Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1749 insertions, 0 deletions

diff --git a/third_party/rust/itertools/tests/quick.rs b/third_party/rust/itertools/tests/quick.rs
new file mode 100644
index 0000000000..0adcf1ad72
--- /dev/null
+++ b/third_party/rust/itertools/tests/quick.rs
@@ -0,0 +1,1749 @@
+//! The purpose of these tests is to cover corner cases of iterators
+//! and adaptors.
+//!
+//! In particular we test the tedious size_hint and exact size correctness.
+
+use quickcheck as qc;
+use std::default::Default;
+use std::num::Wrapping;
+use std::ops::Range;
+use std::cmp::{max, min, Ordering};
+use std::collections::{HashMap, HashSet};
+use itertools::Itertools;
+use itertools::{
+    multizip,
+    EitherOrBoth,
+    iproduct,
+    izip,
+};
+use itertools::free::{
+    cloned,
+    enumerate,
+    multipeek,
+    peek_nth,
+    put_back,
+    put_back_n,
+    rciter,
+    zip,
+    zip_eq,
+};
+
+use rand::Rng;
+use rand::seq::SliceRandom;
+use quickcheck::TestResult;
+
+/// Trait for size hint modifier types
+trait HintKind: Copy + Send + qc::Arbitrary {
+    fn loosen_bounds(&self, org_hint: (usize, Option<usize>)) -> (usize, Option<usize>);
+}
+
+/// Exact size hint variant that leaves hints unchanged
+#[derive(Clone, Copy, Debug)]
+struct Exact {}
+
+impl HintKind for Exact {
+    fn loosen_bounds(&self, org_hint: (usize, Option<usize>)) -> (usize, Option<usize>) {
+        org_hint
+    }
+}
+
+impl qc::Arbitrary for Exact {
+    fn arbitrary<G: qc::Gen>(_: &mut G) -> Self {
+        Exact {}
+    }
+}
+
+/// Inexact size hint variant to simulate imprecise (but valid) size hints
+///
+/// Will always decrease the lower bound and increase the upper bound
+/// of the size hint by set amounts.
+#[derive(Clone, Copy, Debug)]
+struct Inexact {
+    underestimate: usize,
+    overestimate: usize,
+}
+
+impl HintKind for Inexact {
+    fn loosen_bounds(&self, org_hint: (usize, Option<usize>)) -> (usize, Option<usize>) {
+        let (org_lower, org_upper) = org_hint;
+        (org_lower.saturating_sub(self.underestimate),
+         org_upper.and_then(move |x| x.checked_add(self.overestimate)))
+    }
+}
+
+impl qc::Arbitrary for Inexact {
+    fn arbitrary<G: qc::Gen>(g: &mut G) -> Self {
+        let ue_value = usize::arbitrary(g);
+        let oe_value = usize::arbitrary(g);
+        // Compensate for quickcheck using extreme values too rarely
+        let ue_choices = &[0, ue_value, usize::max_value()];
+        let oe_choices = &[0, oe_value, usize::max_value()];
+        Inexact {
+            underestimate: *ue_choices.choose(g).unwrap(),
+            overestimate: *oe_choices.choose(g).unwrap(),
+        }
+    }
+
+    fn shrink(&self) -> Box<dyn Iterator<Item=Self>> {
+        let underestimate_value = self.underestimate;
+        let overestimate_value = self.overestimate;
+        Box::new(
+            underestimate_value.shrink().flat_map(move |ue_value|
+                overestimate_value.shrink().map(move |oe_value|
+                    Inexact {
+                        underestimate: ue_value,
+                        overestimate: oe_value,
+                    }
+                )
+            )
+        )
+    }
+}
+
+/// Our base iterator that we can impl Arbitrary for
+///
+/// By default we'll return inexact bounds estimates for size_hint
+/// to make tests harder to pass.
+///
+/// NOTE: Iter is tricky and is not fused, to help catch bugs.
+/// At the end it will return None once, then return Some(0),
+/// then return None again.
+#[derive(Clone, Debug)]
+struct Iter<T, SK: HintKind = Inexact> {
+    iterator: Range<T>,
+    // fuse/done flag
+    fuse_flag: i32,
+    hint_kind: SK,
+}
+
+impl<T, HK> Iter<T, HK> where HK: HintKind
+{
+    fn new(it: Range<T>, hint_kind: HK) -> Self {
+        Iter {
+            iterator: it,
+            fuse_flag: 0,
+            hint_kind,
+        }
+    }
+}
+
+impl<T, HK> Iterator for Iter<T, HK>
+    where Range<T>: Iterator,
+          <Range<T> as Iterator>::Item: Default,
+          HK: HintKind,
+{
+    type Item = <Range<T> as Iterator>::Item;
+
+    fn next(&mut self) -> Option<Self::Item>
+    {
+        let elt = self.iterator.next();
+        if elt.is_none() {
+            self.fuse_flag += 1;
+            // check fuse flag
+            if self.fuse_flag == 2 {
+                return Some(Default::default())
+            }
+        }
+        elt
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>)
+    {
+        let org_hint = self.iterator.size_hint();
+        self.hint_kind.loosen_bounds(org_hint)
+    }
+}
+
+impl<T, HK> DoubleEndedIterator for Iter<T, HK>
+    where Range<T>: DoubleEndedIterator,
+          <Range<T> as Iterator>::Item: Default,
+          HK: HintKind
+{
+    fn next_back(&mut self) -> Option<Self::Item> { self.iterator.next_back() }
+}
+
+impl<T> ExactSizeIterator for Iter<T, Exact> where Range<T>: ExactSizeIterator,
+    <Range<T> as Iterator>::Item: Default,
+{ }
+
+impl<T, HK> qc::Arbitrary for Iter<T, HK>
+    where T: qc::Arbitrary,
+          HK: HintKind,
+{
+    fn arbitrary<G: qc::Gen>(g: &mut G) -> Self
+    {
+        Iter::new(T::arbitrary(g)..T::arbitrary(g), HK::arbitrary(g))
+    }
+
+    fn shrink(&self) -> Box<dyn Iterator<Item=Iter<T, HK>>>
+    {
+        let r = self.iterator.clone();
+        let hint_kind = self.hint_kind;
+        Box::new(
+            r.start.shrink().flat_map(move |a|
+                r.end.shrink().map(move |b|
+                    Iter::new(a.clone()..b, hint_kind)
+                )
+            )
+        )
+    }
+}
+
+/// A meta-iterator which yields `Iter<i32>`s whose start/endpoints are
+/// increased or decreased linearly on each iteration.
