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

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.

src/libcore/benches/iter.rs

@@ -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()
     })
 }
 

src/libcore/iter/traits/iterator.rs

@@ -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 })
     })
 }
 

src/libcore/tests/iter.rs

@@ -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]