From 994e712162af1d568a1751c32626478f2c613cc2 Mon Sep 17 00:00:00 2001 From: Michal Nazarewicz Date: Mon, 16 Jan 2023 17:56:01 +0100 Subject: [PATCH] Implement DoubleEnded and ExactSize for Take and Take MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Repeat iterator always returns the same element and behaves the same way backwards and forwards. Take iterator can trivially implement backwards iteration over Repeat inner iterator by simply doing forwards iteration. DoubleEndedIterator is not currently implemented for Take> because Repeat doesn’t implement ExactSizeIterator which is a required bound on DEI implementation for Take. Similarly, since Repeat is an infinite iterator which never stops, Take can trivially know how many elements it’s going to return. This allows implementing ExactSizeIterator on Take>. While at it, observe that ExactSizeIterator can also be implemented for Take> so add that implementation too. Since in contrast to Repeat, RepeatWhile doesn’t guarante to always return the same value, DoubleEndedIterator isn’t implemented. Those changes render core::iter::repeat_n somewhat redundant. Issue: https://github.com/rust-lang/rust/issues/104434 Issue: https://github.com/rust-lang/rust/issues/104729 --- library/core/src/iter/adapters/take.rs | 57 +++++++++++++++ library/core/tests/iter/adapters/take.rs | 90 ++++++++++++++++++++++++ 2 files changed, 147 insertions(+) diff --git a/library/core/src/iter/adapters/take.rs b/library/core/src/iter/adapters/take.rs index 297dd0acaddc1..4c8f9fe16da0f 100644 --- a/library/core/src/iter/adapters/take.rs +++ b/library/core/src/iter/adapters/take.rs @@ -317,3 +317,60 @@ impl SpecTake for Take { } } } + +#[stable(feature = "exact_size_take_repeat", since = "CURRENT_RUSTC_VERSION")] +impl DoubleEndedIterator for Take> { + #[inline] + fn next_back(&mut self) -> Option { + self.next() + } + + #[inline] + fn nth_back(&mut self, n: usize) -> Option { + self.nth(n) + } + + #[inline] + fn try_rfold(&mut self, init: Acc, fold: Fold) -> R + where + Self: Sized, + Fold: FnMut(Acc, Self::Item) -> R, + R: Try, + { + self.try_fold(init, fold) + } + + #[inline] + fn rfold(self, init: Acc, fold: Fold) -> Acc + where + Self: Sized, + Fold: FnMut(Acc, Self::Item) -> Acc, + { + self.fold(init, fold) + } + + #[inline] + #[rustc_inherit_overflow_checks] + fn advance_back_by(&mut self, n: usize) -> Result<(), NonZero> { + self.advance_by(n) + } +} + +// Note: It may be tempting to impl DoubleEndedIterator for Take. +// One must fight that temptation since such implementation wouldn’t be correct +// because we have no way to return value of nth invocation of repeater followed +// by n-1st without remembering all results. + +#[stable(feature = "exact_size_take_repeat", since = "CURRENT_RUSTC_VERSION")] +impl ExactSizeIterator for Take> { + fn len(&self) -> usize { + self.n + } +} + +#[stable(feature = "exact_size_take_repeat", since = "CURRENT_RUSTC_VERSION")] +impl A, A> ExactSizeIterator for Take> { + fn len(&self) -> usize { + self.n + } +} diff --git a/library/core/tests/iter/adapters/take.rs b/library/core/tests/iter/adapters/take.rs index 39afa2cbfcaf2..65a8a93b4a916 100644 --- a/library/core/tests/iter/adapters/take.rs +++ b/library/core/tests/iter/adapters/take.rs @@ -170,3 +170,93 @@ fn test_byref_take_consumed_items() { assert_eq!(count, 70); assert_eq!(inner, 90..90); } + +#[test] +fn test_exact_size_take_repeat() { + let mut iter = core::iter::repeat(42).take(40); + assert_eq!((40, Some(40)), iter.size_hint()); + assert_eq!(40, iter.len()); + + assert_eq!(Some(42), iter.next()); + assert_eq!((39, Some(39)), iter.size_hint()); + assert_eq!(39, iter.len()); + + assert_eq!(Some(42), iter.next_back()); + assert_eq!((38, Some(38)), iter.size_hint()); + assert_eq!(38, iter.len()); + + assert_eq!(Some(42), iter.nth(3)); + assert_eq!((34, Some(34)), iter.size_hint()); + assert_eq!(34, iter.len()); + + assert_eq!(Some(42), iter.nth_back(3)); + assert_eq!((30, Some(30)), iter.size_hint()); + assert_eq!(30, iter.len()); + + assert_eq!(Ok(()), iter.advance_by(10)); + assert_eq!((20, Some(20)), iter.size_hint()); + assert_eq!(20, iter.len()); + + assert_eq!(Ok(()), iter.advance_back_by(10)); + assert_eq!((10, Some(10)), iter.size_hint()); + assert_eq!(10, iter.len()); +} + +#[test] +fn test_exact_size_take_repeat_with() { + let mut counter = 0; + let mut iter = core::iter::repeat_with(move || { + counter += 1; + counter + }) + .take(40); + assert_eq!((40, Some(40)), iter.size_hint()); + assert_eq!(40, iter.len()); + + assert_eq!(Some(1), iter.next()); + assert_eq!((39, Some(39)), iter.size_hint()); + assert_eq!(39, iter.len()); + + assert_eq!(Some(5), iter.nth(3)); + assert_eq!((35, Some(35)), iter.size_hint()); + assert_eq!(35, iter.len()); + + assert_eq!(Ok(()), iter.advance_by(10)); + assert_eq!((25, Some(25)), iter.size_hint()); + assert_eq!(25, iter.len()); + + assert_eq!(Some(16), iter.next()); + assert_eq!((24, Some(24)), iter.size_hint()); + assert_eq!(24, iter.len()); +} + +// This is https://github.com/rust-lang/rust/issues/104729 with all uses of +// repeat(0) were replaced by repeat(0).take(20). +#[test] +fn test_reverse_on_zip() { + let vec_1 = [1; 10]; + + let zipped_iter = vec_1.iter().copied().zip(core::iter::repeat(0).take(20)); + + // Forward + for (one, zero) in zipped_iter { + assert_eq!((1, 0), (one, zero)); + } + + let rev_vec_iter = vec_1.iter().rev(); + let rev_repeat_iter = std::iter::repeat(0).take(20).rev(); + + // Manual reversed zip + let rev_zipped_iter = rev_vec_iter.zip(rev_repeat_iter); + + for (&one, zero) in rev_zipped_iter { + assert_eq!((1, 0), (one, zero)); + } + + let zipped_iter = vec_1.iter().zip(core::iter::repeat(0).take(20)); + + // Cannot call rev here for automatic reversed zip constuction + for (&one, zero) in zipped_iter.rev() { + assert_eq!((1, 0), (one, zero)); + } +}