+#[derive(Clone, Debug)]
+struct ShiftRange<HK = Inexact> {
+    range_start: i32,
+    range_end: i32,
+    start_step: i32,
+    end_step: i32,
+    iter_count: u32,
+    hint_kind: HK,
+}
+
+impl<HK> Iterator for ShiftRange<HK> where HK: HintKind {
+    type Item = Iter<i32, HK>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.iter_count == 0 {
+            return None;
+        }
+
+        let iter = Iter::new(self.range_start..self.range_end, self.hint_kind);
+
+        self.range_start += self.start_step;
+        self.range_end += self.end_step;
+        self.iter_count -= 1;
+
+        Some(iter)
+    }
+}
+
+impl ExactSizeIterator for ShiftRange<Exact> { }
+
+impl<HK> qc::Arbitrary for ShiftRange<HK>
+    where HK: HintKind
+{
+    fn arbitrary<G: qc::Gen>(g: &mut G) -> Self {
+        const MAX_STARTING_RANGE_DIFF: i32 = 32;
+        const MAX_STEP_MODULO: i32 = 8;
+        const MAX_ITER_COUNT: u32 = 3;
+
+        let range_start = qc::Arbitrary::arbitrary(g);
+        let range_end = range_start + g.gen_range(0, MAX_STARTING_RANGE_DIFF + 1);
+        let start_step = g.gen_range(-MAX_STEP_MODULO, MAX_STEP_MODULO + 1);
+        let end_step = g.gen_range(-MAX_STEP_MODULO, MAX_STEP_MODULO + 1);
+        let iter_count = g.gen_range(0, MAX_ITER_COUNT + 1);
+        let hint_kind = qc::Arbitrary::arbitrary(g);
+
+        ShiftRange {
+            range_start,
+            range_end,
+            start_step,
+            end_step,
+            iter_count,
+            hint_kind,
+        }
+    }
+}
+
+fn correct_count<I, F>(get_it: F) -> bool
+where
+    I: Iterator,
+    F: Fn() -> I
+{
+    let mut counts = vec![get_it().count()];
+
+    'outer: loop {
+        let mut it = get_it();
+
+        for _ in 0..(counts.len() - 1) {
+            #[allow(clippy::manual_assert)]
+            if it.next().is_none() {
+                panic!("Iterator shouldn't be finished, may not be deterministic");
+            }
+        }
+
+        if it.next().is_none() {
+            break 'outer;
+        }
+
+        counts.push(it.count());
+    }
+
+    let total_actual_count = counts.len() - 1;
+
+    for (i, returned_count) in counts.into_iter().enumerate() {
+        let actual_count = total_actual_count - i;
+        if actual_count != returned_count {
+            println!("Total iterations: {} True count: {} returned count: {}", i, actual_count, returned_count);
+
+            return false;
+        }
+    }
+
+    true
+}
+
+fn correct_size_hint<I: Iterator>(mut it: I) -> bool {
+    // record size hint at each iteration
+    let initial_hint = it.size_hint();
+    let mut hints = Vec::with_capacity(initial_hint.0 + 1);
+    hints.push(initial_hint);
+    while let Some(_) = it.next() {
+        hints.push(it.size_hint())
+    }
+
+    let mut true_count = hints.len(); // start off +1 too much
+
+    // check all the size hints
+    for &(low, hi) in &hints {
+        true_count -= 1;
+        if low > true_count ||
+            (hi.is_some() && hi.unwrap() < true_count)
+        {
+            println!("True size: {:?}, size hint: {:?}", true_count, (low, hi));
+            //println!("All hints: {:?}", hints);
+            return false
+        }
+    }
+    true
+}
+
+fn exact_size<I: ExactSizeIterator>(mut it: I) -> bool {
+    // check every iteration
+    let (mut low, mut hi) = it.size_hint();
+    if Some(low) != hi { return false; }
+    while let Some(_) = it.next() {
+        let (xlow, xhi) = it.size_hint();
+        if low != xlow + 1 { return false; }
+        low = xlow;
+        hi = xhi;
+        if Some(low) != hi { return false; }
+    }
+    let (low, hi) = it.size_hint();
+    low == 0 && hi == Some(0)
+}
+
+// Exact size for this case, without ExactSizeIterator
+fn exact_size_for_this<I: Iterator>(mut it: I) -> bool {
+    // check every iteration
+    let (mut low, mut hi) = it.size_hint();
+    if Some(low) != hi { return false; }
+    while let Some(_) = it.next() {
+        let (xlow, xhi) = it.size_hint();
+        if low != xlow + 1 { return false; }
+        low = xlow;
+        hi = xhi;
+        if Some(low) != hi { return false; }
+    }
+    let (low, hi) = it.size_hint();
+    low == 0 && hi == Some(0)
+}
+
+/*
+ * NOTE: Range<i8> is broken!
+ * (all signed ranges are)
+#[quickcheck]
+fn size_range_i8(a: Iter<i8>) -> bool {
+    exact_size(a)
+}
+
+#[quickcheck]
+fn size_range_i16(a: Iter<i16>) -> bool {
+    exact_size(a)
+}
+
+#[quickcheck]
+fn size_range_u8(a: Iter<u8>) -> bool {
+    exact_size(a)
+}
+ */
+
+macro_rules! quickcheck {
+    // accept several property function definitions
+    // The property functions can use pattern matching and `mut` as usual
+    // in the function arguments, but the functions can not be generic.
+    {$($(#$attr:tt)* fn $fn_name:ident($($arg:tt)*) -> $ret:ty { $($code:tt)* })*} => (
+        $(
+            #[test]
+            $(#$attr)*
+            fn $fn_name() {
+                fn prop($($arg)*) -> $ret {
+                    $($code)*
+                }
+                ::quickcheck::quickcheck(quickcheck!(@fn prop [] $($arg)*));
+            }
+        )*
+    );
+    // parse argument list (with patterns allowed) into prop as fn(_, _) -> _
+    (@fn $f:ident [$($t:tt)*]) => {
+        $f as fn($($t),*) -> _
+    };
+    (@fn $f:ident [$($p:tt)*] : $($tail:tt)*) => {
+        quickcheck!(@fn $f [$($p)* _] $($tail)*)
+    };
+    (@fn $f:ident [$($p:tt)*] $t:tt $($tail:tt)*) => {
+        quickcheck!(@fn $f [$($p)*] $($tail)*)
+    };
+}
+
+quickcheck! {
+
+    fn size_product(a: Iter<u16>, b: Iter<u16>) -> bool {
+        correct_size_hint(a.cartesian_product(b))
+    }
+    fn size_product3(a: Iter<u16>, b: Iter<u16>, c: Iter<u16>) -> bool {
+        correct_size_hint(iproduct!(a, b, c))
+    }
+
+    fn correct_cartesian_product3(a: Iter<u16>, b: Iter<u16>, c: Iter<u16>,
+                                  take_manual: usize) -> ()
+    {
+        // test correctness of iproduct through regular iteration (take)
+        // and through fold.
+        let ac = a.clone();
+        let br = &b.clone();
+        let cr = &c.clone();
+        let answer: Vec<_> = ac.flat_map(move |ea| br.clone().flat_map(move |eb| cr.clone().map(move |ec| (ea, eb, ec)))).collect();
+        let mut product_iter = iproduct!(a, b, c);
+        let mut actual = Vec::new();
+
+        actual.extend((&mut product_iter).take(take_manual));
+        if actual.len() == take_manual {
+            product_iter.fold((), |(), elt| actual.push(elt));
+        }
+        assert_eq!(answer, actual);
+    }
+
+    fn size_multi_product(a: ShiftRange) -> bool {
+        correct_size_hint(a.multi_cartesian_product())
+    }
+    fn correct_multi_product3(a: ShiftRange, take_manual: usize) -> () {
+        // Fix no. of iterators at 3
+        let a = ShiftRange { iter_count: 3, ..a };
+
+        // test correctness of MultiProduct through regular iteration (take)
+        // and through fold.
