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use crate::convert::TryFrom;
use crate::{
intrinsics,
iter::{from_fn, TrustedLen, TrustedRandomAccess},
ops::{Range, Try},
};
/// An iterator for stepping iterators by a custom amount.
///
/// This `struct` is created by the [`step_by`] method on [`Iterator`]. See
/// its documentation for more.
///
/// [`step_by`]: Iterator::step_by
/// [`Iterator`]: trait.Iterator.html
#[must_use = "iterators are lazy and do nothing unless consumed"]
#[stable(feature = "iterator_step_by", since = "1.28.0")]
#[derive(Clone, Debug)]
pub struct StepBy<I> {
/// This field is guaranteed to be preprocessed by the specialized `SpecRangeSetup::setup`
/// in the constructor.
/// For most iterators that processing is a no-op, but for Range<{integer}> types it is lossy
/// which means the inner iterator cannot be returned to user code.
/// Additionally this type-dependent preprocessing means specialized implementations
/// cannot be used interchangeably.
iter: I,
step: usize,
first_take: bool,
}
impl<I> StepBy<I> {
#[inline]
pub(in crate::iter) fn new(iter: I, step: usize) -> StepBy<I> {
assert!(step != 0);
let iter = <I as SpecRangeSetup<I>>::setup(iter, step);
StepBy { iter, step: step - 1, first_take: true }
}
}
#[stable(feature = "iterator_step_by", since = "1.28.0")]
impl<I> Iterator for StepBy<I>
where
I: Iterator,
{
type Item = I::Item;
#[inline]
fn next(&mut self) -> Option<Self::Item> {
self.spec_next()
}
#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
self.spec_size_hint()
}
#[inline]
fn nth(&mut self, n: usize) -> Option<Self::Item> {
self.spec_nth(n)
}
fn try_fold<Acc, F, R>(&mut self, acc: Acc, f: F) -> R
where
F: FnMut(Acc, Self::Item) -> R,
R: Try<Output = Acc>,
{
self.spec_try_fold(acc, f)
}
#[inline]
fn fold<Acc, F>(self, acc: Acc, f: F) -> Acc
where
F: FnMut(Acc, Self::Item) -> Acc,
{
self.spec_fold(acc, f)
}
}
impl<I> StepBy<I>
where
I: ExactSizeIterator,
{
// The zero-based index starting from the end of the iterator of the
// last element. Used in the `DoubleEndedIterator` implementation.
fn next_back_index(&self) -> usize {
let rem = self.iter.len() % (self.step + 1);
if self.first_take { if rem == 0 { self.step } else { rem - 1 } } else { rem }
}
}
#[stable(feature = "double_ended_step_by_iterator", since = "1.38.0")]
impl<I> DoubleEndedIterator for StepBy<I>
where
I: DoubleEndedIterator + ExactSizeIterator,
{
#[inline]
fn next_back(&mut self) -> Option<Self::Item> {
self.spec_next_back()
}
#[inline]
fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
self.spec_nth_back(n)
}
fn try_rfold<Acc, F, R>(&mut self, init: Acc, f: F) -> R
where
F: FnMut(Acc, Self::Item) -> R,
R: Try<Output = Acc>,
{
self.spec_try_rfold(init, f)
}
#[inline]
fn rfold<Acc, F>(self, init: Acc, f: F) -> Acc
where
Self: Sized,
F: FnMut(Acc, Self::Item) -> Acc,
{
self.spec_rfold(init, f)
}
}
// StepBy can only make the iterator shorter, so the len will still fit.
#[stable(feature = "iterator_step_by", since = "1.28.0")]
impl<I> ExactSizeIterator for StepBy<I> where I: ExactSizeIterator {}
// SAFETY: This adapter is shortening. TrustedLen requires the upper bound to be calculated correctly.
// These requirements can only be satisfied when the upper bound of the inner iterator's upper
// bound is never `None`. I: TrustedRandomAccess happens to provide this guarantee while
// I: TrustedLen would not.
// This also covers the Range specializations since the ranges also implement TRA
#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<I> TrustedLen for StepBy<I> where I: Iterator + TrustedRandomAccess {}
trait SpecRangeSetup<T> {
fn setup(inner: T, step: usize) -> T;
}
impl<T> SpecRangeSetup<T> for T {
#[inline]
default fn setup(inner: T, _step: usize) -> T {
inner
}
}
/// Specialization trait to optimize `StepBy<Range<{integer}>>` iteration.
