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Rollup merge of #59138 - timvermeulen:simplify_select_fold1, r=sfackler

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Simplify Iterator::{min, max}

This PR simplifies the `select_fold1` helper method used to implmement `Iterator::{min, min_by, min_by_key, max, max_by, max_by_key}` by removing the projection argument, which was only used by the implementations of `min_by_key` and `max_by_key`.

I also added tests to ensure that the stability as mentioned in the comments of `min` and `max` is preserved, and fixed the `iter::{bench_max, bench_max_by_key}` benchmarks which the compiler presumably was able to collapse into closed-form expressions. None of the benchmark results were impacted, I suspect their generated assembly didn't change.
This commit is contained in:
Mazdak Farrokhzad 2019-03-13 03:33:57 +01:00 committed by GitHub
commit 73feddb9d4
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3 changed files with 49 additions and 58 deletions
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4
src/libcore/benches/iter.rs
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@ -35,7 +35,7 @@ fn scatter(x: i32) -> i32 { (x * 31) % 127 }
fn bench_max_by_key(b: &mut Bencher) {
b.iter(|| {
let it = 0..100;
it.max_by_key(|&x| scatter(x))
it.map(black_box).max_by_key(|&x| scatter(x))
})
}
@ -56,7 +56,7 @@ fn bench_max_by_key2(b: &mut Bencher) {
fn bench_max(b: &mut Bencher) {
b.iter(|| {
let it = 0..100;
it.map(scatter).max()
it.map(black_box).map(scatter).max()
})
}

75
src/libcore/iter/traits/iterator.rs
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@ -2008,12 +2008,7 @@ pub trait Iterator {
#[stable(feature = "rust1", since = "1.0.0")]
fn max(self) -> Option<Self::Item> where Self: Sized, Self::Item: Ord
{
select_fold1(self,
|_| (),
// switch to y even if it is only equal, to preserve
// stability.
|_, x, _, y| *x <= *y)
.map(|(_, x)| x)
self.max_by(Ord::cmp)
}
/// Returns the minimum element of an iterator.
@ -2038,12 +2033,7 @@ pub trait Iterator {
#[stable(feature = "rust1", since = "1.0.0")]
fn min(self) -> Option<Self::Item> where Self: Sized, Self::Item: Ord
{
select_fold1(self,
|_| (),
// only switch to y if it is strictly smaller, to
// preserve stability.
|_, x, _, y| *x > *y)
.map(|(_, x)| x)
self.min_by(Ord::cmp)
}
/// Returns the element that gives the maximum value from the
@ -2062,15 +2052,11 @@ pub trait Iterator {
/// ```
#[inline]
#[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
fn max_by_key<B: Ord, F>(self, mut f: F) -> Option<Self::Item>
where Self: Sized, F: FnMut(&Self::Item) -> B,
{
select_fold1(self,
f,
// switch to y even if it is only equal, to preserve
// stability.
|x_p, _, y_p, _| x_p <= y_p)
.map(|(_, x)| x)
// switch to y even if it is only equal, to preserve stability.
select_fold1(self.map(|x| (f(&x), x)), |(x_p, _), (y_p, _)| x_p <= y_p).map(|(_, x)| x)
}
/// Returns the element that gives the maximum value with respect to the
@ -2092,12 +2078,8 @@ pub trait Iterator {
fn max_by<F>(self, mut compare: F) -> Option<Self::Item>
where Self: Sized, F: FnMut(&Self::Item, &Self::Item) -> Ordering,
{
select_fold1(self,
|_| (),
// switch to y even if it is only equal, to preserve
// stability.
|_, x, _, y| Ordering::Greater != compare(x, y))
.map(|(_, x)| x)
// switch to y even if it is only equal, to preserve stability.
select_fold1(self, |x, y| compare(x, y) != Ordering::Greater)
}
/// Returns the element that gives the minimum value from the
@ -2115,15 +2097,11 @@ pub trait Iterator {
/// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
/// ```
#[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
fn min_by_key<B: Ord, F>(self, mut f: F) -> Option<Self::Item>
where Self: Sized, F: FnMut(&Self::Item) -> B,
{
select_fold1(self,
f,
// only switch to y if it is strictly smaller, to
// preserve stability.
|x_p, _, y_p, _| x_p > y_p)
.map(|(_, x)| x)
// only switch to y if it is strictly smaller, to preserve stability.
select_fold1(self.map(|x| (f(&x), x)), |(x_p, _), (y_p, _)| x_p > y_p).map(|(_, x)| x)
}
/// Returns the element that gives the minimum value with respect to the
@ -2145,12 +2123,8 @@ pub trait Iterator {
fn min_by<F>(self, mut compare: F) -> Option<Self::Item>
where Self: Sized, F: FnMut(&Self::Item, &Self::Item) -> Ordering,
{
select_fold1(self,
|_| (),
// switch to y even if it is strictly smaller, to
// preserve stability.
|_, x, _, y| Ordering::Greater == compare(x, y))
.map(|(_, x)| x)
// only switch to y if it is strictly smaller, to preserve stability.
select_fold1(self, |x, y| compare(x, y) == Ordering::Greater)
}
@ -2693,34 +2667,23 @@ pub trait Iterator {
}
}
/// Select an element from an iterator based on the given "projection"
/// and "comparison" function.
/// Select an element from an iterator based on the given "comparison"
/// function.
///
/// This is an idiosyncratic helper to try to factor out the
/// commonalities of {max,min}{,_by}. In particular, this avoids
/// having to implement optimizations several times.
#[inline]
fn select_fold1<I, B, FProj, FCmp>(mut it: I,
mut f_proj: FProj,
mut f_cmp: FCmp) -> Option<(B, I::Item)>
where I: Iterator,
FProj: FnMut(&I::Item) -> B,
FCmp: FnMut(&B, &I::Item, &B, &I::Item) -> bool
fn select_fold1<I, F>(mut it: I, mut f: F) -> Option<I::Item>
where
I: Iterator,
F: FnMut(&I::Item, &I::Item) -> bool,
{
// start with the first element as our selection. This avoids
// having to use `Option`s inside the loop, translating to a
// sizeable performance gain (6x in one case).
it.next().map(|first| {
let first_p = f_proj(&first);
it.fold((first_p, first), |(sel_p, sel), x| {
let x_p = f_proj(&x);
if f_cmp(&sel_p, &sel, &x_p, &x) {
(x_p, x)
} else {
(sel_p, sel)
}
})
it.fold(first, |sel, x| if f(&sel, &x) { x } else { sel })
})
}