+        let mut iters = a.clone();
+        let i0 = iters.next().unwrap();
+        let i1r = &iters.next().unwrap();
+        let i2r = &iters.next().unwrap();
+        let answer: Vec<_> = i0.flat_map(move |ei0| i1r.clone().flat_map(move |ei1| i2r.clone().map(move |ei2| vec![ei0, ei1, ei2]))).collect();
+        let mut multi_product = a.clone().multi_cartesian_product();
+        let mut actual = Vec::new();
+
+        actual.extend((&mut multi_product).take(take_manual));
+        if actual.len() == take_manual {
+            multi_product.fold((), |(), elt| actual.push(elt));
+        }
+        assert_eq!(answer, actual);
+
+        assert_eq!(answer.into_iter().last(), a.multi_cartesian_product().last());
+    }
+
+    #[allow(deprecated)]
+    fn size_step(a: Iter<i16, Exact>, s: usize) -> bool {
+        let mut s = s;
+        if s == 0 {
+            s += 1; // never zero
+        }
+        let filt = a.clone().dedup();
+        correct_size_hint(filt.step(s)) &&
+            exact_size(a.step(s))
+    }
+
+    #[allow(deprecated)]
+    fn equal_step(a: Iter<i16>, s: usize) -> bool {
+        let mut s = s;
+        if s == 0 {
+            s += 1; // never zero
+        }
+        let mut i = 0;
+        itertools::equal(a.clone().step(s), a.filter(|_| {
+            let keep = i % s == 0;
+            i += 1;
+            keep
+        }))
+    }
+
+    #[allow(deprecated)]
+    fn equal_step_vec(a: Vec<i16>, s: usize) -> bool {
+        let mut s = s;
+        if s == 0 {
+            s += 1; // never zero
+        }
+        let mut i = 0;
+        itertools::equal(a.iter().step(s), a.iter().filter(|_| {
+            let keep = i % s == 0;
+            i += 1;
+            keep
+        }))
+    }
+
+    fn size_multipeek(a: Iter<u16, Exact>, s: u8) -> bool {
+        let mut it = multipeek(a);
+        // peek a few times
+        for _ in 0..s {
+            it.peek();
+        }
+        exact_size(it)
+    }
+
+    fn size_peek_nth(a: Iter<u16, Exact>, s: u8) -> bool {
+        let mut it = peek_nth(a);
+        // peek a few times
+        for n in 0..s {
+            it.peek_nth(n as usize);
+        }
+        exact_size(it)
+    }
+
+    fn equal_merge(mut a: Vec<i16>, mut b: Vec<i16>) -> bool {
+        a.sort();
+        b.sort();
+        let mut merged = a.clone();
+        merged.extend(b.iter().cloned());
+        merged.sort();
+        itertools::equal(&merged, a.iter().merge(&b))
+    }
+    fn size_merge(a: Iter<u16>, b: Iter<u16>) -> bool {
+        correct_size_hint(a.merge(b))
+    }
+    fn size_zip(a: Iter<i16, Exact>, b: Iter<i16, Exact>, c: Iter<i16, Exact>) -> bool {
+        let filt = a.clone().dedup();
+        correct_size_hint(multizip((filt, b.clone(), c.clone()))) &&
+            exact_size(multizip((a, b, c)))
+    }
+    fn size_zip_rc(a: Iter<i16>, b: Iter<i16>) -> bool {
+        let rc = rciter(a);
+        correct_size_hint(multizip((&rc, &rc, b)))
+    }
+
+    fn size_zip_macro(a: Iter<i16, Exact>, b: Iter<i16, Exact>, c: Iter<i16, Exact>) -> bool {
+        let filt = a.clone().dedup();
+        correct_size_hint(izip!(filt, b.clone(), c.clone())) &&
+            exact_size(izip!(a, b, c))
+    }
+    fn equal_kmerge(mut a: Vec<i16>, mut b: Vec<i16>, mut c: Vec<i16>) -> bool {
+        use itertools::free::kmerge;
+        a.sort();
+        b.sort();
+        c.sort();
+        let mut merged = a.clone();
+        merged.extend(b.iter().cloned());
+        merged.extend(c.iter().cloned());
+        merged.sort();
+        itertools::equal(merged.into_iter(), kmerge(vec![a, b, c]))
+    }
+
+    // Any number of input iterators
+    fn equal_kmerge_2(mut inputs: Vec<Vec<i16>>) -> bool {
+        use itertools::free::kmerge;
+        // sort the inputs
+        for input in &mut inputs {
+            input.sort();
+        }
+        let mut merged = inputs.concat();
+        merged.sort();
+        itertools::equal(merged.into_iter(), kmerge(inputs))
+    }
+
+    // Any number of input iterators
+    fn equal_kmerge_by_ge(mut inputs: Vec<Vec<i16>>) -> bool {
+        // sort the inputs
+        for input in &mut inputs {
+            input.sort();
+            input.reverse();
+        }
+        let mut merged = inputs.concat();
+        merged.sort();
+        merged.reverse();
+        itertools::equal(merged.into_iter(),
+                         inputs.into_iter().kmerge_by(|x, y| x >= y))
+    }
+
+    // Any number of input iterators
+    fn equal_kmerge_by_lt(mut inputs: Vec<Vec<i16>>) -> bool {
+        // sort the inputs
+        for input in &mut inputs {
+            input.sort();
+        }
+        let mut merged = inputs.concat();
+        merged.sort();
+        itertools::equal(merged.into_iter(),
+                         inputs.into_iter().kmerge_by(|x, y| x < y))
+    }
+
+    // Any number of input iterators
+    fn equal_kmerge_by_le(mut inputs: Vec<Vec<i16>>) -> bool {
+        // sort the inputs
+        for input in &mut inputs {
+            input.sort();
+        }
+        let mut merged = inputs.concat();
+        merged.sort();
+        itertools::equal(merged.into_iter(),
+                         inputs.into_iter().kmerge_by(|x, y| x <= y))
+    }
+    fn size_kmerge(a: Iter<i16>, b: Iter<i16>, c: Iter<i16>) -> bool {
+        use itertools::free::kmerge;
+        correct_size_hint(kmerge(vec![a, b, c]))
+    }
+    fn equal_zip_eq(a: Vec<i32>, b: Vec<i32>) -> bool {
+        let len = std::cmp::min(a.len(), b.len());
+        let a = &a[..len];
+        let b = &b[..len];
+        itertools::equal(zip_eq(a, b), zip(a, b))
+    }
+    fn size_zip_longest(a: Iter<i16, Exact>, b: Iter<i16, Exact>) -> bool {
+        let filt = a.clone().dedup();
+        let filt2 = b.clone().dedup();
+        correct_size_hint(filt.zip_longest(b.clone())) &&
+        correct_size_hint(a.clone().zip_longest(filt2)) &&
+            exact_size(a.zip_longest(b))
+    }
+    fn size_2_zip_longest(a: Iter<i16>, b: Iter<i16>) -> bool {
+        let it = a.clone().zip_longest(b.clone());
+        let jt = a.clone().zip_longest(b.clone());
+        itertools::equal(a,
+                         it.filter_map(|elt| match elt {
+                             EitherOrBoth::Both(x, _) => Some(x),
+                             EitherOrBoth::Left(x) => Some(x),
+                             _ => None,
+                         }
+                         ))
+            &&
+        itertools::equal(b,
+                         jt.filter_map(|elt| match elt {
+                             EitherOrBoth::Both(_, y) => Some(y),
+                             EitherOrBoth::Right(y) => Some(y),
+                             _ => None,
+                         }
+                         ))
+    }
+    fn size_interleave(a: Iter<i16>, b: Iter<i16>) -> bool {
+        correct_size_hint(a.interleave(b))
+    }
+    fn exact_interleave(a: Iter<i16, Exact>, b: Iter<i16, Exact>) -> bool {
+        exact_size_for_this(a.interleave(b))
+    }
+    fn size_interleave_shortest(a: Iter<i16>, b: Iter<i16>) -> bool {
+        correct_size_hint(a.interleave_shortest(b))
+    }
+    fn exact_interleave_shortest(a: Vec<()>, b: Vec<()>) -> bool {
+        exact_size_for_this(a.iter().interleave_shortest(&b))
+    }
+    fn size_intersperse(a: Iter<i16>, x: i16) -> bool {
+        correct_size_hint(a.intersperse(x))
+    }
+    fn equal_intersperse(a: Vec<i32>, x: i32) -> bool {
+        let mut inter = false;
+        let mut i = 0;
+        for elt in a.iter().cloned().intersperse(x) {
+            if inter {
+                if elt != x { return false }
+            } else {
+                if elt != a[i] { return false }
+                i += 1;
+            }
+            inter = !inter;
+        }
+        true
+    }
+
+    fn equal_combinations_2(a: Vec<u8>) -> bool {
+        let mut v = Vec::new();
+        for (i, x) in enumerate(&a) {
+            for y in &a[i + 1..] {
+                v.push((x, y));
+            }
+        }
+        itertools::equal(a.iter().tuple_combinations::<(_, _)>(), v)
+    }
+
+    fn collect_tuple_matches_size(a: Iter<i16>) -> bool {
+        let size = a.clone().count();
+        a.collect_tuple::<(_, _, _)>().is_some() == (size == 3)
+    }
+
+    fn correct_permutations(vals: HashSet<i32>, k: usize) -> () {
+        // Test permutations only on iterators of distinct integers, to prevent
+        // false positives.
