Merge #372

372: Implement missing specializations on the PutBack adaptor and on the MergeJoinBy Iterator r=jswrenn a=Ten0

Resolves #371

`count`, `last` and `nth` of the `MergeJoinBy` iterator are made faster when one of the iterators is completely consumed by directly calling the methods of the underlying only iterator left (there is benefit if the underlying iterator also specialized these methods).

This is in particular useful when you want to count the number of different elements in the union of two sorted known-size iterators (`count`).

Those methods are also specialized on the `PutBack` adaptor for the same performance reasons.
The `nth` specialization on the `MergeJoinBy` iterator depends on the `nth` specialization on the `PutBack` adaptor working.

Co-authored-by: Thomas BESSOU <[email protected]>

Author: bors[bot]

src/adaptors/mod.rs

@@ -233,6 +233,28 @@ impl<I> Iterator for PutBack<I>
         size_hint::add_scalar(self.iter.size_hint(), self.top.is_some() as usize)
     }
 
+    fn count(self) -> usize {
+        self.iter.count() + (self.top.is_some() as usize)
+    }
+
+    fn last(self) -> Option<Self::Item> {
+        self.iter.last().or(self.top)
+    }
+
+    fn nth(&mut self, n: usize) -> Option<Self::Item> {
+        match self.top {
+            None => self.iter.nth(n),
+            ref mut some => {
+                if n == 0 {
+                    some.take()
+                } else {
+                    *some = None;
+                    self.iter.nth(n - 1)
+                }
+            }
+        }
+    }
+
     fn all<G>(&mut self, mut f: G) -> bool
         where G: FnMut(Self::Item) -> bool
     {

src/merge_join.rs

@@ -32,10 +32,10 @@ pub struct MergeJoinBy<I: Iterator, J: Iterator, F> {
 }
 
 impl<I, J, F> Clone for MergeJoinBy<I, J, F>
-    where I: Clone + Iterator,
-          I::Item: Clone,
-          J: Clone + Iterator,
-          J::Item: Clone,
+    where I: Iterator,
+          J: Iterator,
+          PutBack<Fuse<I>>: Clone,
+          PutBack<Fuse<J>>: Clone,
           F: Clone,
 {
     clone_fields!(left, right, cmp_fn);
@@ -94,4 +94,74 @@ impl<I, J, F> Iterator for MergeJoinBy<I, J, F>
 
         (lower, upper)
     }
+
+    fn count(mut self) -> usize {
+        let mut count = 0;
+        loop {
+            match (self.left.next(), self.right.next()) {
+                (None, None) => break count,
+                (Some(_left), None) => break count + 1 + self.left.into_parts().1.count(),
+                (None, Some(_right)) => break count + 1 + self.right.into_parts().1.count(),
+                (Some(left), Some(right)) => {
+                    count += 1;
+                    match (self.cmp_fn)(&left, &right) {
+                        Ordering::Equal => {}
+                        Ordering::Less => self.right.put_back(right),
+                        Ordering::Greater => self.left.put_back(left),
+                    }
+                }
+            }
+        }
+    }
+
+    fn last(mut self) -> Option<Self::Item> {
+        let mut previous_element = None;
+        loop {
+            match (self.left.next(), self.right.next()) {
+                (None, None) => break previous_element,
+                (Some(left), None) => {
+                    break Some(EitherOrBoth::Left(
+                        self.left.into_parts().1.last().unwrap_or(left),
+                    ))
+                }
+                (None, Some(right)) => {
+                    break Some(EitherOrBoth::Right(
+                        self.right.into_parts().1.last().unwrap_or(right),
+                    ))
+                }
+                (Some(left), Some(right)) => {
+                    previous_element = match (self.cmp_fn)(&left, &right) {
+                        Ordering::Equal => Some(EitherOrBoth::Both(left, right)),
+                        Ordering::Less => {
+                            self.right.put_back(right);
+                            Some(EitherOrBoth::Left(left))
+                        }
+                        Ordering::Greater => {
+                            self.left.put_back(left);
+                            Some(EitherOrBoth::Right(right))
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
+        loop {
+            if n == 0 {
+                break self.next();
+            }
+            n -= 1;
+            match (self.left.next(), self.right.next()) {
+                (None, None) => break None,
+                (Some(_left), None) => break self.left.nth(n).map(EitherOrBoth::Left),
+                (None, Some(_right)) => break self.right.nth(n).map(EitherOrBoth::Right),
+                (Some(left), Some(right)) => match (self.cmp_fn)(&left, &right) {
+                    Ordering::Equal => {}
+                    Ordering::Less => self.right.put_back(right),
+                    Ordering::Greater => self.left.put_back(left),
+                },
+            }
+        }
+    }
 }