///
/// # Safety
///
/// Technically this is safe to implement (look ma, no unsafe!), but in reality
/// a lot of unsafe code relies on ranges over integers being correct.
///
/// For correctness *all* public StepBy methods must be specialized
/// because `setup` drastically alters the meaning of the struct fields so that mixing
/// different implementations would lead to incorrect results.
unsafe trait StepByImpl<I> {
type Item;
fn spec_next(&mut self) -> Option<Self::Item>;
fn spec_size_hint(&self) -> (usize, Option<usize>);
fn spec_nth(&mut self, n: usize) -> Option<Self::Item>;
fn spec_try_fold<Acc, F, R>(&mut self, acc: Acc, f: F) -> R
where
F: FnMut(Acc, Self::Item) -> R,
R: Try<Output = Acc>;
fn spec_fold<Acc, F>(self, acc: Acc, f: F) -> Acc
where
F: FnMut(Acc, Self::Item) -> Acc;
}
/// Specialization trait for double-ended iteration.
///
/// See also: `StepByImpl`
///
/// # Safety
///
/// The specializations must be implemented together with `StepByImpl`
/// where applicable. I.e. if `StepBy` does support backwards iteration
/// for a given iterator and that is specialized for forward iteration then
/// it must also be specialized for backwards iteration.
unsafe trait StepByBackImpl<I> {
type Item;
fn spec_next_back(&mut self) -> Option<Self::Item>
where
I: DoubleEndedIterator + ExactSizeIterator;
fn spec_nth_back(&mut self, n: usize) -> Option<Self::Item>
where
I: DoubleEndedIterator + ExactSizeIterator;
fn spec_try_rfold<Acc, F, R>(&mut self, init: Acc, f: F) -> R
where
I: DoubleEndedIterator + ExactSizeIterator,
F: FnMut(Acc, Self::Item) -> R,
R: Try<Output = Acc>;
fn spec_rfold<Acc, F>(self, init: Acc, f: F) -> Acc
where
I: DoubleEndedIterator + ExactSizeIterator,
F: FnMut(Acc, Self::Item) -> Acc;
}
unsafe impl<I: Iterator> StepByImpl<I> for StepBy<I> {
type Item = I::Item;
#[inline]
default fn spec_next(&mut self) -> Option<I::Item> {
let step_size = if self.first_take { 0 } else { self.step };
self.first_take = false;
self.iter.nth(step_size)
}
#[inline]
default fn spec_size_hint(&self) -> (usize, Option<usize>) {
#[inline]
fn first_size(step: usize) -> impl Fn(usize) -> usize {
move |n| if n == 0 { 0 } else { 1 + (n - 1) / (step + 1) }
}
#[inline]
fn other_size(step: usize) -> impl Fn(usize) -> usize {
move |n| n / (step + 1)
}
let (low, high) = self.iter.size_hint();
if self.first_take {
let f = first_size(self.step);
(f(low), high.map(f))
} else {
let f = other_size(self.step);
(f(low), high.map(f))
}
}
#[inline]
default fn spec_nth(&mut self, mut n: usize) -> Option<I::Item> {
if self.first_take {
self.first_take = false;
let first = self.iter.next();
if n == 0 {
return first;
}
n -= 1;
}
// n and self.step are indices, we need to add 1 to get the amount of elements
// When calling `.nth`, we need to subtract 1 again to convert back to an index
// step + 1 can't overflow because `.step_by` sets `self.step` to `step - 1`
let mut step = self.step + 1;
// n + 1 could overflow
// thus, if n is usize::MAX, instead of adding one, we call .nth(step)
if n == usize::MAX {
self.iter.nth(step - 1);
} else {
n += 1;
}
// overflow handling
loop {
let mul = n.checked_mul(step);
{
if intrinsics::likely(mul.is_some()) {
return self.iter.nth(mul.unwrap() - 1);
}
}
let div_n = usize::MAX / n;
let div_step = usize::MAX / step;
let nth_n = div_n * n;
let nth_step = div_step * step;
let nth = if nth_n > nth_step {
step -= div_n;
nth_n
} else {
n -= div_step;
nth_step
};
self.iter.nth(nth - 1);
}
}
default fn spec_try_fold<Acc, F, R>(&mut self, mut acc: Acc, mut f: F) -> R
where
F: FnMut(Acc, Self::Item) -> R,
R: Try<Output = Acc>,
{
#[inline]