+
+        const MAX_N: usize = 5;
+
+        let n = min(vals.len(), MAX_N);
+        let vals: HashSet<i32> = vals.into_iter().take(n).collect();
+
+        let perms = vals.iter().permutations(k);
+
+        let mut actual = HashSet::new();
+
+        for perm in perms {
+            assert_eq!(perm.len(), k);
+
+            let all_items_valid = perm.iter().all(|p| vals.contains(p));
+            assert!(all_items_valid, "perm contains value not from input: {:?}", perm);
+
+            // Check that all perm items are distinct
+            let distinct_len = {
+                let perm_set: HashSet<_> = perm.iter().collect();
+                perm_set.len()
+            };
+            assert_eq!(perm.len(), distinct_len);
+
+            // Check that the perm is new
+            assert!(actual.insert(perm.clone()), "perm already encountered: {:?}", perm);
+        }
+    }
+
+    fn permutations_lexic_order(a: usize, b: usize) -> () {
+        let a = a % 6;
+        let b = b % 6;
+
+        let n = max(a, b);
+        let k = min (a, b);
+
+        let expected_first: Vec<usize> = (0..k).collect();
+        let expected_last: Vec<usize> = ((n - k)..n).rev().collect();
+
+        let mut perms = (0..n).permutations(k);
+
+        let mut curr_perm = match perms.next() {
+            Some(p) => p,
+            None => { return; }
+        };
+
+        assert_eq!(expected_first, curr_perm);
+
+        for next_perm in perms {
+            assert!(
+                next_perm > curr_perm,
+                "next perm isn't greater-than current; next_perm={:?} curr_perm={:?} n={}",
+                next_perm, curr_perm, n
+            );
+
+            curr_perm = next_perm;
+        }
+
+        assert_eq!(expected_last, curr_perm);
+
+    }
+
+    fn permutations_count(n: usize, k: usize) -> bool {
+        let n = n % 6;
+
+        correct_count(|| (0..n).permutations(k))
+    }
+
+    fn permutations_size(a: Iter<i32>, k: usize) -> bool {
+        correct_size_hint(a.take(5).permutations(k))
+    }
+
+    fn permutations_k0_yields_once(n: usize) -> () {
+        let k = 0;
+        let expected: Vec<Vec<usize>> = vec![vec![]];
+        let actual = (0..n).permutations(k).collect_vec();
+
+        assert_eq!(expected, actual);
+    }
+}
+
+quickcheck! {
+    fn dedup_via_coalesce(a: Vec<i32>) -> bool {
+        let mut b = a.clone();
+        b.dedup();
+        itertools::equal(
+            &b,
+            a
+                .iter()
+                .coalesce(|x, y| {
+                    if x==y {
+                        Ok(x)
+                    } else {
+                        Err((x, y))
+                    }
+                })
+                .fold(vec![], |mut v, n| {
+                    v.push(n);
+                    v
+                })
+        )
+    }
+}
+
+quickcheck! {
+    fn equal_dedup(a: Vec<i32>) -> bool {
+        let mut b = a.clone();
+        b.dedup();
+        itertools::equal(&b, a.iter().dedup())
+    }
+}
+
+quickcheck! {
+    fn equal_dedup_by(a: Vec<(i32, i32)>) -> bool {
+        let mut b = a.clone();
+        b.dedup_by(|x, y| x.0==y.0);
+        itertools::equal(&b, a.iter().dedup_by(|x, y| x.0==y.0))
+    }
+}
+
+quickcheck! {
+    fn size_dedup(a: Vec<i32>) -> bool {
+        correct_size_hint(a.iter().dedup())
+    }
+}
+
+quickcheck! {
+    fn size_dedup_by(a: Vec<(i32, i32)>) -> bool {
+        correct_size_hint(a.iter().dedup_by(|x, y| x.0==y.0))
+    }
+}
+
+quickcheck! {
+    fn exact_repeatn((n, x): (usize, i32)) -> bool {
+        let it = itertools::repeat_n(x, n);
+        exact_size(it)
+    }
+}
+
+quickcheck! {
+    fn size_put_back(a: Vec<u8>, x: Option<u8>) -> bool {
+        let mut it = put_back(a.into_iter());
+        match x {
+            Some(t) => it.put_back(t),
+            None => {}
+        }
+        correct_size_hint(it)
+    }
+}
+
+quickcheck! {
+    fn size_put_backn(a: Vec<u8>, b: Vec<u8>) -> bool {
+        let mut it = put_back_n(a.into_iter());
+        for elt in b {
+            it.put_back(elt)
+        }
+        correct_size_hint(it)
+    }
+}
+
+quickcheck! {
+    fn size_tee(a: Vec<u8>) -> bool {
+        let (mut t1, mut t2) = a.iter().tee();
+        t1.next();
+        t1.next();
+        t2.next();
+        exact_size(t1) && exact_size(t2)
+    }
+}
+
+quickcheck! {
+    fn size_tee_2(a: Vec<u8>) -> bool {
+        let (mut t1, mut t2) = a.iter().dedup().tee();
+        t1.next();
+        t1.next();
+        t2.next();
+        correct_size_hint(t1) && correct_size_hint(t2)
+    }
+}
+
+quickcheck! {
+    fn size_take_while_ref(a: Vec<u8>, stop: u8) -> bool {
+        correct_size_hint(a.iter().take_while_ref(|x| **x != stop))
+    }
+}
+
+quickcheck! {
+    fn equal_partition(a: Vec<i32>) -> bool {
+        let mut a = a;
+        let mut ap = a.clone();
+        let split_index = itertools::partition(&mut ap, |x| *x >= 0);
+        let parted = (0..split_index).all(|i| ap[i] >= 0) &&
+            (split_index..a.len()).all(|i| ap[i] < 0);
+
+        a.sort();
+        ap.sort();
+        parted && (a == ap)
+    }
+}
+
+quickcheck! {
+    fn size_combinations(it: Iter<i16>) -> bool {
+        correct_size_hint(it.tuple_combinations::<(_, _)>())
+    }
+}
+
+quickcheck! {
+    fn equal_combinations(it: Iter<i16>) -> bool {
+        let values = it.clone().collect_vec();
+        let mut cmb = it.tuple_combinations();
+        for i in 0..values.len() {
+            for j in i+1..values.len() {
+                let pair = (values[i], values[j]);
+                if pair != cmb.next().unwrap() {
+                    return false;
+                }
+            }
+        }
+        cmb.next() == None
+    }
+}
+
+quickcheck! {
+    fn size_pad_tail(it: Iter<i8>, pad: u8) -> bool {
+        correct_size_hint(it.clone().pad_using(pad as usize, |_| 0)) &&
+            correct_size_hint(it.dropping(1).rev().pad_using(pad as usize, |_| 0))
+    }
+}
+
+quickcheck! {
+    fn size_pad_tail2(it: Iter<i8, Exact>, pad: u8) -> bool {
+        exact_size(it.pad_using(pad as usize, |_| 0))
+    }
+}
+
+quickcheck! {
+    fn size_powerset(it: Iter<u8, Exact>) -> bool {
+        // Powerset cardinality gets large very quickly, limit input to keep test fast.
