator`]. See its
/// documentation for more.
///
/// [`map`]: Iterator::map
/// [`Iterator`]: trait.Iterator.html
///
/// # Notes about side effects
///
/// The [`map`] iterator implements [`DoubleEndedIterator`], meaning that
/// you can also [`map`] backwards:
///
/// ```rust
/// let v: Vec<i32> = [1, 2, 3].into_iter().map(|x| x + 1).rev().collect();
///
/// assert_eq!(v, [4, 3, 2]);
/// ```
///
/// [`DoubleEndedIterator`]: trait.DoubleEndedIterator.html
///
/// But if your closure has state, iterating backwards may act in a way you do
/// not expect. Let's go through an example. First, in the forward direction:
///
/// ```rust
/// let mut c = 0;
///
/// for pair in ['a', 'b', 'c'].into_iter()
///                                .map(|letter| { c += 1; (letter, c) }) {
///     println!("{pair:?}");
/// }
/// ```
///
/// This will print `('a', 1), ('b', 2), ('c', 3)`.
///
/// Now consider this twist where we add a call to `rev`. This version will
/// print `('c', 1), ('b', 2), ('a', 3)`. Note that the letters are reversed,
/// but the values of the counter still go in order. This is because `map()` is
/// still being called lazily on each item, but we are popping items off the
/// back of the vector now, instead of shifting them from the front.
///
/// ```rust
/// let mut c = 0;
///
/// for pair in ['a', 'b', 'c'].into_iter()
///                                .map(|letter| { c += 1; (letter, c) })
///                                .rev() {
///     println!("{pair:?}");
/// }
/// ```
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "rust1", since = "1.0.0")]
#[derive(Clone)]
pub struct Map<I, F> {
    // Used for `SplitWhitespace` and `SplitAsciiWhitespace` `as_str` methods
    pub(crate) iter: I,
    f: F,
}

impl<I, F> Map<I, F> {
    pub(in crate::iter) fn new(iter: I, f: F) -> Map<I, F> {
        Map { iter, f }
    }
}

#[stable(feature = "core_impl_debug", since = "1.9.0")]
impl<I: fmt::Debug, F> fmt::Debug for Map<I, F> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Map").field("iter", &self.iter).finish()
    }
}

fn map_fold<T, B, Acc>(
    mut f: impl FnMut(T) -> B,
    mut g: impl FnMut(Acc, B) -> Acc,
) -> impl FnMut(Acc, T) -> Acc {
    move |acc, elt| g(acc, f(elt))
}

fn map_try_fold<'a, T, B, Acc, R>(
    f: &'a mut impl FnMut(T) -> B,
    mut g: impl FnMut(Acc, B) -> R + 'a,
) -> impl FnMut(Acc, T) -> R + 'a {
    move |acc, elt| g(acc, f(elt))
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I: Iterator, F> Iterator for Map<I, F>
where
    F: FnMut(I::Item) -> B,
{
    type Item = B;

    #[inline]
    fn next(&mut self) -> Option<B> {
        self.iter.next().map(&mut self.f)
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }

    fn try_fold<Acc, G, R>(&mut self, init: Acc, g: G) -> R
    where
        Self: Sized,
        G: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>,
    {
        self.iter.try_fold(init, map_try_fold(&mut self.f, g))
    }

    fn fold<Acc, G>(self, init: Acc, g: G) -> Acc
    where
        G: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.fold(init, map_fold(self.f, g))
    }

    #[inline]
    unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> B
    where
        Self: TrustedRandomAccessNoCoerce,
    {
        // SAFETY: the caller must uphold the contract for
        // `Iterator::__iterator_get_unchecked`.
        unsafe { (self.f)(try_get_unchecked(&mut self.iter, idx)) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for Map<I, F>
where
    F: FnMut(I::Item) -> B,
{
    #[inline]
    fn next_back(&mut self) -> Option<B> {
        self.iter.next_back().map(&mut self.f)
    }

    fn try_rfold<Acc, G, R>(&mut self, init: Acc, g: G) -> R
    where
        Self: Sized,
        G: FnMut(Acc, Self::Item) -> R,
        R: Try<Output = Acc>,
    {
        self.iter.try_rfold(init, map_try_fold(&mut self.f, g))
    }

    fn rfold<Acc, G>(self, init: Acc, g: G) -> Acc
    where
        G: FnMut(Acc, Self::Item) -> Acc,
    {
        self.iter.rfold(init, map_fold(self.f, g))
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<B, I: ExactSizeIterator, F> ExactSizeIterator for Map<I, F>
where
    F: FnMut(I::Item) -> B,
{
    fn len(&self) -> usize {
        self.iter.len()
    }

    fn is_empty(&self) -> bool {
        self.iter.is_empty()
    }
}

#[stable(feature = "fused", since = "1.26.0")]
impl<B, I: FusedIterator, F> FusedIterator for Map<I, F> where F: FnMut(I::Item) -> B {}

#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<B, I, F> TrustedLen for Map<I, F>
where
    I: TrustedLen,
    F: FnMut(I::Item) -> B,
{
}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<I, F> TrustedRandomAccess for Map<I, F> where I: TrustedRandomAccess {}

#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<I, F> TrustedRandomAccessNoCoerce for Map<I, F>
where
    I: TrustedRandomAccessNoCoerce,
{
    const MAY_HAVE_SIDE_EFFECT: bool = true;
}

#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<I, F> SourceIter for Map<I, F>
where
    I: SourceIter,
{
    type Source = I::Source;

    #[inline]
    unsafe fn as_inner(&mut self) -> &mut I::Source {
        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
        unsafe { SourceIter::as_inner(&mut self.iter) }
    }
}

#[unstable(issue = "none", feature = "inplace_iteration")]
unsafe impl<B, I: InPlaceIterable, F> InPlaceIterable for Map<I, F> where F: FnMut(I::Item) -> B {}