28
src/libcore/tests/iter.rs
View File

@ -1082,12 +1082,39 @@ fn test_iterator_product_result() {
assert_eq!(v.iter().cloned().product::<Result<i32, _>>(), Err(()));
}
/// A wrapper struct that implements `Eq` and `Ord` based on the wrapped
/// integer modulo 3. Used to test that `Iterator::max` and `Iterator::min`
/// return the correct element if some of them are equal.
#[derive(Debug)]
struct Mod3(i32);
impl PartialEq for Mod3 {
fn eq(&self, other: &Self) -> bool {
self.0 % 3 == other.0 % 3
}
}
impl Eq for Mod3 {}
impl PartialOrd for Mod3 {
fn partial_cmp(&self, other: &Self) -> Option<core::cmp::Ordering> {
Some(self.cmp(other))
}
}
impl Ord for Mod3 {
fn cmp(&self, other: &Self) -> core::cmp::Ordering {
(self.0 % 3).cmp(&(other.0 % 3))
}
}
#[test]
fn test_iterator_max() {
let v: &[_] = &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
assert_eq!(v[..4].iter().cloned().max(), Some(3));
assert_eq!(v.iter().cloned().max(), Some(10));
assert_eq!(v[..0].iter().cloned().max(), None);
assert_eq!(v.iter().cloned().map(Mod3).max().map(|x| x.0), Some(8));
}
#[test]
@ -1096,6 +1123,7 @@ fn test_iterator_min() {
assert_eq!(v[..4].iter().cloned().min(), Some(0));
assert_eq!(v.iter().cloned().min(), Some(0));
assert_eq!(v[..0].iter().cloned().min(), None);
assert_eq!(v.iter().cloned().map(Mod3).min().map(|x| x.0), Some(0));
}
#[test]