+        correct_size_hint(it.take(12).powerset())
+    }
+}
+
+quickcheck! {
+    fn size_duplicates(it: Iter<i8>) -> bool {
+        correct_size_hint(it.duplicates())
+    }
+}
+
+quickcheck! {
+    fn size_unique(it: Iter<i8>) -> bool {
+        correct_size_hint(it.unique())
+    }
+
+    fn count_unique(it: Vec<i8>, take_first: u8) -> () {
+        let answer = {
+            let mut v = it.clone();
+            v.sort(); v.dedup();
+            v.len()
+        };
+        let mut iter = cloned(&it).unique();
+        let first_count = (&mut iter).take(take_first as usize).count();
+        let rest_count = iter.count();
+        assert_eq!(answer, first_count + rest_count);
+    }
+}
+
+quickcheck! {
+    fn fuzz_group_by_lazy_1(it: Iter<u8>) -> bool {
+        let jt = it.clone();
+        let groups = it.group_by(|k| *k);
+        itertools::equal(jt, groups.into_iter().flat_map(|(_, x)| x))
+    }
+}
+
+quickcheck! {
+    fn fuzz_group_by_lazy_2(data: Vec<u8>) -> bool {
+        let groups = data.iter().group_by(|k| *k / 10);
+        let res = itertools::equal(data.iter(), groups.into_iter().flat_map(|(_, x)| x));
+        res
+    }
+}
+
+quickcheck! {
+    fn fuzz_group_by_lazy_3(data: Vec<u8>) -> bool {
+        let grouper = data.iter().group_by(|k| *k / 10);
+        let groups = grouper.into_iter().collect_vec();
+        let res = itertools::equal(data.iter(), groups.into_iter().flat_map(|(_, x)| x));
+        res
+    }
+}
+
+quickcheck! {
+    fn fuzz_group_by_lazy_duo(data: Vec<u8>, order: Vec<(bool, bool)>) -> bool {
+        let grouper = data.iter().group_by(|k| *k / 3);
+        let mut groups1 = grouper.into_iter();
+        let mut groups2 = grouper.into_iter();
+        let mut elts = Vec::<&u8>::new();
+        let mut old_groups = Vec::new();
+
+        let tup1 = |(_, b)| b;
+        for &(ord, consume_now) in &order {
+            let iter = &mut [&mut groups1, &mut groups2][ord as usize];
+            match iter.next() {
+                Some((_, gr)) => if consume_now {
+                    for og in old_groups.drain(..) {
+                        elts.extend(og);
+                    }
+                    elts.extend(gr);
+                } else {
+                    old_groups.push(gr);
+                },
+                None => break,
+            }
+        }
+        for og in old_groups.drain(..) {
+            elts.extend(og);
+        }
+        for gr in groups1.map(&tup1) { elts.extend(gr); }
+        for gr in groups2.map(&tup1) { elts.extend(gr); }
+        itertools::assert_equal(&data, elts);
+        true
+    }
+}
+
+quickcheck! {
+    fn equal_chunks_lazy(a: Vec<u8>, size: u8) -> bool {
+        let mut size = size;
+        if size == 0 {
+            size += 1;
+        }
+        let chunks = a.iter().chunks(size as usize);
+        let it = a.chunks(size as usize);
+        for (a, b) in chunks.into_iter().zip(it) {
+            if !itertools::equal(a, b) {
+                return false;
+            }
+        }
+        true
+    }
+}
+
+quickcheck! {
+    fn equal_tuple_windows_1(a: Vec<u8>) -> bool {
+        let x = a.windows(1).map(|s| (&s[0], ));
+        let y = a.iter().tuple_windows::<(_,)>();
+        itertools::equal(x, y)
+    }
+
+    fn equal_tuple_windows_2(a: Vec<u8>) -> bool {
+        let x = a.windows(2).map(|s| (&s[0], &s[1]));
+        let y = a.iter().tuple_windows::<(_, _)>();
+        itertools::equal(x, y)
+    }
+
+    fn equal_tuple_windows_3(a: Vec<u8>) -> bool {
+        let x = a.windows(3).map(|s| (&s[0], &s[1], &s[2]));
+        let y = a.iter().tuple_windows::<(_, _, _)>();
+        itertools::equal(x, y)
+    }
+
+    fn equal_tuple_windows_4(a: Vec<u8>) -> bool {
+        let x = a.windows(4).map(|s| (&s[0], &s[1], &s[2], &s[3]));
+        let y = a.iter().tuple_windows::<(_, _, _, _)>();
+        itertools::equal(x, y)
+    }
+
+    fn equal_tuples_1(a: Vec<u8>) -> bool {
+        let x = a.chunks(1).map(|s| (&s[0], ));
+        let y = a.iter().tuples::<(_,)>();
+        itertools::equal(x, y)
+    }
+
+    fn equal_tuples_2(a: Vec<u8>) -> bool {
+        let x = a.chunks(2).filter(|s| s.len() == 2).map(|s| (&s[0], &s[1]));
+        let y = a.iter().tuples::<(_, _)>();
+        itertools::equal(x, y)
+    }
+
+    fn equal_tuples_3(a: Vec<u8>) -> bool {
+        let x = a.chunks(3).filter(|s| s.len() == 3).map(|s| (&s[0], &s[1], &s[2]));
+        let y = a.iter().tuples::<(_, _, _)>();
+        itertools::equal(x, y)
+    }
+
+    fn equal_tuples_4(a: Vec<u8>) -> bool {
+        let x = a.chunks(4).filter(|s| s.len() == 4).map(|s| (&s[0], &s[1], &s[2], &s[3]));
+        let y = a.iter().tuples::<(_, _, _, _)>();
+        itertools::equal(x, y)
+    }
+
+    fn exact_tuple_buffer(a: Vec<u8>) -> bool {
+        let mut iter = a.iter().tuples::<(_, _, _, _)>();
+        (&mut iter).last();
+        let buffer = iter.into_buffer();
+        assert_eq!(buffer.len(), a.len() % 4);
+        exact_size(buffer)
+    }
+}
+
+// with_position
+quickcheck! {
+    fn with_position_exact_size_1(a: Vec<u8>) -> bool {
+        exact_size_for_this(a.iter().with_position())
+    }
+    fn with_position_exact_size_2(a: Iter<u8, Exact>) -> bool {
+        exact_size_for_this(a.with_position())
+    }
+}
+
+quickcheck! {
+    fn correct_group_map_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let count = a.len();
+        let lookup = a.into_iter().map(|i| (i % modulo, i)).into_group_map();
+
+        assert_eq!(lookup.values().flat_map(|vals| vals.iter()).count(), count);
+
+        for (&key, vals) in lookup.iter() {
+            assert!(vals.iter().all(|&val| val % modulo == key));
+        }
+    }
+}
+
+/// A peculiar type: Equality compares both tuple items, but ordering only the
+/// first item.  This is so we can check the stability property easily.
