core/iter/adapters/
filter_map.rs

1use crate::iter::adapters::SourceIter;
2use crate::iter::{FusedIterator, InPlaceIterable, TrustedFused};
3use crate::mem::{ManuallyDrop, MaybeUninit};
4use crate::num::NonZero;
5use crate::ops::{ControlFlow, Try};
6use crate::{array, fmt};
7
8/// An iterator that uses `f` to both filter and map elements from `iter`.
9///
10/// This `struct` is created by the [`filter_map`] method on [`Iterator`]. See its
11/// documentation for more.
12///
13/// [`filter_map`]: Iterator::filter_map
14/// [`Iterator`]: trait.Iterator.html
15#[must_use = "iterators are lazy and do nothing unless consumed"]
16#[stable(feature = "rust1", since = "1.0.0")]
17#[derive(Clone)]
18pub struct FilterMap<I, F> {
19    iter: I,
20    f: F,
21}
22impl<I, F> FilterMap<I, F> {
23    pub(in crate::iter) fn new(iter: I, f: F) -> FilterMap<I, F> {
24        FilterMap { iter, f }
25    }
26}
27
28#[stable(feature = "core_impl_debug", since = "1.9.0")]
29impl<I: fmt::Debug, F> fmt::Debug for FilterMap<I, F> {
30    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
31        f.debug_struct("FilterMap").field("iter", &self.iter).finish()
32    }
33}
34
35fn filter_map_fold<T, B, Acc>(
36    mut f: impl FnMut(T) -> Option<B>,
37    mut fold: impl FnMut(Acc, B) -> Acc,
38) -> impl FnMut(Acc, T) -> Acc {
39    move |acc, item| match f(item) {
40        Some(x) => fold(acc, x),
41        None => acc,
42    }
43}
44
45fn filter_map_try_fold<'a, T, B, Acc, R: Try<Output = Acc>>(
46    f: &'a mut impl FnMut(T) -> Option<B>,
47    mut fold: impl FnMut(Acc, B) -> R + 'a,
48) -> impl FnMut(Acc, T) -> R + 'a {
49    move |acc, item| match f(item) {
50        Some(x) => fold(acc, x),
51        None => try { acc },
52    }
53}
54
55#[stable(feature = "rust1", since = "1.0.0")]
56impl<B, I: Iterator, F> Iterator for FilterMap<I, F>
57where
58    F: FnMut(I::Item) -> Option<B>,
59{
60    type Item = B;
61
62    #[inline]
63    fn next(&mut self) -> Option<B> {
64        self.iter.find_map(&mut self.f)
65    }
66
67    #[inline]
68    fn next_chunk<const N: usize>(
69        &mut self,
70    ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>> {
71        let mut array: [MaybeUninit<Self::Item>; N] = [const { MaybeUninit::uninit() }; N];
72
73        struct Guard<'a, T> {
74            array: &'a mut [MaybeUninit<T>],
75            initialized: usize,
76        }
77
78        impl<T> Drop for Guard<'_, T> {
79            #[inline]
80            fn drop(&mut self) {
81                if const { crate::mem::needs_drop::<T>() } {
82                    // SAFETY: self.initialized is always <= N, which also is the length of the array.
83                    unsafe {
84                        self.array.get_unchecked_mut(..self.initialized).assume_init_drop();
85                    }
86                }
87            }
88        }
89
90        let mut guard = Guard { array: &mut array, initialized: 0 };
91
92        let result = self.iter.try_for_each(|element| {
93            let idx = guard.initialized;
94            let val = (self.f)(element);
95            guard.initialized = idx + val.is_some() as usize;
96
97            // SAFETY: Loop conditions ensure the index is in bounds.