tests/specializations.rs

@@ -0,0 +1,154 @@
+extern crate itertools;
+
+#[macro_use]
+extern crate quickcheck;
+
+use itertools::{EitherOrBoth, Itertools};
+
+use std::fmt::Debug;
+use std::ops::BitXor;
+
+struct Unspecialized<I>(I);
+impl<I> Iterator for Unspecialized<I>
+where
+    I: Iterator,
+{
+    type Item = I::Item;
+
+    #[inline(always)]
+    fn next(&mut self) -> Option<I::Item> {
+        self.0.next()
+    }
+
+    #[inline(always)]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
+}
+
+fn check_specialized<'a, V, IterItem, Iter, F>(iterator: &Iter, mapper: F)
+where
+    V: Eq + Debug,
+    IterItem: 'a,
+    Iter: Iterator<Item = IterItem> + Clone + 'a,
+    F: Fn(Box<Iterator<Item = IterItem> + 'a>) -> V,
+{
+    assert_eq!(
+        mapper(Box::new(Unspecialized(iterator.clone()))),
+        mapper(Box::new(iterator.clone()))
+    )
+}
+
+fn check_specialized_count_last_nth_sizeh<'a, IterItem, Iter>(
+    it: &Iter,
+    known_expected_size: Option<usize>,
+) where
+    IterItem: 'a + Eq + Debug,
+    Iter: Iterator<Item = IterItem> + Clone + 'a,
+{
+    let size = it.clone().count();
+    if let Some(expected_size) = known_expected_size {
+        assert_eq!(size, expected_size);
+    }
+    check_specialized(it, |i| i.count());
+    check_specialized(it, |i| i.last());
+    for n in 0..size + 2 {
+        check_specialized(it, |mut i| i.nth(n));
+    }
+    let mut it_sh = it.clone();
+    for n in 0..size + 2 {
+        let len = it_sh.clone().count();
+        let (min, max) = it_sh.size_hint();
+        assert_eq!((size - n.min(size)), len);
+        assert!(min <= len);
+        if let Some(max) = max {
+            assert!(len <= max);
+        }
+        it_sh.next();
+    }
+}
+
+fn check_specialized_fold_xor<'a, IterItem, Iter>(it: &Iter)
+where
+    IterItem: 'a
+        + BitXor
+        + Eq
+        + Debug
+        + BitXor<<IterItem as BitXor>::Output, Output = <IterItem as BitXor>::Output>
+        + Clone,
+    <IterItem as BitXor>::Output:
+        BitXor<Output = <IterItem as BitXor>::Output> + Eq + Debug + Clone,
+    Iter: Iterator<Item = IterItem> + Clone + 'a,
+{
+    check_specialized(it, |mut i| {
+        let first = i.next().map(|f| f.clone() ^ (f.clone() ^ f));
+        i.fold(first, |acc, v: IterItem| acc.map(move |a| v ^ a))
+    });
+}
+
+fn put_back_test(test_vec: Vec<i32>, known_expected_size: Option<usize>) {
+    {
+        // Lexical lifetimes support
+        let pb = itertools::put_back(test_vec.iter());
+        check_specialized_count_last_nth_sizeh(&pb, known_expected_size);
+        check_specialized_fold_xor(&pb);
+    }
+
+    let mut pb = itertools::put_back(test_vec.into_iter());
+    pb.put_back(1);
+    check_specialized_count_last_nth_sizeh(&pb, known_expected_size.map(|x| x + 1));
+    check_specialized_fold_xor(&pb)
+}
+
+#[test]
+fn put_back() {
+    put_back_test(vec![7, 4, 1], Some(3));
+}
+
+quickcheck! {
+    fn put_back_qc(test_vec: Vec<i32>) -> () {
+        put_back_test(test_vec, None)
+    }
+}
+
+fn merge_join_by_test(i1: Vec<usize>, i2: Vec<usize>, known_expected_size: Option<usize>) {
+    let i1 = i1.into_iter();
+    let i2 = i2.into_iter();
+    let mjb = i1.clone().merge_join_by(i2.clone(), std::cmp::Ord::cmp);
+    check_specialized_count_last_nth_sizeh(&mjb, known_expected_size);
+    // Rust 1.24 compatibility:
+    fn eob_left_z(eob: EitherOrBoth<usize, usize>) -> usize {
+        eob.left().unwrap_or(0)
+    }
+    fn eob_right_z(eob: EitherOrBoth<usize, usize>) -> usize {
+        eob.left().unwrap_or(0)
+    }
+    fn eob_both_z(eob: EitherOrBoth<usize, usize>) -> usize {
+        let (a, b) = eob.both().unwrap_or((0, 0));
+        assert_eq!(a, b);
+        a
+    }
+    check_specialized_fold_xor(&mjb.clone().map(eob_left_z));
+    check_specialized_fold_xor(&mjb.clone().map(eob_right_z));
+    check_specialized_fold_xor(&mjb.clone().map(eob_both_z));
+
+    // And the other way around
+    let mjb = i2.merge_join_by(i1, std::cmp::Ord::cmp);
+    check_specialized_count_last_nth_sizeh(&mjb, known_expected_size);
+    check_specialized_fold_xor(&mjb.clone().map(eob_left_z));
+    check_specialized_fold_xor(&mjb.clone().map(eob_right_z));
+    check_specialized_fold_xor(&mjb.clone().map(eob_both_z));
+}
+
+#[test]
+fn merge_join_by() {
+    let i1 = vec![1, 3, 5, 7, 8, 9];
+    let i2 = vec![0, 3, 4, 5];
+    merge_join_by_test(i1, i2, Some(8));
+}
+
+quickcheck! {
+    fn merge_join_by_qc(i1: Vec<usize>, i2: Vec<usize>) -> () {
+        merge_join_by_test(i1, i2, None)
+    }
+}