+#[derive(Clone, Debug, PartialEq, Eq)]
+struct Val(u32, u32);
+
+impl PartialOrd<Val> for Val {
+    fn partial_cmp(&self, other: &Val) -> Option<Ordering> {
+        self.0.partial_cmp(&other.0)
+    }
+}
+
+impl Ord for Val {
+    fn cmp(&self, other: &Val) -> Ordering {
+        self.0.cmp(&other.0)
+    }
+}
+
+impl qc::Arbitrary for Val {
+    fn arbitrary<G: qc::Gen>(g: &mut G) -> Self {
+        let (x, y) = <(u32, u32)>::arbitrary(g);
+        Val(x, y)
+    }
+    fn shrink(&self) -> Box<dyn Iterator<Item = Self>> {
+        Box::new((self.0, self.1).shrink().map(|(x, y)| Val(x, y)))
+    }
+}
+
+quickcheck! {
+    fn minmax(a: Vec<Val>) -> bool {
+        use itertools::MinMaxResult;
+
+
+        let minmax = a.iter().minmax();
+        let expected = match a.len() {
+            0 => MinMaxResult::NoElements,
+            1 => MinMaxResult::OneElement(&a[0]),
+            _ => MinMaxResult::MinMax(a.iter().min().unwrap(),
+                                      a.iter().max().unwrap()),
+        };
+        minmax == expected
+    }
+}
+
+quickcheck! {
+    fn minmax_f64(a: Vec<f64>) -> TestResult {
+        use itertools::MinMaxResult;
+
+        if a.iter().any(|x| x.is_nan()) {
+            return TestResult::discard();
+        }
+
+        let min = cloned(&a).fold1(f64::min);
+        let max = cloned(&a).fold1(f64::max);
+
+        let minmax = cloned(&a).minmax();
+        let expected = match a.len() {
+            0 => MinMaxResult::NoElements,
+            1 => MinMaxResult::OneElement(min.unwrap()),
+            _ => MinMaxResult::MinMax(min.unwrap(), max.unwrap()),
+        };
+        TestResult::from_bool(minmax == expected)
+    }
+}
+
+quickcheck! {
+    #[allow(deprecated)]
+    fn tree_fold1_f64(mut a: Vec<f64>) -> TestResult {
+        fn collapse_adjacent<F>(x: Vec<f64>, mut f: F) -> Vec<f64>
+            where F: FnMut(f64, f64) -> f64
+        {
+            let mut out = Vec::new();
+            for i in (0..x.len()).step(2) {
+                if i == x.len()-1 {
+                    out.push(x[i])
+                } else {
+                    out.push(f(x[i], x[i+1]));
+                }
+            }
+            out
+        }
+
+        if a.iter().any(|x| x.is_nan()) {
+            return TestResult::discard();
+        }
+
+        let actual = a.iter().cloned().tree_fold1(f64::atan2);
+
+        while a.len() > 1 {
+            a = collapse_adjacent(a, f64::atan2);
+        }
+        let expected = a.pop();
+
+        TestResult::from_bool(actual == expected)
+    }
+}
+
+quickcheck! {
+    fn exactly_one_i32(a: Vec<i32>) -> TestResult {
+        let ret = a.iter().cloned().exactly_one();
+        match a.len() {
+            1 => TestResult::from_bool(ret.unwrap() == a[0]),
+            _ => TestResult::from_bool(ret.unwrap_err().eq(a.iter().cloned())),
+        }
+    }
+}
+
+quickcheck! {
+    fn at_most_one_i32(a: Vec<i32>) -> TestResult {
+        let ret = a.iter().cloned().at_most_one();
+        match a.len() {
+            0 => TestResult::from_bool(ret.unwrap() == None),
+            1 => TestResult::from_bool(ret.unwrap() == Some(a[0])),
+            _ => TestResult::from_bool(ret.unwrap_err().eq(a.iter().cloned())),
+        }
+    }
+}
+
+quickcheck! {
+    fn consistent_grouping_map_with_by(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+
+        let lookup_grouping_map = a.iter().copied().map(|i| (i % modulo, i)).into_grouping_map().collect::<Vec<_>>();
+        let lookup_grouping_map_by = a.iter().copied().into_grouping_map_by(|i| i % modulo).collect::<Vec<_>>();
+
+        assert_eq!(lookup_grouping_map, lookup_grouping_map_by);
+    }
+
+    fn correct_grouping_map_by_aggregate_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo < 2 { 2 } else { modulo } as u64; // Avoid `% 0`
+        let lookup = a.iter()
+            .map(|&b| b as u64) // Avoid overflows
+            .into_grouping_map_by(|i| i % modulo)
+            .aggregate(|acc, &key, val| {
+                assert!(val % modulo == key);
+                if val % (modulo - 1) == 0 {
+                    None
+                } else {
+                    Some(acc.unwrap_or(0) + val)
+                }
+            });
+        
+        let group_map_lookup = a.iter()
+            .map(|&b| b as u64)
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .filter_map(|(key, vals)| {
+                vals.into_iter().fold(None, |acc, val| {
+                    if val % (modulo - 1) == 0 {
+                        None
+                    } else {
+                        Some(acc.unwrap_or(0) + val)
+                    }
+                }).map(|new_val| (key, new_val))
+            })
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for m in 0..modulo {
+            assert_eq!(
+                lookup.get(&m).copied(), 
+                a.iter()
+                    .map(|&b| b as u64)
+                    .filter(|&val| val % modulo == m)
+                    .fold(None, |acc, val| {
+                        if val % (modulo - 1) == 0 {
+                            None
+                        } else {
+                            Some(acc.unwrap_or(0) + val)
+                        }
+                    })
+            );
+        }
+    }
+
+    fn correct_grouping_map_by_fold_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo } as u64; // Avoid `% 0`
+        let lookup = a.iter().map(|&b| b as u64) // Avoid overflows
+            .into_grouping_map_by(|i| i % modulo)
+            .fold(0u64, |acc, &key, val| {
+                assert!(val % modulo == key);
+                acc + val
+            });
+
+        let group_map_lookup = a.iter()
+            .map(|&b| b as u64)
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().sum()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &sum) in lookup.iter() {
+            assert_eq!(sum, a.iter().map(|&b| b as u64).filter(|&val| val % modulo == key).sum::<u64>());
+        }
+    }
+
+    fn correct_grouping_map_by_fold_first_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo } as u64; // Avoid `% 0`
+        let lookup = a.iter().map(|&b| b as u64) // Avoid overflows
+            .into_grouping_map_by(|i| i % modulo)
+            .fold_first(|acc, &key, val| {
+                assert!(val % modulo == key);
+                acc + val
+            });
+
+        // TODO: Swap `fold1` with stdlib's `fold_first` when it's stabilized
