From f9259d1b73f01c4c690dfd7429bb3ce25a34874f Mon Sep 17 00:00:00 2001 From: Yuri Astrakhan Date: Tue, 9 May 2023 01:04:32 -0400 Subject: [PATCH] Boost intersperse(_with) performance I did some benchmark digging into the `intersperse` and `intersperse_with` code as part of the https://internals.rust-lang.org/t/add-iterate-with-separators-iterator-function/18781/13 discussion, and as a result I optimized them a bit, without relying on the peekable iterator. --- library/core/src/iter/adapters/intersperse.rs | 126 ++++++++++-------- 1 file changed, 74 insertions(+), 52 deletions(-) diff --git a/library/core/src/iter/adapters/intersperse.rs b/library/core/src/iter/adapters/intersperse.rs index d8bbd424cf2..654e3077771 100644 --- a/library/core/src/iter/adapters/intersperse.rs +++ b/library/core/src/iter/adapters/intersperse.rs @@ -1,4 +1,5 @@ -use super::Peekable; +use core::fmt; +use core::iter::{Fuse, FusedIterator}; /// An iterator adapter that places a separator between all elements. /// @@ -11,8 +12,16 @@ where I::Item: Clone, { separator: I::Item, - iter: Peekable, - needs_sep: bool, + next_item: Option, + iter: Fuse, +} + +#[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")] +impl FusedIterator for Intersperse +where + I: FusedIterator, + I::Item: Clone, +{ } impl Intersperse @@ -20,7 +29,8 @@ where I::Item: Clone, { pub(in crate::iter) fn new(iter: I, separator: I::Item) -> Self { - Self { iter: iter.peekable(), separator, needs_sep: false } + let mut iter = iter.fuse(); + Self { separator, next_item: iter.next(), iter } } } @@ -33,27 +43,31 @@ where type Item = I::Item; #[inline] - fn next(&mut self) -> Option { - if self.needs_sep && self.iter.peek().is_some() { - self.needs_sep = false; - Some(self.separator.clone()) + fn next(&mut self) -> Option { + if let Some(v) = self.next_item.take() { + Some(v) } else { - self.needs_sep = true; - self.iter.next() + let next_item = self.iter.next(); + if next_item.is_some() { + self.next_item = next_item; + Some(self.separator.clone()) + } else { + None + } } } + fn size_hint(&self) -> (usize, Option) { + intersperse_size_hint(&self.iter, self.next_item.is_some()) + } + fn fold(self, init: B, f: F) -> B where Self: Sized, F: FnMut(B, Self::Item) -> B, { let separator = self.separator; - intersperse_fold(self.iter, init, f, move || separator.clone(), self.needs_sep) - } - - fn size_hint(&self) -> (usize, Option) { - intersperse_size_hint(&self.iter, self.needs_sep) + intersperse_fold(self.iter, init, f, move || separator.clone(), self.next_item) } } @@ -67,38 +81,46 @@ where I: Iterator, { separator: G, - iter: Peekable, - needs_sep: bool, + next_item: Option, + iter: Fuse, } #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")] -impl crate::fmt::Debug for IntersperseWith +impl FusedIterator for IntersperseWith where - I: Iterator + crate::fmt::Debug, - I::Item: crate::fmt::Debug, - G: crate::fmt::Debug, + I: FusedIterator, + G: FnMut() -> I::Item, { - fn fmt(&self, f: &mut crate::fmt::Formatter<'_>) -> crate::fmt::Result { +} + +#[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")] +impl fmt::Debug for IntersperseWith +where + I: Iterator + fmt::Debug, + I::Item: fmt::Debug, + G: fmt::Debug, +{ + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { f.debug_struct("IntersperseWith") .field("separator", &self.separator) .field("iter", &self.iter) - .field("needs_sep", &self.needs_sep) + .field("next_item", &self.next_item) .finish() } } #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")] -impl crate::clone::Clone for IntersperseWith +impl Clone for IntersperseWith where - I: Iterator + crate::clone::Clone, - I::Item: crate::clone::Clone, + I: Iterator + Clone, + I::Item: Clone, G: Clone, { fn clone(&self) -> Self { - IntersperseWith { + Self { separator: self.separator.clone(), iter: self.iter.clone(), - needs_sep: self.needs_sep.clone(), + next_item: self.next_item.clone(), } } } @@ -109,7 +131,8 @@ where G: FnMut() -> I::Item, { pub(in crate::iter) fn new(iter: I, separator: G) -> Self { - Self { iter: iter.peekable(), separator, needs_sep: false } + let mut iter = iter.fuse(); + Self { separator, next_item: iter.next(), iter } } } @@ -122,47 +145,50 @@ where type Item = I::Item; #[inline] - fn next(&mut self) -> Option { - if self.needs_sep && self.iter.peek().is_some() { - self.needs_sep = false; - Some((self.separator)()) + fn next(&mut self) -> Option { + if let Some(v) = self.next_item.take() { + Some(v) } else { - self.needs_sep = true; - self.iter.next() + let next_item = self.iter.next(); + if next_item.is_some() { + self.next_item = next_item; + Some((self.separator)()) + } else { + None + } } } + fn size_hint(&self) -> (usize, Option) { + intersperse_size_hint(&self.iter, self.next_item.is_some()) + } + fn fold(self, init: B, f: F) -> B where Self: Sized, F: FnMut(B, Self::Item) -> B, { - intersperse_fold(self.iter, init, f, self.separator, self.needs_sep) - } - - fn size_hint(&self) -> (usize, Option) { - intersperse_size_hint(&self.iter, self.needs_sep) + intersperse_fold(self.iter, init, f, self.separator, self.next_item) } } -fn intersperse_size_hint(iter: &I, needs_sep: bool) -> (usize, Option) +fn intersperse_size_hint(iter: &I, next_is_elem: bool) -> (usize, Option) where I: Iterator, { let (lo, hi) = iter.size_hint(); - let next_is_elem = !needs_sep; ( - lo.saturating_sub(next_is_elem as usize).saturating_add(lo), - hi.and_then(|hi| hi.saturating_sub(next_is_elem as usize).checked_add(hi)), + lo.saturating_add(next_is_elem as usize).saturating_add(lo), + hi.map(|hi| hi.saturating_add(next_is_elem as usize).saturating_add(hi)), ) } fn intersperse_fold( - mut iter: I, + iter: I, init: B, mut f: F, mut separator: G, - needs_sep: bool, + mut next_item: Option, ) -> B where I: Iterator, @@ -171,12 +197,8 @@ where { let mut accum = init; - if !needs_sep { - if let Some(x) = iter.next() { - accum = f(accum, x); - } else { - return accum; - } + if let Some(x) = next_item.take() { + accum = f(accum, x); } iter.fold(accum, |mut accum, x| {