98
99            unsafe {
100                let opt_payload_at: *const MaybeUninit<B> =
101                    (&raw const val).byte_add(core::mem::offset_of!(Option<B>, Some.0)).cast();
102                let dst = guard.array.as_mut_ptr().add(idx);
103                crate::ptr::copy_nonoverlapping(opt_payload_at, dst, 1);
104                crate::mem::forget(val);
105            };
106
107            if guard.initialized < N { ControlFlow::Continue(()) } else { ControlFlow::Break(()) }
108        });
109
110        let guard = ManuallyDrop::new(guard);
111
112        match result {
113            ControlFlow::Break(()) => {
114                // SAFETY: The loop above is only explicitly broken when the array has been fully initialized
115                Ok(unsafe { MaybeUninit::array_assume_init(array) })
116            }
117            ControlFlow::Continue(()) => {
118                let initialized = guard.initialized;
119                // SAFETY: The range is in bounds since the loop breaks when reaching N elements.
120                Err(unsafe { array::IntoIter::new_unchecked(array, 0..initialized) })
121            }
122        }
123    }
124
125    #[inline]
126    fn size_hint(&self) -> (usize, Option<usize>) {
127        let (_, upper) = self.iter.size_hint();
128        (0, upper) // can't know a lower bound, due to the predicate
129    }
130
131    #[inline]
132    fn try_fold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
133    where
134        Self: Sized,
135        Fold: FnMut(Acc, Self::Item) -> R,
136        R: Try<Output = Acc>,
137    {
138        self.iter.try_fold(init, filter_map_try_fold(&mut self.f, fold))
139    }
140
141    #[inline]
142    fn fold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
143    where
144        Fold: FnMut(Acc, Self::Item) -> Acc,
145    {
146        self.iter.fold(init, filter_map_fold(self.f, fold))
147    }
148}
149
150#[stable(feature = "rust1", since = "1.0.0")]
151impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for FilterMap<I, F>
152where
153    F: FnMut(I::Item) -> Option<B>,
154{
155    #[inline]
156    fn next_back(&mut self) -> Option<B> {
157        #[inline]
158        fn find<T, B>(
159            f: &mut impl FnMut(T) -> Option<B>,
160        ) -> impl FnMut((), T) -> ControlFlow<B> + '_ {
161            move |(), x| match f(x) {
162                Some(x) => ControlFlow::Break(x),
163                None => ControlFlow::Continue(()),
164            }
165        }
166
167        self.iter.try_rfold((), find(&mut self.f)).break_value()
168    }
169
170    #[inline]
171    fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R
172    where
173        Self: Sized,
174        Fold: FnMut(Acc, Self::Item) -> R,
175        R: Try<Output = Acc>,
176    {
177        self.iter.try_rfold(init, filter_map_try_fold(&mut self.f, fold))
178    }
179
180    #[inline]
181    fn rfold<Acc, Fold>(self, init: Acc, fold: Fold) -> Acc
182    where
183        Fold: FnMut(Acc, Self::Item) -> Acc,
184    {
185        self.iter.rfold(init, filter_map_fold(self.f, fold))
186    }
187}
188
189#[stable(feature = "fused", since = "1.26.0")]
190impl<B, I: FusedIterator, F> FusedIterator for FilterMap<I, F> where F: FnMut(I::Item) -> Option<B> {}
191
192#[unstable(issue = "none", feature = "trusted_fused")]
193unsafe impl<I: TrustedFused, F> TrustedFused for FilterMap<I, F> {}
194
195#[unstable(issue = "none", feature = "inplace_iteration")]
196unsafe impl<I, F> SourceIter for FilterMap<I, F>
197where
198    I: SourceIter,
199{
200    type Source = I::Source;
201
202    #[inline]
203    unsafe fn as_inner(&mut self) -> &mut I::Source {
204        // SAFETY: unsafe function forwarding to unsafe function with the same requirements
205        unsafe { SourceIter::as_inner(&mut self.iter) }
206    }
207}
208
209#[unstable(issue = "none", feature = "inplace_iteration")]
210unsafe impl<I: InPlaceIterable, F> InPlaceIterable for FilterMap<I, F> {
211    const EXPAND_BY: Option<NonZero<usize>> = I::EXPAND_BY;
212    const MERGE_BY: Option<NonZero<usize>> = I::MERGE_BY;
213}
core/iter/adapters/filter_map.rs

core/iter/adapters/
filter_map.rs