+        let group_map_lookup = a.iter()
+            .map(|&b| b as u64)
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().fold1(|acc, val| acc + val).unwrap()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &sum) in lookup.iter() {
+            assert_eq!(sum, a.iter().map(|&b| b as u64).filter(|&val| val % modulo == key).sum::<u64>());
+        }
+    }
+
+    fn correct_grouping_map_by_collect_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup_grouping_map = a.iter().copied().into_grouping_map_by(|i| i % modulo).collect::<Vec<_>>();
+        let lookup_group_map = a.iter().copied().map(|i| (i % modulo, i)).into_group_map();
+
+        assert_eq!(lookup_grouping_map, lookup_group_map);
+    }
+
+    fn correct_grouping_map_by_max_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).max();
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().max().unwrap()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &max) in lookup.iter() {
+            assert_eq!(Some(max), a.iter().copied().filter(|&val| val % modulo == key).max());
+        }
+    }
+
+    fn correct_grouping_map_by_max_by_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).max_by(|_, v1, v2| v1.cmp(v2));
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().max_by(|v1, v2| v1.cmp(v2)).unwrap()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &max) in lookup.iter() {
+            assert_eq!(Some(max), a.iter().copied().filter(|&val| val % modulo == key).max_by(|v1, v2| v1.cmp(v2)));
+        }
+    }
+
+    fn correct_grouping_map_by_max_by_key_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).max_by_key(|_, &val| val);
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().max_by_key(|&val| val).unwrap()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &max) in lookup.iter() {
+            assert_eq!(Some(max), a.iter().copied().filter(|&val| val % modulo == key).max_by_key(|&val| val));
+        }
+    }
+    
+    fn correct_grouping_map_by_min_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).min();
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().min().unwrap()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &min) in lookup.iter() {
+            assert_eq!(Some(min), a.iter().copied().filter(|&val| val % modulo == key).min());
+        }
+    }
+
+    fn correct_grouping_map_by_min_by_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).min_by(|_, v1, v2| v1.cmp(v2));
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().min_by(|v1, v2| v1.cmp(v2)).unwrap()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &min) in lookup.iter() {
+            assert_eq!(Some(min), a.iter().copied().filter(|&val| val % modulo == key).min_by(|v1, v2| v1.cmp(v2)));
+        }
+    }
+
+    fn correct_grouping_map_by_min_by_key_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).min_by_key(|_, &val| val);
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().min_by_key(|&val| val).unwrap()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &min) in lookup.iter() {
+            assert_eq!(Some(min), a.iter().copied().filter(|&val| val % modulo == key).min_by_key(|&val| val));
+        }
+    }
+    
+    fn correct_grouping_map_by_minmax_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).minmax();
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().minmax()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &minmax) in lookup.iter() {
+            assert_eq!(minmax, a.iter().copied().filter(|&val| val % modulo == key).minmax());
+        }
+    }
+
+    fn correct_grouping_map_by_minmax_by_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).minmax_by(|_, v1, v2| v1.cmp(v2));
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().minmax_by(|v1, v2| v1.cmp(v2))))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &minmax) in lookup.iter() {
+            assert_eq!(minmax, a.iter().copied().filter(|&val| val % modulo == key).minmax_by(|v1, v2| v1.cmp(v2)));
+        }
+    }
+
+    fn correct_grouping_map_by_minmax_by_key_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo }; // Avoid `% 0`
+        let lookup = a.iter().copied().into_grouping_map_by(|i| i % modulo).minmax_by_key(|_, &val| val);
+
+        let group_map_lookup = a.iter().copied()
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().minmax_by_key(|&val| val)))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &minmax) in lookup.iter() {
+            assert_eq!(minmax, a.iter().copied().filter(|&val| val % modulo == key).minmax_by_key(|&val| val));
+        }
+    }
+
+    fn correct_grouping_map_by_sum_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = if modulo == 0 { 1 } else { modulo } as u64; // Avoid `% 0`
+        let lookup = a.iter().map(|&b| b as u64) // Avoid overflows
+            .into_grouping_map_by(|i| i % modulo)
+            .sum();
+
+        let group_map_lookup = a.iter().map(|&b| b as u64)
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().sum()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &sum) in lookup.iter() {
+            assert_eq!(sum, a.iter().map(|&b| b as u64).filter(|&val| val % modulo == key).sum::<u64>());
+        }
+    }
+
+    fn correct_grouping_map_by_product_modulo_key(a: Vec<u8>, modulo: u8) -> () {
+        let modulo = Wrapping(if modulo == 0 { 1 } else { modulo } as u64); // Avoid `% 0`
+        let lookup = a.iter().map(|&b| Wrapping(b as u64)) // Avoid overflows
+            .into_grouping_map_by(|i| i % modulo)
+            .product();
+
+        let group_map_lookup = a.iter().map(|&b| Wrapping(b as u64))
+            .map(|i| (i % modulo, i))
+            .into_group_map()
+            .into_iter()
+            .map(|(key, vals)| (key, vals.into_iter().product::<Wrapping<u64>>()))
+            .collect::<HashMap<_,_>>();
+        assert_eq!(lookup, group_map_lookup);
+
+        for (&key, &prod) in lookup.iter() {
+            assert_eq!(
+                prod,
+                a.iter()
+                    .map(|&b| Wrapping(b as u64))
+                    .filter(|&val| val % modulo == key)
+                    .product::<Wrapping<u64>>()
+            );
+        }
+    }
+
+    // This should check that if multiple elements are equally minimum or maximum
+    // then `max`, `min` and `minmax` pick the first minimum and the last maximum.
+    // This is to be consistent with `std::iter::max` and `std::iter::min`.