fn nth<I: Iterator>(iter: &mut I, step: usize) -> impl FnMut() -> Option<I::Item> + '_ {
move || iter.nth(step)
}
if self.first_take {
self.first_take = false;
match self.iter.next() {
None => return try { acc },
Some(x) => acc = f(acc, x)?,
}
}
from_fn(nth(&mut self.iter, self.step)).try_fold(acc, f)
}
default fn spec_fold<Acc, F>(mut self, mut acc: Acc, mut f: F) -> Acc
where
F: FnMut(Acc, Self::Item) -> Acc,
{
#[inline]
fn nth<I: Iterator>(iter: &mut I, step: usize) -> impl FnMut() -> Option<I::Item> + '_ {
move || iter.nth(step)
}
if self.first_take {
self.first_take = false;
match self.iter.next() {
None => return acc,
Some(x) => acc = f(acc, x),
}
}
from_fn(nth(&mut self.iter, self.step)).fold(acc, f)
}
}
unsafe impl<I: DoubleEndedIterator + ExactSizeIterator> StepByBackImpl<I> for StepBy<I> {
type Item = I::Item;
#[inline]
default fn spec_next_back(&mut self) -> Option<Self::Item> {
self.iter.nth_back(self.next_back_index())
}
#[inline]
default fn spec_nth_back(&mut self, n: usize) -> Option<I::Item> {
// `self.iter.nth_back(usize::MAX)` does the right thing here when `n`
// is out of bounds because the length of `self.iter` does not exceed
// `usize::MAX` (because `I: ExactSizeIterator`) and `nth_back` is
// zero-indexed
let n = n.saturating_mul(self.step + 1).saturating_add(self.next_back_index());
self.iter.nth_back(n)
}
default fn spec_try_rfold<Acc, F, R>(&mut self, init: Acc, mut f: F) -> R
where
F: FnMut(Acc, Self::Item) -> R,
R: Try<Output = Acc>,
{
#[inline]
fn nth_back<I: DoubleEndedIterator>(
iter: &mut I,
step: usize,
) -> impl FnMut() -> Option<I::Item> + '_ {
move || iter.nth_back(step)
}
match self.next_back() {
None => try { init },
Some(x) => {
let acc = f(init, x)?;
from_fn(nth_back(&mut self.iter, self.step)).try_fold(acc, f)
}
}
}
#[inline]
default fn spec_rfold<Acc, F>(mut self, init: Acc, mut f: F) -> Acc
where
Self: Sized,
F: FnMut(Acc, I::Item) -> Acc,
{
#[inline]
fn nth_back<I: DoubleEndedIterator>(
iter: &mut I,
step: usize,
) -> impl FnMut() -> Option<I::Item> + '_ {
move || iter.nth_back(step)
}
match self.next_back() {
None => init,
Some(x) => {
let acc = f(init, x);
from_fn(nth_back(&mut self.iter, self.step)).fold(acc, f)
}
}
}
}
/// For these implementations, `SpecRangeSetup` calculates the number
/// of iterations that will be needed and stores that in `iter.end`.
///
/// The various iterator implementations then rely on that to not need
/// overflow checking, letting loops just be counted instead.
///
/// These only work for unsigned types, and will need to be reworked
/// if you want to use it to specialize on signed types.
///
/// Currently these are only implemented for integers up to usize due to
/// correctness issues around ExactSizeIterator impls on 16bit platforms.
/// And since ExactSizeIterator is a prerequisite for backwards iteration
/// and we must consistently specialize backwards and forwards iteration
/// that makes the situation complicated enough that it's not covered
/// for now.
macro_rules! spec_int_ranges {
($($t:ty)*) => ($(
const _: () = assert!(usize::BITS >= <$t>::BITS);
impl SpecRangeSetup<Range<$t>> for Range<$t> {
#[inline]
fn setup(mut r: Range<$t>, step: usize) -> Range<$t> {
let inner_len = r.size_hint().0;
// If step exceeds $t::MAX, then the count will be at most 1 and
// thus always fit into $t.
let yield_count = inner_len.div_ceil(step);
// Turn the range end into an iteration counter
r.end = yield_count as $t;
r
}
}
unsafe impl StepByImpl<Range<$t>> for StepBy<Range<$t>> {
#[inline]
fn spec_next(&mut self) -> Option<$t> {
// if a step size larger than the type has been specified fall back to
// t::MAX, in which case remaining will be at most 1.
// The `+ 1` can't overflow since the constructor substracted 1 from the original value.
let step = <$t>::try_from(self.step + 1).unwrap_or(<$t>::MAX);
let remaining = self.iter.end;
if remaining > 0 {
let val = self.iter.start;
// this can only overflow during the last step, after which the value
// will not be used
self.iter.start = val.wrapping_add(step);
self.iter.end = remaining - 1;
Some(val)
} else {
None
}
}
#[inline]
fn spec_size_hint(&self) -> (usize, Option<usize>) {
let remaining = self.iter.end as usize;
(remaining, Some(remaining))
}
// The methods below are all copied from the Iterator trait default impls.
// We have to repeat them here so that the specialization overrides the StepByImpl defaults
#[inline]
fn spec_nth(&mut self, n: usize) -> Option<Self::Item> {
self.advance_by(n).ok()?;
self.next()
}
#[inline]
fn spec_try_fold<Acc, F, R>(&mut self, init: Acc, mut f: F) -> R
where
F: FnMut(Acc, Self::Item) -> R,
R: Try<Output = Acc>
{
let mut accum = init;
while let Some(x) = self.next() {
accum = f(accum, x)?;
}
try { accum }
}
#[inline]
fn spec_fold<Acc, F>(self, init: Acc, mut f: F) -> Acc
where
F: FnMut(Acc, Self::Item) -> Acc
{
// if a step size larger than the type has been specified fall back to
// t::MAX, in which case remaining will be at most 1.
let step = <$t>::try_from(self.step + 1).unwrap_or(<$t>::MAX);
let remaining = self.iter.end;
let mut acc = init;
let mut val = self.iter.start;
for _ in 0..remaining {
acc = f(acc, val);
// this can only overflow during the last step, after which the value
// will no longer be used
val = val.wrapping_add(step);
}
acc
}
}
)*)
}
macro_rules! spec_int_ranges_r {
($($t:ty)*) => ($(
const _: () = assert!(usize::BITS >= <$t>::BITS);
unsafe impl StepByBackImpl<Range<$t>> for StepBy<Range<$t>> {
#[inline]
fn spec_next_back(&mut self) -> Option<Self::Item>
where Range<$t>: DoubleEndedIterator + ExactSizeIterator,
{
let step = (self.step + 1) as $t;
let remaining = self.iter.end;
if remaining > 0 {
let start = self.iter.start;
self.iter.end = remaining - 1;
Some(start + step * (remaining - 1))
} else {
None
}
}
// The methods below are all copied from the Iterator trait default impls.
// We have to repeat them here so that the specialization overrides the StepByImplBack defaults
#[inline]
fn spec_nth_back(&mut self, n: usize) -> Option<Self::Item>
where Self: DoubleEndedIterator,
{
if self.advance_back_by(n).is_err() {
return None;
}
self.next_back()
}
#[inline]
fn spec_try_rfold<Acc, F, R>(&mut self, init: Acc, mut f: F) -> R
where
Self: DoubleEndedIterator,
F: FnMut(Acc, Self::Item) -> R,
R: Try<Output = Acc>
{
let mut accum = init;
while let Some(x) = self.next_back() {
accum = f(accum, x)?;
}
try { accum }
}
#[inline]
fn spec_rfold<Acc, F>(mut self, init: Acc, mut f: F) -> Acc
where
Self: DoubleEndedIterator,
F: FnMut(Acc, Self::Item) -> Acc
{
let mut accum = init;
while let Some(x) = self.next_back() {
accum = f(accum, x);
}
accum
}
}
)*)
}
#[cfg(target_pointer_width = "64")]
spec_int_ranges!(u8 u16 u32 u64 usize);
// DoubleEndedIterator requires ExactSizeIterator, which isn't implemented for Range<u64>
#[cfg(target_pointer_width = "64")]
spec_int_ranges_r!(u8 u16 u32 usize);
#[cfg(target_pointer_width = "32")]
spec_int_ranges!(u8 u16 u32 usize);
#[cfg(target_pointer_width = "32")]
spec_int_ranges_r!(u8 u16 u32 usize);
#[cfg(target_pointer_width = "16")]
spec_int_ranges!(u8 u16 usize);
#[cfg(target_pointer_width = "16")]
spec_int_ranges_r!(u8 u16 usize);