+    fn correct_grouping_map_by_min_max_minmax_order_modulo_key() -> () {
+        use itertools::MinMaxResult;
+
+        let lookup = (0..=10)
+            .into_grouping_map_by(|_| 0)
+            .max_by(|_, _, _| Ordering::Equal);
+
+        assert_eq!(lookup[&0], 10);
+
+        let lookup = (0..=10)
+            .into_grouping_map_by(|_| 0)
+            .min_by(|_, _, _| Ordering::Equal);
+
+        assert_eq!(lookup[&0], 0);
+        
+        let lookup = (0..=10)
+            .into_grouping_map_by(|_| 0)
+            .minmax_by(|_, _, _| Ordering::Equal);
+
+        assert_eq!(lookup[&0], MinMaxResult::MinMax(0, 10));
+    }
+}
+
+quickcheck! {
+    fn counts(nums: Vec<isize>) -> TestResult {
+        let counts = nums.iter().counts();
+        for (&item, &count) in counts.iter() {
+            #[allow(clippy::absurd_extreme_comparisons)]
+            if count <= 0 {
+                return TestResult::failed();
+            }
+            if count != nums.iter().filter(|&x| x == item).count() {
+                return TestResult::failed();
+            }
+        }
+        for item in nums.iter() {
+            if !counts.contains_key(item) {
+                return TestResult::failed();
+            }
+        }
+        TestResult::passed()
+    }
+}
+
+quickcheck! {
+    fn test_double_ended_zip_2(a: Vec<u8>, b: Vec<u8>) -> TestResult {
+        let mut x =
+          multizip((a.clone().into_iter(), b.clone().into_iter()))
+            .collect_vec();
+        x.reverse();
+
+        let y =
+          multizip((a.into_iter(), b.into_iter()))
+          .rfold(Vec::new(), |mut vec, e| { vec.push(e); vec });
+
+        TestResult::from_bool(itertools::equal(x, y))
+    }
+
+    fn test_double_ended_zip_3(a: Vec<u8>, b: Vec<u8>, c: Vec<u8>) -> TestResult {
+        let mut x =
+          multizip((a.clone().into_iter(), b.clone().into_iter(), c.clone().into_iter()))
+            .collect_vec();
+        x.reverse();
+
+        let y =
+          multizip((a.into_iter(), b.into_iter(), c.into_iter()))
+          .rfold(Vec::new(), |mut vec, e| { vec.push(e); vec });
+
+        TestResult::from_bool(itertools::equal(x, y))
+    }
+}
+
+
+fn is_fused<I: Iterator>(mut it: I) -> bool
+{
+    for _ in it.by_ref() {}
+    for _ in 0..10{
+        if it.next().is_some(){
+            return false;
+        }
+    }
+    true
+}
+
+quickcheck! {
+    fn fused_combination(a: Iter<i16>) -> bool
+    {
+        is_fused(a.clone().combinations(1)) &&
+        is_fused(a.combinations(3))
+    }
+
+    fn fused_combination_with_replacement(a: Iter<i16>) -> bool
+    {
+        is_fused(a.clone().combinations_with_replacement(1)) &&
+        is_fused(a.combinations_with_replacement(3))
+    }
+
+    fn fused_tuple_combination(a: Iter<i16>) -> bool
+    {
+        is_fused(a.clone().fuse().tuple_combinations::<(_,)>()) &&
+        is_fused(a.fuse().tuple_combinations::<(_,_,_)>())
+    }
+
+    fn fused_unique(a: Iter<i16>) -> bool
+    {
+        is_fused(a.fuse().unique())
+    }
+
+    fn fused_unique_by(a: Iter<i16>) -> bool
+    {
+        is_fused(a.fuse().unique_by(|x| x % 100))
+    }
+
+    fn fused_interleave_shortest(a: Iter<i16>, b: Iter<i16>) -> bool
+    {
+        !is_fused(a.clone().interleave_shortest(b.clone())) &&
+        is_fused(a.fuse().interleave_shortest(b.fuse()))
+    }
+    
+    fn fused_product(a: Iter<i16>, b: Iter<i16>) -> bool
+    {
+        is_fused(a.fuse().cartesian_product(b.fuse()))
+    }
+
+    fn fused_merge(a: Iter<i16>, b: Iter<i16>) -> bool
+    {
+        is_fused(a.fuse().merge(b.fuse()))
+    }
+
+    fn fused_filter_ok(a: Iter<i16>) -> bool
+    {
+        is_fused(a.map(|x| if x % 2 == 0 {Ok(x)} else {Err(x)} )
+                 .filter_ok(|x| x % 3 == 0)
+                 .fuse())
+    }
+
+    fn fused_filter_map_ok(a: Iter<i16>) -> bool
+    {
+        is_fused(a.map(|x| if x % 2 == 0 {Ok(x)} else {Err(x)} )
+                 .filter_map_ok(|x| if x % 3 == 0 {Some(x / 3)} else {None})
+                 .fuse())
+    }
+
+    fn fused_positions(a: Iter<i16>) -> bool
+    {
+        !is_fused(a.clone().positions(|x|x%2==0)) &&
+        is_fused(a.fuse().positions(|x|x%2==0))
+    }
+
+    fn fused_update(a: Iter<i16>) -> bool
+    {
+        !is_fused(a.clone().update(|x|*x+=1)) &&
+        is_fused(a.fuse().update(|x|*x+=1))
+    }
+
+    fn fused_tuple_windows(a: Iter<i16>) -> bool
+    {
+        is_fused(a.fuse().tuple_windows::<(_,_)>())
+    }
+
+    fn fused_pad_using(a: Iter<i16>) -> bool
+    {
+        is_fused(a.fuse().pad_using(100,|_|0))
+    }
+}
+
+quickcheck! {
+    fn min_set_contains_min(a: Vec<(usize, char)>) -> bool {
+        let result_set = a.iter().min_set();
+        if let Some(result_element) = a.iter().min() {
+            result_set.contains(&result_element)
+        } else {
+            result_set.is_empty()
+        }
+    }
+
+    fn min_set_by_contains_min(a: Vec<(usize, char)>) -> bool {
+        let compare = |x: &&(usize, char), y: &&(usize, char)| x.1.cmp(&y.1);
+        let result_set = a.iter().min_set_by(compare);
+        if let Some(result_element) = a.iter().min_by(compare) {
+            result_set.contains(&result_element)
+        } else {
+            result_set.is_empty()
+        }
+    }
+
+    fn min_set_by_key_contains_min(a: Vec<(usize, char)>) -> bool {
+        let key = |x: &&(usize, char)| x.1;
+        let result_set = a.iter().min_set_by_key(&key);
+        if let Some(result_element) = a.iter().min_by_key(&key) {
+            result_set.contains(&result_element)
+        } else {
+            result_set.is_empty()
+        }
+    }
+
+    fn max_set_contains_max(a: Vec<(usize, char)>) -> bool {
+        let result_set = a.iter().max_set();
+        if let Some(result_element) = a.iter().max() {
+            result_set.contains(&result_element)
+        } else {
+            result_set.is_empty()
+        }
+    }
+
+    fn max_set_by_contains_max(a: Vec<(usize, char)>) -> bool {
+        let compare = |x: &&(usize, char), y: &&(usize, char)| x.1.cmp(&y.1);
+        let result_set = a.iter().max_set_by(compare);
+        if let Some(result_element) = a.iter().max_by(compare) {
+            result_set.contains(&result_element)
+        } else {
+            result_set.is_empty()
+        }
+    }
+
+    fn max_set_by_key_contains_max(a: Vec<(usize, char)>) -> bool {
+        let key = |x: &&(usize, char)| x.1;
+        let result_set = a.iter().max_set_by_key(&key);
+        if let Some(result_element) = a.iter().max_by_key(&key) {
+            result_set.contains(&result_element)
+        } else {
+            result_set.is_empty()
+        }
+    }
+}